1
Introduction
1.1
History
Formaldehyde was described in the year 1855 by the Russian scientist Alexander Michailowitsch
Butlerow. The technical synthesis by dehydration of methanol was achieved in 1867
by the German chemist August Wilhelm von Hofmann. The versatility that makes it suitable
for use in various industrial applications was soon discovered, and the compound was
one of the first to be indexed by Chemical Abstracts Service (CAS). In 1944, Walker
published the first edition of his classic work Formaldehyde.(1) Between 1900 and
1930, formaldehyde-based resins became important adhesives for wood and wood composites.
The first commercial particle board was produced during World War II in Bremen, Germany.
Since 1950, particle board has become an attractive alternative to solid wood for
the manufacturing of furniture. Particle board and other wood-based panels were subsequently
also used for the construction of housing. Adverse health effects from exposure to
formaldehyde in prefabricated houses, especially irritation of the eyes and upper
airways, were first reported in the mid-1960s. Formaldehyde emissions from particle
boards bonded with urea formaldehyde resin were soon identified as the cause of the
complaints. As a consequence, a guideline value of 0.1 ppm was proposed in 1977 by
the former German Federal Agency of Health to limit human exposure in dwellings. Criteria
for the limitation and regulation of formaldehyde emissions from wood-based materials
were established in 1981 in Germany and Denmark. The first regulations followed in
the United States in 1985 or thereabouts. In Germany and the United States, large-scale
test chambers were used for the evaluation of emissions. Although the chamber method
is very reliable, it is also time-consuming and expensive. This meant there was a
strong demand for simple laboratory test methods.(2)
1.2
Formaldehyde as a Priority Indoor Pollutant
Discussion about formaldehyde as a possible carcinogen started in 1980 when the carcinogenicity
of formaldehyde in rats and mice after long-term inhalation exposure was reported.
3,4
These publications and the results of studies of human exposure assessment for formaldehyde
triggered an avalanche of scientific work as well as stories in the yellow press.
Although electronic databases and powerful search engines are now available, it is
still difficult to survey all papers in the technical and medical literature. Notwithstanding
this, formaldehyde is definitely the most common and the best-known indoor air pollutant.
Over the years, the release of formaldehyde from building products has been decreasing.
On the other hand, formaldehyde concentrations in ambient air are increasing continuously,
especially in the urban environment. For this reason, formaldehyde slipped out of
the primary focus of indoor research in the 1990s, although special formaldehyde-related
events occasionally come to the attention of the general public. Well-known examples
are reports about increasedformaldehyde emission from furniture coatings in Germany
(1992) and high formaldehyde concentrations in mobile homes in the United States (2006).
However, in 2004, formaldehyde discussions were generally taken up again when formaldehyde
was considered as carcinogenic for humans. As a consequence, various authorities and
institutions have proposed new indoor air guidelines, giving values that are nearly
ubiquitous. Although a prioritized ranking of chemicals and exposures that cause concern
is difficult and uncertain, the Scientific Committee on Health and Environmental Risks
(SCHER)(5) states that formaldehyde (like carbon monoxide, nitrogen dioxide, benzene,
naphthalene, environmental tobacco smoke (ETS), radon, lead, and organophosphate pesticides)
is a compound of concern in the indoor environment.
1.3
Review of Literature
In this article, the current status of indoor-related formaldehyde research is summarized.
This review is based on a literature search carried out using the “Web of Science”
(ISI). The keywords “formaldehyde” and “indoor” gave 1240 hits for the period from
1990 to 2008. The results were cross-checked by searching Elsevier’s “ScienceDirect”
(1850 hits), “Blackwell Synergy” (174 hits for the Indoor Air journal alone), the
American Chemical Society, PubMed, SpringerLink, and Informaworld. Other references
known to the authors such as standards (DIN, VDI, CEN, ISO, ASHRAE) and conference
proceedings were also included.
2
General Description
2.1
Physical and Chemical Properties
Formaldehyde is produced on a large scale by the oxidation of methane or methanol
in the presence of a catalyst.(6) At room temperature, it is a colorless gas that
is flammable and highly reactive. The compound is soluble in water, ethanol, diethyl
ether, and acetone. In aqueous solution, methylene glycol [CH2(OH)2] and polymethylene
glycols [H(CH2O)
n
OH] are formed.(2) Formaldehyde is commonly purchased as a 37% solution in water,
known as formalin, with 10% methanol as a stabilizer. The annual production of 37%
formaldehyde is about 20 million tons worldwide.(7) In a recent review article, Tang
et al.(8) estimate a global output of 32 million tons of formaldehyde in 2006, with
the highest producers being China (34%), the United States (14%), and Germany (8%).
More than 65% of the total formaldehyde is used to synthesize resins. The name paraformaldehyde
describes a polymeric structure with 8−100 formaldehyde units per molecule.(1) The
cyclic trimer of formaldehyde C3H6O3 is called 1,3,5-trioxane. Formaldehyde has a
dipolar resonance structure (see Table 1), which makes the molecule a typical electrophile.
According to Roffael(2) and Walker,(1) the most important reactions apart from polymerization
are as follows:
Table 1
Physical and Chemical Properties of Formaldehyde
parameter
ref
structure
synonyms
methanal, methyl aldehyde, methyl oxide
CAS registry no.
50-00-0
molecular formula
HCHO, CH2O
SMILES
C=O
molecular wt
30.03 g mol−1
(422)
melting pointa
−92 °C
(422)
boiling point
−21 °C
(422)
dipole moment
2.33 D
(422)
solubility
soluble in water, ethanol, ether, acetone
Henry’s law constant
2.5 × 103 M atm−1 (25 °C)
(9)
log(K
ow)b
−0.83
k
OH•
c
9.37 × 10−12 cm3 molecule−1 s−1 (298 K)
(11)
k
O3
c
2.09 × 10−24 cm3 molecule−1 s−1 (298 K)
(15)
k
NO3
c
5.80 × 10−16 cm3 molecule−1 s−1 (298 K)
(12)
conversion factor
0.1 ppm = 124.8 μg m−3 (293 K, 1013 mbar)
1 μg m−3 = 0.815 ppb (293 K, 1013 mbar)
a
In some publications, a boiling point of −118 °C is given.
b
Calculated with SPARC (http://ibmlc2.chem.uga.edu/sparc/).
c
See also NIST Kinetics Database (http://kinetics.nist.gov).
• Reaction with ammonia to form hexamethylene tetramine
• Cannizzaro reaction
• Aldol reaction
• Tischenko reaction
The Henry’s law constant is 2.5 × 103 M atm−1 at 298 K (6.3 × 103 M atm−1 if diol
formation is taken into account).(9) The calculated octanol/water partition coefficient
is log(K
ow) = −0.83. The World Health Organization (WHO) has published a value of log(K
ow) = −1.(10) The reaction rate constant with the OH-radical is k
OH = 9.3 × 10−12 cm3 molecule−1 s−1 at 298 K.(11) Assuming an atmospheric OH concentration
of 106 molecules cm−3, this gives an HCHO lifetime against the OH reaction of 31 h.
In the gas phase, formaldehyde shows a structured absorption spectrum between 260
and 360 nm.(12) The lifetimes against the photolytic processes HCHO → H2 + CO and
HCHO → H + HCO, calculated for the latitude of 50°, are 6.9 and 2.1 h, respectively.(13)
Atkinson(14) has calculated formaldehyde lifetimes in the atmosphere with respect
to photolysis (τ = 4 h), reaction with the OH radical (τ = 1.2 days), reaction with
the NO3 radical (τ = 80 days), and reaction with O3 (τ > 4.5 years). The gas-phase
reaction of ozone with formaldehyde has been studied by Braslavsky and Heicklen.(15)
2.2
Toxicology
The high solubility of formaldehyde in water causes rapid absorption in the respiratory
and gastrointestinal tract. Here, it can be oxidized to form formate and exhaled as
carbon dioxide or incorporated in biological matrices. The biological half-life is
extremely short at about 1 min.(16) As an electrophile, formaldehyde can react with
nucleophilic biogenic compounds in the body.(17) Formaldehyde itself is produced in
small amounts from methanol via the enzyme alcohol dehydrogenase (ADH),
18,19
which is a human metabolite and can be measured in urine.
20,21
According to a report published by “Health Canada”, which is based on human clinical
studies and animal experiments, the primary effects of acute exposure to formaldehyde
are irritation of the mucosa of the upper respiratory tract and the eyes.(22) The
RD50 values (exposure concentration producing a 50% respiratory rate decrease as an
indication of respiratory tract irritation) of male mice are 3.1−5.3 ppm for an exposure
time of 5−10 min.(23) The lowest observable adverse effect levels (LOAEL) for human
sensory irritation range from 0.4 ppm (rhinitis) to 3 ppm (eye, nasal, and throat
irritation).(23) A recent study of formaldehyde and sensory irritation in humans showed
that eye irritation is the most sensitive parameter. A no observed effect level (NOEL)
of 0.5 ppm was derived in the case of constant exposure.(24)
Different threshold values are available for the odor perception of formaldehyde.
Devos et al.(25) have calculated a standardized human olfactory threshold of 0.87
ppm (1.07 mg m−3). The WHO has estimated absolute odor thresholds (defined as the
concentration at which 50% of the panel detects the odor) between 0.06 and 0.22 mg
m−3.(10) In the INDEX report
17,26
very low odor thresholds of 0.03 and 0.035 mg m−3 are specified, which refer to an
updated WHO report(27) and an unavailable paper from 1917 cited in Devos et al.,(25)
respectively.
In 2004, the International Agency for Research on Cancer (IARC) has classified formaldehyde
as carcinogenic for humans (Group 1).(28) This evaluation is based on information
regarding the relationship between nasopharyngeal cancer and leukemia related to the
exposure to formaldehyde. In the European Union, formaldehyde is classified under
Category 3 as a suspected carcinogen (Directive 2001/58/CEE). Since 1991, the U.S.
EPA has regarded formaldehyde as a probable human carcinogen (B1) (http://cfpub.epa.gov/ncea/iris).
2.3
Application of Formaldehyde
Formaldehyde is a chemical feedstock for numerous industrial processes. It is also
used as a preservative, disinfectant, and biocide. As far as the indoor environment
is concerned, its use as a component of thermosetting adhesives is of particular significance.
The reactions described below are described in detail by different authors.
1,2,29−31
Urea-formaldehyde (UF) adhesives (so-called aminoplasts) are still the most commonly
used products in the manufacturing of wood-based materials and furniture due to their
rapid curing, their compatibility with additives, and their low price. In the first
step, mono-, di-, and trimethylolurea are formed from formaldehyde and urea in a Mannich
reaction. This is followed by condensation reactions to build up the polymer (see
eqs 5 and 6). UF adhesives have poor water resistance: the presence of water results
in a hydrolysis of the C−N bond and, as a consequence, the release of formaldehyde.
Melamine−urea−formaldehyde (MUF) adhesives are similar to UF adhesives. They are produced
by mixing portions of UF and melamine−formaldehyde (MF) or by cocondensation of all
monomers in one batch. Equation 7 shows the first step of the melamine−formaldehyde
reaction.
Phenol−formaldehyde (PF) adhesives (so-called phenoplasts) are made by electrophilic
substitution to methylol phenol in the first step, as shown in eq 8. In alkaline solution,
the reaction results in highly viscous resins of low molecular weight, called resols.
A novolac with a high degree of cross-linking is formed in acidic solution. PF adhesives
are very stable and water-resistant and have a high adherence to wood. In the past,
plastics made of PF resins were also known as Bakelite and were, among other things,
used as casings for telephones, radios, etc.
Melamine−urea−phenol−formaldehyde (MUPF) adhesives are used for the production of
moisture-proofed wood-based products and for construction materials. Like MUF adhesives,
they are produced by the addition of small amounts of phenol.
Figure 1 provides an overview of the industrial utilization of formaldehyde. Indoor-related
applications of formaldehyde in the past and present have been summarized by a number
of authors.
32,33
A brief overview is given below:
Figure 1
Survey of industrial applications for formaldehyde and formaldehyde products.
• Wood-based products (particle board, oriented-strand board (OSB), high-density fiber
board (HDF), medium-density fiber board (MDF), plywood)
• Cork products (flooring materials)
• Insulation materials made of UF foam, mineral wool, or glass wool
• Paper products
• Coating materials, paints, and lacquers containing formaldehyde as preservative
• Textiles
• Cleaning and caring products
• Disinfectants and preservatives
• Photoprocessing chemicals
• Cosmetics.
3
Sources of Formaldehyde
3.1
Outdoor Sources
3.1.1
Formaldehyde as a Natural Compound
A number of natural and anthropogenic outdoor sources are known for formaldehyde.(34)
Like other VOCs, it is a biogenic compound and part of plant physiological and plant/atmosphere
exchange processes.(35) In 1927, Freudenberg and Harder identified formaldehyde as
a decomposition product of lignin.(36) Müller et al.(37) found formaldehyde within
and above a coniferous forest in Germany. Trapp et al.(38) mention formaldehyde as
a degradation product of isoprene in a eucalyptus forest in Portugal. Carter and Atkinson(39)
proposed a scheme for the formation of formaldehyde from isoprene via reaction with
OH and NO. Kesselmeier et al.(40) have measured several parts per billion (ppb) of
formaldehyde in a remote forest site in central Amazonia. Smidt et al.(41) have been
able to detect low formaldehyde concentrations of 0.24−0.52 ppb in forests in the
Austrian Alps (920 m) and 0.16−0.30 ppb at a mountaintop site (1758 m). Long-term
measurements at rural European monitoring sites were carried out by Solberg et al.(42)
Meyer and Boehme(43) have shown that formaldehyde is released from solid wood. Seco
et al.(44) have reviewed VOC emission and uptake by plants. They point out that formaldehyde
seems to be a product of methanol oxidation, but the exact origin within plants remains
unclear. Other possible mechanisms, such as 5,10-methylene-tetrahydrofolate dissociation,
glyoxylate decarboxylation, or oxidative demethylation reactions, have been proposed
by Hanson and Roje.(45) Formaldehyde is also produced in the marine environment.(46)
3.1.2
Atmospheric Reactions
Thousands of organic compounds are released into the atmosphere from biogenic sources.
According to Atkinson and Arey,(47) these organic compounds include isoprene, monoterpenes,
sesquiterpenes, and a number of oxygenated compounds. In the troposphere, they react
with hydroxyl (OH) radicals, nitrate (NO3) radicals, and ozone (O3), and they play
an important role in the chemistry of the lower troposphere. The gas-phase reaction
of ozone with unsaturated hydrocarbons is known to produce aldehydes, ketones, and
acids as main components. As shown in Figure 2, an ozonide is formed from the reaction
of the ozone with the double bond. The two decomposition pathways of the ozonide are
of equal importance for alkenes of the structure RCH=CH2, R1CH=CHR, or R1R2C=CR3R4,
but for alkenes with the structure R1R2C=CH2 or R1R2C=CHR3, the ozonide decomposes
preferentially via pathway 3 while forming formaldehyde or R3CHO.(48) Grosjean and
Grosjean(49) have identified formaldehyde in a number of alkene−ozone reactions. Grosjean
et al.(50) have studied atmospheric oxidation reactions of biogenic hydrocarbons in
a test chamber and have measured formaldehyde concentrations up to 26 ppb at 22 °C
with excess cyclohexane to scavenge OH from the reaction of ozone (0.07−0.1 ppm) with
β-pinene (1.0 ppm), d-limonene (1.2 ppm), and trans-caryophyllene (0.2−0.5 ppm), respectively.
Formaldehyde formation from ozonolysis of carvone, carveol, geraniol, and citral has
been reported by Nunes et al.(51) Griesbaum et al.(52) have identified formaldehyde
by means of NMR spectroscopy as a byproduct of the gas-phase ozonolysis of terpenes.
Relatively high formaldehyde outdoor concentrations can be found in the urban air
of heavily polluted megacities. Here, HCHO is directly released into the atmosphere
or produced by photochemical gas-phase reaction of hydroxyl radicals with so-called
nonmethane hydrocarbons (NMHC). During one ozone episode in the city of Beijing, Duan
et al.(53) measured a concentration of 36 μg m−3 formaldehyde in urban air. The rate
constant for the reaction of the hydroxyl radical with methane is low (k
OH(CH4) = 6.3 × 10−15 cm3 molecule−1 s−1). This means that the formation of formaldehyde
from methane is only important in remote areas.
Figure 2
Formation of carbonyl compounds from alkene−ozone reactions.
12,421
3.1.3
Outdoor Combustion
The combustion of wood is also a natural source of formaldehyde.(54) Hedberg et al.(55)
have studied birch combustion and report formaldehyde emission rates of 180−710 mg/kg
wood. This is in accordance with data by Schauer et al.(56) for oak (759 mg/kg), pine
(1165 mg/kg), and eucalyptus (599 mg/kg). Enhanced formaldehyde concentrations can
be found under the influence of wildfire activity.(57) Reisen and Brown(58) have measured
levels up to 0.57 ppm for the personal exposure of Australian firefighters. Formaldehyde
is a known component of automobile exhaust gas.(59) Public interest in biodiesel fuel
has recently stimulated fresh discussion of that topic. Machado Correra and Arbilla(60)
and also Guarieiro et al.(61) have shown that carbonyl emissions are dependent on
the biodiesel content and that the biodiesel ester molecules are probably the source
of these carbonyls. However, Peng et al.(62) arrive at a different conclusion and
attribute lower formaldehyde emissions to more complete combustion and increases in
engine performance.
3.1.4
Formaldehyde Release into the Atmosphere
The WHO(27) pointed out that industrial formaldehyde releases can occur at any stage
of the production, use, storage, transportation, or disposal of products with residual
formaldehyde. Emissions have been detected from chemical manufacturing plants, pulp
and paper mills, forestry product plants, tire and rubber plants, coal processing
plants, textile mills, automotive manufacturing plants, and the metal products industry.
Hauptmann et al.(7) have evaluated data from different references. On the basis of
data from Canada, they provide the following estimated breakdown of emissions into
outdoor air: traffic (70%), aircraft (11%), shipping (7%), the formaldehyde processing
industry (10%), and power plants and waste incineration (<1%). It is difficult to
determine the global emission of formaldehyde. The WHO report(27) mentions total releases
of 8960 t/a from U.S. industries into the environment for the year 1992. The nationwide
emission estimate for the United States of 4500 t/a made by Nazaroff and Alvarez-Cohen(63)
is based on a U.S. EPA report from 1993.
3.2
Indoor Sources
Generally speaking, exposure to formaldehyde is higher indoors than outdoors. This
is mainly due to the stronger sources and low air exchange rates in the indoor environment.(64)
A special situation arises for workplaces, which are not, however, treated in detail
in this review. A general overview of formaldehyde sources, which might contribute
to increased indoor concentrations, is shown in Figure 3. More specific information
on different sources is provided in Table 2.
Figure 3
Possible indoor- and outdoor-related formaldehyde sources.
Table 2
Potential Formaldehyde Sources in the Indoor Environment as Determined in Different
Studies
source
comments
ref
Wood and Wood-Based Products
solid wood
oak, Douglas fir, beech, spruce, pine
(43)
particle board
effect of hot-pressing
(412)
particle board
recycled wood-waste sprayed with PMDI/PF
(423)
particle board, MDF
comparison of standard methods
(133)
particle board
effect of aging
(424)
particle board
effect of humidity and temperature
(425)
particle board
(168)
oriented-strand board
comparison of analytical techniques
(246)
wood-based composites
laminate, engineered flooring, MDF, particle board
(247,426)
wood based panels
effect of loading and ventilation
(427)
wood panels
interlaboratory comparison
(178)
particle board, plywood
with carpet and insulation
(428,429)
pressed wood products
(179,430)
wood-based flooring materials
effect of ozone, infrared, sunlight, UV-A, UV-B
(256)
Insulation Materials
mineral wool
(249)
mineral wool
interlaboratory comparison
(431)
Flooring Materials
carpet
interaction of ozone
(78,85)
laminate
effect of temperature
(248)
Cork products
natural cork and cork tiles
(415)
building finishing materials
effect of temperature
(432)
Coating Materials
latex paint
(74,75)
latex paint
presence of ozone
(88)
water-based paint
emission of biocides
(76)
natural paint
presence of ozone
(408)
photocatalytic paint
effect of irradiation
(93−95)
Combustion
wood burning
wood-heated homes in Quebec, Canada
(433)
wood burning
wood-heated homes in Sweden
(434)
cooking stoves
(435)
cooking
residential cooking activities in a test house
(97)
burning of incense
measured in temples
(436)
cigarette smoking
(98,100,261)
mosquito coils and candles
(437)
Miscellaneous
personal computers
(438)
laser printers, photocopiers
(439−441)
miscellaneous building materials
pine wood, gypsum board, wallpaper, carpet, PVC, linoleum, paint, and presence of
ozone
(71)
furniture and home equipment
parquet, sofa, table, chair, carpet, book shelves
(244)
miscellaneous building materials
plywood, particle board, hard board, carpet, barrier materials
(243)
miscellaneous building materials
test house study
(442)
textiles, permanent-press fabrics
effect of aging, temperature, humidity
(73,443,444)
wall coverings
paper, acrylic, PVC
(70)
cleaning products, air fresheners
(445)
car air freshener
presence of ozone
(84)
chemical products
formaldehyde and formaldehyde releasers
(446)
cleaning activities
(447)
household products
presence of ozone
(82,448)
consumer products
55 materials studied
(73)
miscellaneous materials
carpet, wall, floor, cooking oil, and presence of ozone
(72)
miscellaneous materials
wood-based products, carpet, textiles, heaters, burners, cigarettes
(263)
miscellaneous polymeric materials
PVC, carpet, SBR, wall coverings, rubber foam backing
(449)
VOC mixtures
presence of ozone
(255,450,451)
portable air cleaners
with and without air freshners
(258)
miscellaneous materials
ozone reactions during disinfection
(86,87)
miscellaneous materials
aircraft cabin materials and clothing fabrics
(91)
newspaper/books/journals
(452)
preservative
anatomical dissection course
(288,328)
human metabolites
breath air
(103−106)
3.2.1
Wood-Based Materials
In the past, there was a link between the two items “formaldehyde emission” and “wood-based
products”. This negative image resulted from the high formaldehyde release from UF-bonded
particle board under living conditions in the 1960s and 1970s. Dwellings in which
particle board was used extensively, such as prefabricated houses and mobile homes,
were particularly concerned, and many occupants complained about bad odors and adverse
health effects. Subsequent emissions of formaldehyde are due to the presence of small
amounts of free formaldehyde in the resin and to the reversibility of the urea−formaldehyde
reaction (see eqs 5 and 6). In Germany, the first publication dealing with this topic
appeared in 1962.(65) Formaldehyde emission from UF-bonded particle board may continue
for months or even years,(66) but the emission potential decreases with increasing
age. Since 1970 formaldehyde emission rates from particle board and other wood-based
materials have decreased as a consequence of governmental and voluntary guidelines
and regulations. Wood-based products bonded with PF adhesives show comparatively low
formaldehyde emission potentials because the cross-linking is more stable. Furthermore,
environmentally friendly adhesives using natural tannin have been developed to reduce
the dependence on formaldehyde-based adhesives.
67−69
3.2.2
Flooring Materials
Most laminates used in furniture production are impregnated with modified aminoplastic
resins and finished with lacquer. The emission of formaldehyde from veneered and laminated
wood-based products is mainly caused by adhesives and glueing. However, low-emitting
resins and new manufacturing techniques have distinctly improved such products. Paper
is known to be the main source for formaldehyde emission from wall coverings.(70)
In the case of flooring materials such as carpet, parquet, laminate, PVC, and linoleum,
the emission of formaldehyde is of no or only minor importance in the absence of ozone.
71,72
3.2.3
Insulation Materials
Mineral wool is preferred for insulation purposes in walls or floorings. This product
is made from molten glass, stone, or slag that is spun into fibers. Inorganic rock
or slag is the main component (typically 97%) of stone wool. The remaining 3% organic
content is generally a thermosetting resin binder and oil. Glass wool (GW) is made
from sand or recycled glass, limestone, and soda ash and usually contains 95%−96%
inorganic material. Urea-modified phenol−formaldehyde resins are used as binders,
producing low emissions of formaldehyde during use. Higher emissions are known from
insulation materials made of UF foam, but this form of insulation is of little importance
today.
3.2.4
Coatings
As far as liquid coating materials are concerned, acid-curing lacquers made of modified
urea− and melamine−formaldehyde resins were the strongest formaldehyde source.(73)
Mainly for that reason they have almost completely disappeared from the market in
central Europe. Nevertheless, acid-curing wood finishes are still applied in Scandinavia,
Eastern Europe, and Asia (http://www.kompass.com). High emissions of formaldehyde
from latex paint have been reported by Chang et al.
74,75
Formaldehyde and formaldehyde-releasing compounds such as dimethylol glycol and dimethylol
urea were routinely used as biocides in water-based paints and fungicidal products
but have now been widely replaced by other compounds such as isothiazolinones.(76)
The release of formaldehyde from catalytic paints will be treated in the next section.
3.2.5
Indoor Chemistry: General Aspects
Indoor chemistry is a special but sometimes important source of formaldehyde. Wolkoff
et al.(77) stated that reactions between unsaturated VOCs and ozone can form irritants
that may be responsible for many reported symptoms. One of the first indoor-related
papers on this topic was published in 1992 by Weschler et al.,(78) who reported the
emission of formaldehyde and other aldehydes from carpeting in the presence of ozone,
while concentrations of unsaturated compounds such as 4-phenylcyclohexene (PCH), 4-vinylcyclohexene
(VCH), and styrene decreased. In the indoor environment, we have a situation in which
ozone concentrations are lower, as compared with the outdoor environment,(79) while
concentrations of unsaturated compounds such as terpenes are in contrast distinctly
higher. Many terpenoids present in indoor air, such as limonene (k
O3
= 200 × 10−18 cm3 molecule−1 s−1), myrcene (k
O3
= 470 × 10−18 cm3 molecule−1 s−1), and terpinolene (k
O3
= 1880 × 10−18 cm3 molecule−1 s−1), exhibit high gas-phase reaction constants with
ozone.
80,81
Terpene/ozone reaction rates in indoor and outdoor air can therefore be of the same
order of magnitude. It has been shown recently that many household products contain
terpenes and can rapidly react with ozone under indoor-related conditions.
82−84
Other building products have also been studied for the emission of so-called secondary
products as the result of indoor-related chemical reactions.
71,72,85−88
Furthermore, formaldehyde has been detected as a reaction product of ozone-initiated
chemistry in aircraft cabins
89,90
and as a byproduct of surface reactions with aircraft cabin materials.(91) Wisthaler
et al.(90) and later Petrick and Dubowski(92) have identified oxidation reactions
of squalene, which is a major component of the skin, as a directly human related formaldehyde
source. Short-term formaldehyde emissions under test chamber conditions have been
found with indoor wall paints equipped with modified TiO2 to serve as a catalyst under
daylight or artificial light to reduce indoor air pollutants.
93−95
3.2.6
Indoor Combustion: General Aspects
Thermal treatment and combustion are known to be strong sources of formaldehyde indoors.
Balakrishnan et al. pointed out that about half of the world’s population, especially
the developing countries, relies on traditional fuels such as biomass as the primary
source of domestic energy.(96) Besides CO, NO
x
, SO2, polycyclic aromatic hydrocarbons (PAH), and particulates, formaldehyde is one
of the main components emitted from biomass fuel smoke. Residential cooking activities
were also identified as formaldehyde sources.(97) In developed countries, cigarette
smoke is the primary combustion source indoors.
98−101
3.2.7
Other Indoor-Related Sources
In his recent review on changes in indoor pollutants since the 1950s, Weschler(102)
notes that “easy care” and “permanent press” fabrics, especially T-shirts, pants,
and shirts, were introduced in the 1960s. These fabrics had been treated with formaldehyde
resins and had significant formaldehyde emissions close to the breathing zone. These
resins have since been improved, and such fabrics emit less formaldehyde today. There
are many more potential sources of formaldehyde in the indoor environment, such as
electronic equipment, paper, fabric dyes, inks, cosmetics, objects for anatomy dissection,
etc., but it is not possible to examine them in detail here (see Table 2).
Last but not least, it should be mentioned that traces of formaldehyde have also been
discovered as a product of human metabolic reactions. Lindinger et al.(103) have shown
that formaldehyde is a component of exhaled human breath. Wehinger et al.(104) have
identified increased levels of formaldehyde in exhaled breath samples from primary
lung cancer patients. Moser et al.(105) report formaldehyde concentrations in the
deep lung portion of human breath up to 72.7 ppbv with a median of 4.3 ppbv. Kushch
et al.(106) have examined trace compounds in the exhaled breath of 81 smokers, 210
nonsmokers, and 79 ex-smokers. For formaldehyde, median values of about 10 ppb were
measured and no statistically significant difference between smokers and nonsmokers
was observed. However, the experimental technique of proton-transfer-reaction mass
spectrometry (PTR-MS), being used in all cited studies for the measurement of formaldehyde,
103−106
has been criticized by several authors(485) (see the next section).
4
Sampling and Analysis of Formaldehyde
4.1
Analytical Methods
4.1.1
In-Situ Methods
For the analysis of formaldehyde in the outdoor environment, spectroscopic techniques
are convenient. Vairavamurthy et al.(107) have reviewed four in situ monitoring techniques:
(a) differential optical absorption spectroscopy (DOAS); (b) Fourier transform infrared
absorption (FTIR); (c) laser-induced fluorescence spectroscopy (LIFS); and (d) tunable
diode laser spectroscopy (TDLS). Finlayson-Pitts and Pitts(12) have compared formaldehyde
detection limits for FTIR, TDLS, and matrix isolation IR (see Table 3). Vairavamurthy
et al. also pointed out that in situ techniques usually require long optical paths,
which makes these methods unsuitable for routine applications. Infrared diode laser
spectroscopy was nevertheless used by Hanoune et al. for formaldehyde measurements
at ppb levels in libraries.
108,109
Photoacoustic spectroscopy (PAS) has been used occasionally for the determination
of formaldehyde in indoor air.
110−112
However, this method is susceptible to interference and suffers from high detection
limits. Cihelka et al.(113) used diode lasers in combination with FTIR and photacoustics
and achieved detection limits of <100 ppb. Proton-transfer-reaction mass spectrometry
(PTR-MS) is based on chemical ionization using H3O+ as the primary reactant ion.(486)
The method has been successfully applied in monitoring formaldehyde in outdoor and
indoor air,(103) although de Gouw and Warneke state that formaldehyde is a difficult
compound to detect by PTR-MS.(114) According to Wisthaler et al.,(142) the PTR-MS
method is less sensitive to formaldehyde than other carbonyl compounds due to the
loss of protonated formaldehyde resulting from the reaction with water.(115) Kushch(106)
suggested that m/z 31 ions (protonated formaldehyde) should be corrected for the isotope
effects of NO. Fragments of the reaction products of ethanol and O2
+ or methanol and O2
+ may also be observed. Thekedar et al. have excluded m/z 31 from breath gas analysis
for the reasons that the sensitivity depends strongly on humidity and that formaldehyde
is present in room air samples in much higher concentration than in the exhaled breath.(116)
As a potential alternative to PTR-MS, Spanel and Smith have applied selected ion flow
tube mass spectrometry (SIFT-MS) for the detection of formaldehyde in breath gas.(484)
Hak et al.(117) have made an intercomparison study of four different in situ techniques
for ambient formaldehyde measurement in urban air. Formaldehyde concentrations obtained
using continuously measuring DOAS, FTIR, and Hantzsch instruments agreed within 11%,
while two-hour integrated samples obtained by DNPH (see below) presented concentrations
up to 25% lower. In addition, the authors provide a detailed review of previous formaldehyde
intercomparison studies.
Table 3
Overview of Sampling Methods and Analytical Techniques for the Determination of Formaldehyde
in Air
method
comments
ref
In Situ
FTIR
LODa = 6 ppb at L = 1 km (ν̃b = 2779, 2781.5 cm−1)
(12)
TDLS
LODa = 0.05 ppb at L = 150 m (ν̃b = 2781 cm−1)
(12)
matrix isolation IR
LODa = 0.03 ppb
(12)
DOAS
(107,453)
PTR-MS
relatively low sensitivity for formaldehyde
(103)
SIFT-MS
relatively low sensitivity for formaldehyde
(484)
photoacoustics
LODa = 60 ppb (λc = 3.6 μm), interferences with other pollutants
(110)
Derivatization
acetylacetone (acac)
UV/vis and fluorimetry (Nash reagent)
(132,138,139)
DNPH
HPLC-UV/vis (also can be used for higher aldehydes)
(146,148)
MBTH
LODa = 0.08 ppb, near real-time measurement
(112,124,125)
dimedone
UV/vis and fluorimetry
(454,455)
pararosaniline
UV/vis
(2,107,120,121)
chromotropic acid
UV/vis
(2,107,120)
AHMT
UV/vis
(2,107,120,126)
PFBOAd
headspace GC/MS
(456)
kinetic fluorimetry
formaldehyde as a catalyst for rhodamine B oxidation
(457)
Sensors
microgas sensor
LODa = 0.06 ppm
(156)
microgas sensor
MWCNT or palladium-doped (LODa = 0.03 ppm)
(158,159)
cataluminescence
online monitoring using gas sensor
(157)
biosensor
LODa = 50 ppb, online detection using dehydrogenase reaction
(162)
Comparison of Methods
derivatization
acac vs pararosaniline and chromotropic acid methods
(118)
derivatization
DNPH vs acac
(155)
derivatization + in situ
DNPH vs TDLS
(108)
in situ
DOAS, FTIR, acac, DNPH
(117)
derivatization + in situ
acac, DNPH, DOAS, PTR-MS
(142)
a
LOD = limit of detection.
b
Wavenumber.
c
Center wavelength of pho-acoustic instrument.
d
o-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine.
4.1.2
Derivatization Methods
For indoor applications, batch-sampling methods are more convenient. Here, formaldehyde
is trapped in an absorber or on impregnated filters and cartridges. The compound is
then derivatized and the resulting chromophore can be analyzed by chromatography and/or
spectroscopy. However, a common problem is their nonspecificity for formaldehyde if
no chromatographic separation is applied. Most reagents also react with other carbonyl
compounds, and these byproduct could interfere with the target analyte.(118) Different
methods for the analysis of aldehydes have been reviewed and discussed by several
authors.
2,107,119
Brief descriptions will be given of most of the derivatization methods (see refs (2,107,120)
for chemical reaction schemes), but the chromotropic acid, the acetylacetone, and
the DNPH methods will be considered in more detail. Other techniques are summarized
in Table 3. Titrimetric methods are not discussed in this review, and the reader is
referred to the book by Roffael(2) and the review by Marutzky.(31)
Pararosaniline Method.(121) A magenta dye is formed from formaldehyde and pararosaniline
in the presence of sodium sulfite. Under acidic conditions, this intermediate reacts
with SO2 to form the chromophore. Its strong absorbance at 570 nm is used in UV/vis
detection. Other aldehydes, such as acetaldehyde, acrolein, and propanal, interfere,
but at pH ≤ 1.0, the reaction is specific for formaldehyde. In the presence of atmospheric
SO2, a toxic Hg(II) reagent is required in order to eliminate the sulfite formed from
SO2. Although the sensitivity of the modified pararosaniline method is limited and
it is susceptible to interference, it has in the past been one of the most widely
used techniques for the determination of formaldehyde.
The MBTH method
122−124
(3-methyl-2-benzothiazolinonehydrazone) is a nonselective colorimetric method for
the determination of aliphatic aldehydes of low molecular weight. MBTH reacts on aldehydes
to give an azide. In parallel with this, a reactive cation is formed by oxidation
of MBTH with Fe(III). In a further step, a blue ionic dye is formed. Absorbance is
monitored at 628 nm. This reaction is less sensitive than that of the pararosaniline
method. Other aldehydes undergo an analogous reaction, but the yield is generally
lower. The MBTH method quantifies total aldehydes in ambient air in terms of their
formaldehyde equivalents. Strong reducing agents can interfere with the determination
of aldehydes. An online detection system has been described by Toda et al.(125)
AHMT (4-amino-3-hydrazino-5-mercapto-4H-1,2,4-triazole) reacts with aldehydes in strong
alkaline media to form a colorless intermediate product. This product is oxidized
by atmospheric oxygen to give a magenta-colored dye, detectable by colorimetry at
a wavelength of 550 nm. The sensitivity of the AHMT reagent varies, depending on the
type of reacting aldehyde. On the basis of the AHMT reaction, Kawamura et al.(126)
have developed a sensor to measure formaldehyde concentrations in the range of 0.04−1
ppm with a sampling time of 3 min. The authors explain that their device has been
developed for prevention and control of the “Sick Building Syndrome” (SBS). This statement
has to be questioned, since SBS is a very complex phenomenon (see section ).
Chromotropic Acid Method.(127) In the presence of concentrated sulfuric acid, chromotropic
acid (1,8-dihydroxynaphthalene-3,6-disulfonic acid) reacts with formaldehyde to give
a red-violet hydroxydiphenylmethane derivative (see eq 9). In the second step of the
reaction, a violet quinoid oxidation product is formed with atmospheric oxygen. The
concentrated sulfuric acid is a catalyst for dehydration and oxidation. The absorption
maximum at 580 nm is used for UV/vis detection. The reaction is specific for formaldehyde
when the pH value is <1.0. Interference can be caused by phenols, some other organic
substances, and strong oxidizers. One of its main disadvantages is the low stability
of chromotropic acid in solution. Nevertheless, the method has been standardized in
the USA by NIOSH(128) and is used for the determination of formaldehyde in large-scale
chambers,(129) in small-scale chambers,(130) and in the desiccator method.(131)
The acetylacetone (acac) method as described by Nash(132) is a widely applied(133)
standard procedure and recommended in Europe
134−136
and Japan(137) for the determination of formaldehyde emissions from wood-based materials.
The reaction, which is based on the Hantzsch synthesis, involves the cyclization of
2,4-pentanedione (acac), ammonium acetate, and formaldehyde to form the dihydropyridine
3,5-diacetyl-1,4-dihydrolutidine (DDL) (see eq 10). Quantification can be performed
by UV/vis spectroscopy at 412 nm (ε(H2O) = 7850 L mol−1 cm−1). The molecule also exhibits
fluorescence (ϕ(H2O) = 0.005), thus offering the possibility of very selective fluorimetric
determination at 510 nm,
138,139
since other carbonyl compounds do not form strongly fluorescent Hantzsch products.
107,140
Sampling is carried out by passing air through an absorber where formaldehyde is quantitatively
trapped in distilled water. After addition of 2,4-pentanedione and ammonium acetate,
the reaction to form DDL is completed within 10 min at 40 °C. The long sampling time
of 40 min could be a drawback of the conventional acac method in the case of dynamic
processes where the formaldehyde concentration rapidly changes over time. Portable
instruments are now available which enable reliable in situ measurement of formaldehyde
on a time scale of seconds.
141−143
Acetoacetanilide has been introduced as an alternative to acac for the derivatization
of formaldehyde based on the Hantzsch reaction.(144) The authors state that the reaction
with acetoacetanilide can be carried out at room temperature.
The DNPH method
145,146
is frequently used for the simultaneous analysis of formaldehyde, other aldehydes,
and ketones. In acidic solution, hydrazones are formed from 2,4-dinitrophenylhydrazine
(DNPH) by nucleophilic addition to the carbonyl group, followed by elimination of
water (see eq 11). In sampling, air is pulled through cartridges typically containing
silica gel and coated with an acid solution of DNPH (today XAD-2 is only very rarely
used as adsorbent(147)). To prevent water from condensing on the surface, the cartridge
is sometimes covered with C18-alkyles.(148) After sampling, the cartridge is then
eluted with acetonitrile. This eluate is used directly for HPLC analysis. Chromatographic
separation of the hydrazones is achieved by means of a C18 column and water/acetonitrile
solvent combinations with binary or ternary gradients. UV spectroscopy is used for
detection, with the absorption maxima of different hydrazones ranging from 340 to
427 nm.(146) This method is described in U.S. EPA Method TO-11A,(149) ASTM D 5197(150)
and is accepted as an international standard by ISO.(151) It is also applied in several
standards for the determination of the emission of formaldehyde from building products.
152,153
The data presented in Figure 4 are obtained from a recent study.(154) They show a
very good agreement between the acac and DNPH methods, and both analytical techniques
are equivalent as regards the determination of formaldehyde in indoor air. The statistical
deviations between the acac and DNPH methods increase with higher formaldehyde concentrations.
This is obvious from the plot of residuals (calculated value against measured value)
(inset in Figure 4). A similar observation of increasing standard deviations was made
by Hanoune et al.(108) when comparing the DNPH method with infrared diode laser spectroscopy.
A fair correlation between the acac and DNPH methods was also observed by Trapp and
De Serves(155) at low atmospheric concentrations. On the other hand, Wisthaler et
al.(142) found that DNPH-HPLC data severely underestimated formaldehyde levels in
the atmosphere simulation chamber (SAPHIR). This was explained by a suppressed hydrazine-to-hydrazone
conversion at low humidities. The acac method with derivatization to DDL in aqueous
solution followed by fluorimetric determination is probably the most reliable method
of measuring formaldehyde in indoor air although the fluorescence response is dependent
on temperature and DDL decomposes on exposure to heat or light.(139)
Figure 4
Linear relationship between formaldehyde concentrations in a test house measured by
the acac and the DNPH methods for 78 data points. The inset shows the unweighted residuals
plotted (reprinted from ref (154) with permission from Elsevier).
4.1.3
Sensors
There is an increasing demand for fast and simple formaldehyde indoor monitoring methods,
and this has stimulated research activities in the field of sensor technology. However,
the sensors available still suffer from comparatively high detection limits, which
makes the technique mainly suitable for workplace environments. Lv et al.(156) have
developed a microgas sensor based on a microhot plate, which can detect a concentration
of 0.06 ppm of indoor formaldehyde. Zhou et al.(157) describe a cataluminescence-based
gas sensor using nanosized V2Ti4O13 as a probe for online determination of formaldehyde
in air which has a detection limit of 0.06 mg m−3. A semiconductor gas sensor of tin
oxide doped with hydroxyl-functionalized multiwall carbon nanotubes (MWCNTs) has been
designed and tested by Wang et al.(158) The same authors describe a formaldehyde gas
microsensor of palladium-doped tin dioxide on a silicon substrate.(159) Both sensors
are able to detect formaldehyde concentrations of 0.03 ppm. Other common types of
sensors are based on enzyme reactions. In the presence of oxidized nicotinamide adenine
dinucleotide (NAD+) as a catalyst, formaldehyde is oxidized to form formic acid, which
is detected by electrochemical or photochemical techniques.
160−163
The conductometric biosensor developed by Vianello et al.(162) provides an indoor-related
detection limit of 50 ppb. Seo et al. describe the biosensing of formaldehyde on the
basis of fungal growth.(164)
4.1.4
Future Trends in Formaldehyde Analysis
In the indoor environment, formaldehyde concentrations of interest range between 1
and 1000 ppb. Sampling or rather measuring devices should be transportable, robust,
and easy to calibrate. Most of the spectroscopic techniques, such as DOAS, FTIR, and
TDLS, do not fulfill these requirements, but they are useful for determining sub-ppb
levels in rural or remote areas. Derivatization methods are state-of-the-art for the
indoor analysis of formaldehyde, with the most important being the chromotropic acid,
DNPH, and acetylacetone methods. All three have the drawback of long sampling times,
typically 0.5−2 h. The standardized DNPH method(151) is also used for higher aldehydes.
Nevertheless, the acetylacetone method is easier to use, specific, and highly sensitive
for formaldehyde when combined with fluorescence spectroscopy. This makes the method
very attractive for use in online measuring devices. The development of such techniques
is urgently needed for the study of dynamic processes and is definitely among current
and future trends. The formaldehyde online monitor AL-4021 (see http://www.aero-laser.de)
is based on the acac method, is commercially available, and has been tested by Hak
et al.(117) for measurement of formaldehyde in urban air. In our institute, AL-4021
has been successfully applied for the time-resolved determination of formaldehyde
in test chambers. In Figure 5, the time vs formaldehyde concentration curve of an
MDF board in a 1 m3 chamber (T = 23 °C, r.h. = 45%, n = 1.0 h−1, L = 1.0 m2 m−3) is
shown. The solid line has been recorded by use of AL-4021 with a time resolution of
1 s. The data represented by solid and open circles have been measured with the conventional
acac method by UV/vis and fluorescence detection, respectively.
Figure 5
Formaldehyde emission from a MDF board in a 1 m3 chamber at T = 23 °C, r.h. = 45%,
n = 1 h−1, and L = 1 m2 m−3. Solid line, online detection using an AL-4021 formaldehyde
analyzer; solid circles (●), conventional acac method with UV/vis detection; open
circles (○), conventional acac method with fluorescence detection.
Modern nanotechnology also opens up possibilities for new measurement techniques,
especially sensors. At the present time, detection limits are inadequate, but it is
foreseeable that the next generation of sensors may be suitable for many applications
in active and passive formaldehyde measurement. A comprehensive review covering many
aspects of carbonyl sampling and analysis in indoor air has recently been published
by Barro et al.(165) In the present publication, the reader is referred to Table 3,
where analytical procedures are summarized.
4.2
Emission Testing Using Chambers and Cells
4.2.1
Chambers
The evaluation of the formaldehyde emission potential of individual products and materials
under indoor-related conditions and over defined time scales requires the use of climate-controlled
emission testing systems, so-called emission test chambers, and cells, the size of
which can vary between a few cubic centimeters, and several cubic meters, depending
on the application. The first room-sized test chambers were developed in the mid-1970s
when building authority regulations were being introduced concerning formaldehyde
emissions from wood particle boards (see Figure 6A and B). Large chambers can be regarded
as the “standard meter” in formaldehyde testing, as only with this type are real-life-related
scenarios possible. The formaldehyde test methods used in North America, Europe, and
China are derived from large-chamber studies. Nowadays, a common size for an emission
test chamber is 1 m3 and the interior is usually made of glass(166) (see Figure 6C)
or stainless steel.(167) Chambers smaller than this are used only occasionally for
the testing of formaldehyde. However, Crump et al.(168) have demonstrated a good comparability
between a 1 m3 chamber, a 4.5 L chamber, and a 2.4 L chamber for measuring formaldehyde
emission from wood-based particle and fiber boards. In the United States the so-called
Dynamic Micro Chamber (DMC) is used.(169) The DMC is an apparatus for measuring formaldehyde
emission from composite wood products bonded with urea−formaldehyde adhesives and
employs a combination of a small sample chamber and an electrochemical sensor. So
far, the DMC method has not been developed into an international standard. Large and
small chambers have only recently gained new importance due to the California Air
Resources Board (CARB) (see http://www.arb.ca.gov) approving regulations to reduce
the emission of formaldehyde from wood-based panels,(170) in accordance with ASTM
E 1333 (large chamber) and ASTM D 6007 (small chamber).
Figure 6
Different devices for measuring formaldehyde emission from building products under
indoor-related conditions. (A) 48 m3 stainless steel chamber with particle board;
(B) diagram of the 48 m3 stainless steel chamber; (C) 1 m3 glass chamber with particle
board; (D) field and laboratory emission cell (FLEC) with floor covering.
During an emission investigation, the product or material is tested with regard to
temperature (T), relative humidity (r.h.), air exchange rate (n), air velocity, and
product loading factor (L = ratio of the surface of the product to be investigated
to the volume of the emission test chamber) under standardized conditions in the testing
device. This can be sealed to exclude gas from the outside atmosphere. This procedure
is suitable for measuring the formaldehyde concentration in air and the product specific
emission rate (SER). The latter can be related to sample length (μg m−1 h−1), volume
(μg m−3 h−1), area (μg m−2 h−1), or unit (μg unit−1 h−1). Note that different standards
use different methods for sample preparation, measurement, and expressing the results.
In most cases, the result is presented as an area-specific emission rate
129,153
or as a chamber concentration in the steady state(134) (see section ). The chamber
test is a conventional process in which conditions are selected in such a way that
they reflect those to be found in realistic indoor rooms. In interpreting the results
of test chamber/cell investigations, it must, under certain circumstances, be accepted
as a limiting factor that not all realistic conditions to be found in an indoor room
can be simulated. Groah et al.(171) have compared different protocols and analytical
methods for the large-chamber testing of formaldehyde in Europe and in North America.
The European protocol produced values that were 20% lower than the North American
protocol, but strong linear relationships between the tests could be observed.
4.2.2
Field and Laboratory Emission Cell (FLEC)
It is also desirable to have a measuring system that can be used to carry out emission
testing and quality assurance on location. The relevant principle of a transportable
emission testing cell for mobile application was implemented in Scandinavia for the
first time in 1991 with the so-called field and laboratory emission cell (FLEC).
172,173
The FLEC as an example of a frequently used type of cell is shown in Figure 6D. Air
flow conditions in the FLEC have been described by several authors.
174,175
The device now makes it possible to carry out nondestructive emission testing on surfaces
within the framework of field investigations. The FLEC has been tested for formaldehyde
applications,(176) but a standardized method for measuring formaldehyde emissions
has not yet been established. The general requirements relating to the design and
properties of an emission cell are described in ISO standard 16000-10.(177)
4.2.3
Determination of Emission Rates and Steady-State Concentrations
Air measurement in a chamber or cell initially produces the concentration C(t) at
the time t of the measurement. For better comparability of the measured data, the
specific emission rate (SER) independent of air exchange and loading is to be preferred.
In some publications, the specific emission rate (area or unit) is called the emission
factor (EF).
The time-dependent determination of the emission potential is carried out on the basis
of the balance equation (eq 12), where C(t) = chamber concentration (in μg m−3 or
mg m−3), n = air exchange (in h−1), and L = loading (in m2 m−3).
For a decaying concentration−time function, SER(t) is obtained from eq 12 by transition
to the difference quotient according to eq 13.
where
This means that if n + 1 experimental data are available for concentration, it will
be possible to obtain n − 1 emission rate values by this method. In the steady-state
(dC/dt = 0), eq 12 progresses to eq 15, which is frequently used for the determination
of emission rates.
In the case of wood-based materials, the time vs concentration curve often decreases
continuously and the power function 16, as proposed by Colombo et al.,(178) can be
applied for the interpolation of the data. An example is provided in Figure 7 for
the least-squares fit to eq 16 of the formaldehyde concentration data from a 48 m3
chamber test at T = 23 °C, r.h. = 45%, n = 1 h−1, and L = 1 m2 m−3 (see figure captions
for fit parameters).
Figure 7
Formaldehyde emission from particle board in a large chamber (48 m3) at T = 23 °C,
r.h. = 45%, n = 1 h−1, and L = 1 m2 m−3. The interpolation curve was obtained from
nonlinear regression analysis using eq 16 with A = 1.18, B = 4.76, and D = 0.29.
C(t) is the chamber concentration in milligrams per cubic meter, t is the time in
hours, and A, B, and D are fit parameters. The steady-state concentration is determined
from eq 16 on the basis of a procedure given in European standard EN 717-1. It must
always be kept in mind that this function goes to infinity when approaching zero and
does not take peak values into account.
4.3
Material Testing
Formaldehyde testing in chambers and cells is usually time-consuming and calls for
sophisticated equipment. For pretesting and production control, the so-called derived
methods have been developed. These methods are based on determining the formaldehyde
content or formaldehyde release under special testing conditions. Several derived
methods are available for wood-based panels in particular (see Table 4 and Figure
8). Critical reviews of the methods are provided by Roffael,(2) Marutzky,(31) Marutzky
and Margosian,(179) and Risholm-Sundmann et al.(133) The first correlations between
chamber testing and derived methods were given by Mehlhorn.(180)
Figure 8
Standardized laboratory methods (perforator, gas analysis, flask, desiccator) for
the determination of formaldehyde release from wood-based materials.
Table 4
Standard Methods for Formaldehyde Emission Testing
method
standard
volume
ref
chamber method
EN 717-1
>12 m3, 1 m3, 0.225 m3
(134)
JIS A 1901
20 L to 1 m3
(152)
JIS A 1911
>1 m3 to 80 m3
(153)
ASTM E 1333
>22 m3
(458)
ASTM D 6007
1 m3
(130)
desiccator method
ASTM D 5582
≈ 10.5 La
(131)
JIS A 1460
9−11 L
(137)
JASb
9−11 L
(459)c
gas analysis method
EN 717-2
4 L chamber
(135)
flask method
EN 717-3
500 mL flask
(136)
perforator method
EN 120
(460)
a
Inside diameter is 250 mm.
b
JAS no. depends on product.
c
Refers to JAS 233 for plywood.
4.3.1
Perforator Method
The formaldehyde content of wood-based panels is determined by the perforator method.
The content principally correlates with the emission value, especially for wood composites
of similar structure and density. The method was developed in the late 1960s by the
former European Particleboard Federation (FESYP). Since 1984, it has become established
as European standard EN 120. It is a procedure for extracting small samples of wood-based
panels by means of boiling toluene and is suitable for unlaminated and uncoated wood-based
panels. The extracted formaldehyde is sampled through perforation in water and is
measured in the aqueous solution by a suitable analytical procedure. The original
method using iodine proved to be too unspecific and was later on replaced by the specific
acetylacetone method. The perforator value depends on the moisture content of the
tested samples. Correction factors, based on a reference moisture content, are used
to compensate for this influence.(181) The test procedure needs comparably simple
equipment and has a short total running time of 3 h. For these reasons, it is widely
used for production control in the wood-based panel industry, especially in Europe
and China.
4.3.2
Flask Method
Another simple test for wood-based panels is the flask test. It was developed by Roffael
in 1975.(182) The test is based on storing one to three board pieces with a total
mass close to 20 g in a closed polyethylene bottle with a volume of 400 cm3. The pieces
are stored over 50 mL of distilled water for a defined period of time—usually 24 h—at
a constant temperature of 40 °C. The formaldehyde released is absorbed by the water.
The formaldehyde content of the aqueous solution is determined photometrically at
412 nm by the acetylacetone method and referred to the dry weight of the tested pieces.
A slightly modified version of the method was later standardized as EN 717-3. Disadvantages
of the method are the small quantity of material which can be tested and the unrealistic
ratio of open edges to surfaces of the tested specimens. In spite of these limitations,
the method is most suitable for production control of panels with a similar structure.
Variations of the methods have been developed with larger bottles and modified testing
times.
4.3.3
Desiccator Methods
The so-called desiccator methods
131,137
are based on the same principles as the flask method. Pieces of wood-based panel of
known surface area are positioned over water for 24 h at a constant temperature. Instead
of a small plastic bottle, a glass desiccator with a volume of 9−13 L is used, thus
permitting larger quantities of test material. Analysis of formaldehyde is usually
carried out by either the acetylacetone method(137) or the chromotropic acid method.(131)
A number of variations of the desiccator method exist (see Table 4). In the meantime,
a standard harmonized between the wood-based panel industries of Australia, Japan,
and New Zealand has been accepted by the International Standardization Organization
as ISO/CD 12460-4.
4.3.4
Gas Analysis
An eminently suitable derived formaldehyde test is the gas analysis method, which
determines the accelerated formaldehyde release at an elevated temperature of 60 °C.
It can be used for all types of panels, including coated boards. This test is also
used for testing formaldehyde-emitting impregnated papers, laminates, and insulation
foams. The method requires a specimen of 400 mm × 50 mm × thickness. The sample is
placed in a test tube at a controlled temperature of 60 °C. A gas stream of 1 L per
minute is passed through the tube. The emitted formaldehyde is absorbed by gas-washing
bottles and measured photometrically. The bottles are changed once per hour over a
total testing time of 4 h. Usually the values measured are averaged, ignoring the
first hour. The result is expressed in mg h−1 m−2. This procedure is standardized
as EN 717-2.(135)
4.4
Air Sampling Strategies
Appropriate sampling strategies for the measurement of formaldehyde in indoor air
are discussed by Gavin et al.(183) and in the ISO 16000-2 standard.(184) Discontinuous
methods for measuring room-air components can be subdivided into short-term and long-term
types. While active sampling is suitable not only for short but also—provided the
air flow rate has been reduced correspondingly—for longer measurement intervals, passive
sampling is used mostly for long-term measurement. In discontinuous methods, the measured
value is first determined by subsequent analysis in the laboratory. In the case of
active sampling, air is passed through a sampling device using a pump and the air
volume is accurately determined. A passive sampler is a device which is capable of
taking air samples at a rate controlled by diffusion through a static layer or permeation
through a membrane.(185) The flow of pollutants into the passive sampler is proportional
to the difference of concentrations in the ambient air (C
air) and on the surface of the passive sampler (C
A), as shown in eq 17.
D
i
is the diffusion coefficient for compound i, m
i
is the collected mass of compound i, A is the surface area of the sampler, l is the
length of diffusion, and t is the collection time. In the case of active sampling
with short measurement time intervals (10 min to approximately 4 h), international
guidelines have been drawn up for interior air measurement.(151) Formaldehyde is a
highly volatile compound, which means that the indoor concentration will generally
depend on the source strength and on the air exchange rate. Strong sinks such as gypsum
board may influence the concentration by adsorption and desorption effects.(186) Due
to its high solubility, water and other polar liquids act as permanent sinks for formaldehyde.
For a constant emission source and an air exchange rate n, the time required to reach
a desired percentage of the steady-state concentration C
∞ is given by eq 18.
The solid curve in Figure 9 models the increase in formaldehyde concentration in a
room after ventilation for an air exchange rate of n = 0.3 h−1 in the absence of sinks.
Two sampling strategies can be applied: the solid circles in Figure 9 stand for a
1 h staggered strategy, in which sampling starts immediately after the windows are
closed. The steady-state value must be calculated from eq 18. The open circles in
Figure 9 represent possible sampling times at the steady-state level. This method
depends on a good guess at the air exchange rate.
Figure 9
Modeled increase of the formaldehyde concentration after ventilation for a continuous
source and an air exchange rate of n = 0.3 h−1 in the absence of sinks (see eq 18).
The solid circles (●) represent a staggered measuring strategy; the open circles (○)
represent a steady-state measuring strategy (note: the circles represent possible
sampling times, not data from measurements).
Passive sampling, whose theoretical fundamentals were described in detail and summarized
by Crump,(185) is enjoying increasing popularity in indoor air testing, since it can
be employed without causing any nuisance to room users. Different types of formaldehyde
passive samplers have been employed.
187−193
However, it must be borne in mind that passive collectors are usually left in a room
for days or even weeks at a time without continuous monitoring by the analyst, and
this means that the possibility of tampering cannot be excluded. One of the advantages
of passive sampling is that person-related exposure can be determined in a simple
manner by having the passive collector worn by an individual for a specific period.
194,195
However, the result of passive sampling will depend on temperature, since the accumulation
of molecules is driven by diffusion, which is a function of the gas kinetic properties.
Another critical parameter is the flow rate, and low air velocities will cause undervalued
concentrations. A standard procedure for passive sampling of formaldehyde in indoor
air has been described by ISO.(196)
5
Fomaldehyde Indoor Guidelines
5.1
Guidelines by Category
Several safety and occupational health authorities worldwide have laid down permissible
exposure levels of formaldehyde by inhalation. Most levels are based on results of
epidemiological and toxicological test outcomes obtained from both human and animal
data for a certain exposure time or are based on health hazard assessments in the
relevant toxicological literature. Limit values are basically separated into two main
categories: workplace environments in which occupational exposure occurs and nonoccupational
(i.e., residential) environments. Such occupational threshold limit values (TLV) are
often categorized as time-weighted average (TWA), short-term exposure limit (STEL),
and ceiling (C) values, with the last defining the exposure limit, which should not
be exceeded at any time.
The Occupational Safety and Health Administration (OSHA)(197) has set the STEL for
formaldehyde at 2 ppm in 15 min and the permissible exposure limit time-weighted average
(PEL-TWA) at 0.75 ppm. The TLV-C proposed by the American Conference of Governmental
Industrial Hygienists (ACGIH)(198) is 0.3 ppm. The National Institute for Occupational
Safety and Health (NIOSH)(199) has set a more stringent STEL of 0.1 ppm and a recommended
exposure limit for occupational exposure of 0.016 ppm. Other occupational formaldehyde
guideline values may be found in the papers by Duhayon et al.(200) and Paustenbach
et al.(201) In general, occupational limit values are higher than indoor guideline
values on account of two important factors. The basic difference between these guideline
values depends on the vulnerability of the people staying in these environments. One
of the factors is that nonoccupational indoor environments cover the general population,
including infants, children, the elderly, pregnant women, and people allergic to formaldehyde.
The other factor is that the general population is often exposed to lower formaldehyde
levels over long time periods (i.e., during their lifetimes), while workers are assumed
to be exposed to formaldehyde for about 8 h in a working day and 5 days a week. In
a recent review, Zhang et al.(202) discussed and compared occupational and indoor
formaldehyde guideline values.
As can be seen from Table 5, indoor guideline values can be roughly categorized into
two groups based on exposure durations. The short-time exposure levels are used for
preventing acute health effects on individuals while long-term exposure levels are
used for preventing the chronic health effects of formaldehyde. The most common short-term
exposure limit is 100 μg m−3 as a 0.5 h average value aimed at preventing significant
sensory irritation in the general population and is recommended by the WHO.
Table 5
International Guideline Values and Recommendations for Formaldehyde in Indoor Air
country
year issued
value
comments
Australia
1982(226)
0.1 ppm
120 μg m−3
short-duration
2006(227)
0.08 ppm
100 μg m−3
Canada
1987(220)
0.1 ppm
120 μg m−3
action level
1987
0.05 ppm
60 μg m−3
target level
2005(22)
0.1 ppm
123 μg m−3
1 h
2005
0.04 ppm
50 μg m−3
8 h
China
2003(225)
0.08 ppm
100 μg m−3
1 h average
Denmark
1990(207)
0.15 mg m−3
Finland
2001(209)
30 μg m−3
S1
50 μg m−3
S2
100 μg m−3
S3
France
2008(213)
50 μg m
−3
2 h (proposed)
10 μg m
−3
long-term exposure (proposed)
Germany
1977(216)
0.1 ppm
Singapore
1996(224)
0.1 ppm
120 μg m−3
8 h
Hong Kong
1999
0.025 ppm
30 μg m−3
level 1 (8 h)
0.081 ppm
100 μg m−3
level 2 (8 h)
0.3 ppm
370 μg m−3
level 3 (8 h)
2003(221)
0.025 ppm
30 μg m−3
excellent
0.081 ppm
100 μg m−3
good
Japan
1997(223)
0.08 ppm
100 μg m−3
0.5 h
Korea
2004(222)
0.1 ppm
120 μg m−3
8 h
Norway
1990(210)
0.05 ppm
60 μg m−3
24 h average
1999(211)
0.05 ppm
100 μg m−3
30 min average
Sweden
2000
0.08 ppm
100 μg m−3
adopted from WHO
Poland
1996(215)
0.04 ppm
50 μg m−3
category A: 24 h
0.08 ppm
100 μg m−3
category B: 8−10 h
U.K.
2004(208)
100 μg m−3
0.5 h
USA (California)
1991(217)
0.1 ppm
120 μg m−3
action level
0.05 ppm
60 μg m−3
target level (ALARA)a
1999(203)
0.076 ppm
94 μg m−3
1 h (acute REL)b
2004(219)
0.027 ppm
33 μg m−3
8 h (interim REL)
2005(218)
0.002 ppm
3 μg m−3
annual average (chronic REL)
WHO
1987(228)
0.08 ppm
100 μg m−3
0.5 h average
a
ALARA = as low as reasonably achievable.
b
REL = reference exposure limit.
Long-term exposure values in indoor guidelines are often based on 8 or 24 h time durations.
These time-weighted average (TWA) values were set to protect the public in indoor
environments from the chronic effects of formaldehyde and are considered to offer
adequate protection to individuals exposed to formaldehyde continuously over their
lifetimes. Chronic noncancer health effects have been assessed based on a threshold
concentration or dose which is below a level at which no adverse health effects would
occur. Reference Exposure Levels (RELs), as estimated by the OEHHA,(203) are designed
to protect the most sensitive individuals in the population and include margins of
safety.
Some organizations try to encourage the use of low-emitting products for reducing
particular indoor air pollutants, mainly formaldehyde. An example of this is the U.S.
Green Building Council (USGBC),(204) which published the Leadership in Energy and
Environmental Design (LEED) Green Building Rating System, which is based on voluntarily
participation and aims at facilitating high-performance buildings. Similarly, the
“Standard for the Design of High-Performance, Green Buildings Except Low-Rise Residential
Buildings” set by ASHRAE(205) only accepts urea-formaldehyde being used on the exterior
envelope material of the buildings. European labeling systems are surveyed in a report
published by the European Commission.(206)
5.2
Guidelines by Regions
Table 5 shows the current formaldehyde indoor guideline values set in different countries
by different organizations. Most of the cited documents are published as official
government publications and are available on the Internet. An overview of guideline
values (12 countries in the year 1990) is given in a report published by the European
Commission.(207)
5.2.1
Europe
In the U.K., the Committee on the Medical Effects of Air Pollutants (COMEAP)(208)
recommended a limit value of 100 μg m−3 (0.5 h) for indoor formaldehyde in 2004. Finland(209)
has set up a different system. The indoor climate is classified as S1 (individual
indoor climate), S2 (good indoor climate), and S3 (satisfactory indoor climate), in
which formaldehyde target values were set as 30 μg m−3, 50 μg m−3, and 100 μg m−3,
respectively. An indoor formaldehyde level was specified by the Norwegian Health Directorate
(NHD)(210) in 1990 in the Guidelines for Indoor Air Quality, in which a 24-h average
indoor formaldehyde level was set at 60 μg m−3. Stranger et al.(211) cite a guideline
value of 100 μg m−3 (0.5 h exposure), applicable in Norway since 1999. The Danish
guideline value of 0.15 mg m−3 has not been revised since 1990.(207) Sweden has adopted
the WHO-guideline value, but a further reduction to 60 μg m−3 is currently under discussion.(212)
In France, the French Agency for Environmental and Occupational Health Safety (AFSSET)
has proposed guideline values of 10 μg m−3 and 50 μg m−3 for long-term exposure and
short-term exposure (2 h), respectively.
213,214
The Polish Ministry of Health and Social Welfare(215) issued a decree to reduce the
pollutants emitted by building materials and furnishings in inhabited enclosed areas.
The maximum allowable concentrations for formaldehyde, categorized as Category A (up
to 24 h exposure per day) and Category B (8−10 h exposure per day), are 50 μg m−3
and 100 μg m−3, respectively. Germany established an indoor guideline value of 0.1
ppm in 1977. The Federal Institute for Risk Assessment (BfR) and the Federal Environment
Agency stated in 2006 that a revision of this guideline value is not required.(216)
5.2.2
USA/Canada
In 1991 the California Environmental Protection Agency set indoor formaldehyde levels
at 0.10 ppm as an action level and at 0.05 ppm as a target value.(217) These values
were recently lowered by the Office of Environmental Health Hazard Assessment (OEHHA).
Formaldehyde levels for acute exposure, 8-h exposure, and chronic exposure were set
at 0.076 ppm (94 μg m−3), 0.027 ppm (33 μg m−3), and 0.002 ppm (3 μg m−3), respectively.
203,218,219
Health Canada
22,220
and the Federal Provincial Advisory Committee on Occupational and Environmental Health
(CEOH) laid down indoor air quality guidelines in 1987 and revised them in 1989. In
these guidelines, formaldehyde target and action levels were set at 0.05 ppm (60 μg
m−3) and 0.1 ppm (120 μg m−3), respectively. Following reclassification of formaldehyde
as a carcinogen by the IARC and on the basis of the results of epidemiological and
toxicological studies, Health Canada set new limits in 2006. These new guidelines,
the Proposed Residential Indoor Air Quality Guidelines, specified a short-term (1
h) exposure limit of 0.1 ppm (123 μg m−3) and a long-term (8 h) exposure limit of
0.04 ppm (50 μg m−3).
5.2.3
Asia
The Indoor Air Quality Management Group in Hong Kong published indoor formaldehyde
guidelines in 1999 entitled “Guidance Notes for the Management of Indoor Air Quality
in Office and Public Places”. In these guidelines, the indoor air quality (8 h average)
in offices and public environments was classified into three categories: Level 1 represents
very good indoor air quality, Level 2 represents the recommended indoor air quality
standards for the general public, and Level 3 represents the indoor air quality required
as protection for workers. The indoor formaldehyde concentrations corresponding to
these three levels were 30 μg m−3, 100 μg m−3, and 370 μg m−3. The guidelines were
modified in 2003 in the form of an indoor air quality certification scheme. Two indoor
air quality levels (8 h average) were defined as benchmarks, namely “excellent class”
(<30 μg m−3) and “good class” (<100 μg m−3).(221)
In Korea, the indoor formaldehyde was set at 0.1 ppm (8 h) according to the Air Quality
Standard in Office and Indoor Air Quality Management Act in 2004.(222) The Ministry
of Health and Welfare (MHW) in Japan laid down an indoor air guideline value of 0.08
ppm (0.5 h) in June 1997.(223) Guidelines for good indoor air quality in office premises
were promulgated by the Singapore Ministry of the Environment in 1996.(224) According
to these guidelines, the maximum formaldehyde concentration limit for acceptable indoor
air quality is 0.1 ppm (8 h). In China, indoor air quality became a governmental concern
about a decade ago, and a guideline value termed the “Indoor Air Quality Standard”
of 100 μg m−3 (1 h average) was issued in 2002.(225)
5.2.4
Australia
The National Health and Medical Research Council (NHMRC) recommended an indoor formaldehyde
level of 130 μg m−3 (0.1 ppm) in 1982 due to concerns regarding the urea−formaldehyde
foam insulation (UFFI) used in buildings.(226) Recently, the Australian Government
has specified indoor formaldehyde guideline values for various indoor environments
in the National Industrial Chemicals Notification and Assessment Scheme (NICNAS),
where the recommended indoor air guidance value of formaldehyde for a short duration
was set at 0.08 ppm.(227)
In 1987 the World Health Organization (WHO) recommended a concentration < 0.08 ppm
(100 μg m−3) for short-term exposure (0.5 h) to prevent nose and throat irritation.(228)
Mendell(229) has reviewed chemical emissions as risk factors for respiratory and allergic
effects in children. This author criticizes the wide range of existing formaldehyde
guideline values and suggests a need for research to quantify risk/response relationships
as guidance for the necessary preventive action.
6
Formaldehyde Emission Data
Comparison of product emission rates is difficult because different methods and units
are used. The test chamber is still the standard device for emission testing, and
it is convenient to express the emission rate directly in (mg m−2 h−1) or as a chamber
concentration (ppm, ppb, mg m−3) in the case of the steady state. Moreover, different
test protocols use different L and n. For this reason, the emission rates of different
building products are cited in Table 6 as they appear in the references together with
the information provided about chamber conditions.
Table 6
Compilation of Indoor-Related Formaldehyde Emission Studiesa
material
T (°C)
r.h. (%)
L (m2 m−3)
n (h−1)
HCHO emission
comment
yearref
chipboard
22
35
1.7
0.5
0.70 ppm
1975(230)
particle board
22
30
1.6
0.2
0.40 ppm
1980(234)
27
60
1.6
0.2
1.70 ppm
27
30
1.6
0.5
0.80 ppm
22
60
1.6
0.5
0.90 ppm
particle board
25
50
0.131−0.426
0.25−0.50
0.095−0.29 ppm
10 samples
1987(233)
OSB
23
45
1.0
1.0
0.01−0.09 ppm
31 samples (median: 0.04 ppm)
2008(246)
solid wood
23
45
1.0
1.0
2−9 ppb
beech, Douglas fir, oak, spruce, pine
1996(43)
UF wood products (bare)
22
50
0.46
1.0
8.6−1580 μg m2 h−1
19 samples (median: 164 μg m2 h−1)
1999(73)
UF wood products (bare)
27
50
0.46
0.3
6.8−1170 μg m2 h−1
19 samples (median: 158 μg m2 h−1)
PF wood products (bare)
22
50
0.46
1.0
4.1−9.2 μg m2 h−1
4 samples
PF wood products (bare)
27
50
0.46
0.3
9.5−13 μg m2 h−1
4 samples
UF wood products (coated)
22
50
0.46
1.0
<2.7−460 μg m2 h−1
14 samples (median: 8.6 μg m2 h−1)
UF wood products (coated)
27
50
0.46
0.3
4.6−1300 μg m2 h−1
14 samples (median: 15 μg m2 h−1)
decorative laminates
22
50
1.83
1.0
4.0−51 μg m2 h−1
3 samples
cabinetry materials
23
50
1.9
5.7
8−470 μg m2 h−1
particle board, hard board, plywood
2002(243)
particle board (18 mm)
25
50
2.16
0.5
0.45 mg m2 h−1
7 days after sample installation
2006(247)
MDF (18 mm)
25
50
2.16
0.5
0.33 mg m2 h−1
7 days after sample installation
laminate
25
50
2.16
0.5
0.03 mg m2 h−1
7 days after sample installation
engineered flooring
25
50
2.16
0.5
0.04 mg m2 h−1
7 days after sample installation
floor covering: natural wood
23.2
n.d.
n.d.
n.d
288 μg m2 h−1
1 h exposure to 750 ppb ozone
2009(256)
floor covering: wood-based
23.2
n.d.
n.d.
n.d.
31 μg m2 h−1
1 h exposure to 750 ppb ozone
floor covering: PVCb
n.d.
50
505
514−686
<5−18 μg m2 h−1
new building (mean: 9 μg m2 h−1)
2007(250)
floor covering: parquetb
n.d.
50
505
514−686
<5−10 μg m2 h−1
new building (mean: 7 μg m2 h−1)
ceilingb
n.d.
50
505
514−686
5−96 μg m2 h−1
new building (mean: 42 μg m2 h−1)
wallsb
n.d.
50
505
514−686
<5−11 μg m2 h−1
new building (mean: 7 μg m2 h−1)
mineral wool
23
45
1.0
1.0
0.02−0.10 ppm
5 samples (4 samples <0.05 ppm)
1993(249)
fiberglass products
23
50
0.87−1.04
1.0
16−32 μg m2 h−1
3 samples
1999(73)
wall coverings
23
45
1.0
1.0
0.05−0.035 ppm
10 samples (median: 0.015 ppm)
1993(70)
carpet
23
45
0.4
0.5
8−15 μg m−3
4 samples, 24 h values
2008(251)
latex paint
n.d.
18−58
flow chamber (2.5 L/min)
0.13−6.29 μg h−1
11 samples exposed to ozone
1995(88)
paints
22
50
1.04
1.0
326−663 μg m2 h−1
3 products, initial emission
1999(73)
paints
22
50
1.04
1.0
8.1−9.8 μg m2 h−1
3 products, final emission
carpet
23
50
n.d.
7
4−30 μg m2 h−1
4 samples exposed to ozone
2006(85)
household products
21−23
36−55
n.d.
0.95−1.08
8.2−23.7 ppb
3 products exposed to ozone
2006(83)
textiles
22
50
7.05
1.0
107 μg m2 h−1
permanent-press T-shirts (unwashed)
1999(73)
42 μg m2 h−1
permanent-press T-shirts (washed)
temporary housing units
22−25
49−58
n.d.
0.15−0.39
164−266 μg m2 h−1
morning (unoccupied)
2008(307)
26−30
46−49
257−347 μg m2 h−1
afternoon (unoccupied)
photocopy machines
26−31
30−35
1c
2.0
<500−2600 μg h−1
4 dry-process photocopy machines
1996(441)
building products
23
30−50
8.2−14.7
12
5.5−40.6 μg m2 h−1
14 products exposed to ozone
2007(71)
burning of incense
23
50
0.5
ca. 20−300 μg m−3
3 sticks in 18.26 m3 chamber
2004(260)
ca. 300−1700 μg g−1
10 products tested
wood burning
180−710 mg kg−1
4 samples (birch) tested in wood stove
2002(55)
wood burning
599−1165 mg kg−1
oak, pine, eucalyptus
2001(56)
cigarette smoking
20−25
45−55
0.022
234.1 μg m−3
average of 8 commercial cigarettes
2004(98)
experimental cigarettes
30−57 μg cigarette−1
13 cigarettes with added saccharides
2006(261)
formaldehyde in breath
1.2−72.7 ppb
median: 4.3 ppb (deep lung portion)
2005(105)
a
The emission values are presented as they appear in the references.
b
Measured on site by use of the FLEC.
c
Units per volume.
6.1
Emission Modeling
One of the classic papers on the indoor air pollution caused by formaldehyde emission
from chipboard was published in 1975 by Andersen et al.(230) The authors studied the
influence of climatic parameters on steady-state concentration in climate chambers.
A hyperbolic decrease in formaldehyde concentration occurred as air exchange rates
increased, while the equilibrium concentration was directly proportional to temperature
and humidity. On the basis of these findings, Andersen et al. have formulated a relationship
(eq 19) between the steady-state chamber concentration C
ss and the climatic parameters.
T is the temperature in °C, H is the humidity in g kg−1, L is the loading rate in
m2 m−3, and n is the air exchange in h−1 while a, b, c, S, and R are constants depending
on the type and surface coating of the chipboard. An updated model was developed later
by Mølhave et al.(231) Myers and Nagaoka(232) and also Lehmann(233) have applied the
Hoetjer equation (eq 20) to describe the effect of ventilation and loading rates on
emissions from particle board and MDF in large test chambers.
C
eq is the formaldehyde concentration at n = 0 h−1, and K (m h−1) is the formaldehyde
transfer coefficient. For particle board, K values lie between 0.40 m h−1 and 1.58
m h−1.(233) Similar equations have been derived by Berge et al.(234) and others.(235)
The Berge equation (eq 21) assumes no air exchange and no concentration gradients
in the chamber air. Formaldehyde emission is assumed to increase linearly with increasing
relative humidity of the air. On the basis of previously reported results, an Arrhenius
temperature dependence was selected.(236) Godish(237) pointed out that the Berge model
is a relatively good predictor of formaldehyde concentration under standard conditions
when measured under a variety of environmental conditions (C
x
= standardized concentration, C = measured concentration, H = relative humidity under
test conditions (%), H
0 = relative humidity under standard conditions (%), T = temperature under test conditions
(K), T
0 = temperature under standard conditions (K), R
B = coefficient of temperature (optimum value: 9799 K),(234)A
B = coefficient of humidity (optimum value: 0.0175%−1)(234)). As an example, formaldehyde
concentrations (in ppm) under steady-state conditions have been calculated with eq
21 as a function of humidity and temperature and are shown in Figure 10. The data
have been standardized for 0.05 ppm, 296 K, and 50% relative humidity. The curves
demonstrate the high influence of climatic parameters on formaldehyde concentrations.
Figure 10
Variation of the formaldehyde concentration as a function of humidity and temperature,
calculated with the Berge equation (eq 21). The initial conditions were T
0 = 296 K, H
0 = 50%, and C
x
= 0.05 ppm.
Roffael(2) has expressed the criticism that eqs 19−21 do not take into account the
influence of aging on the formaldehyde release capacity. Hawthorne and Matthews
238,239
have stated that the release of formaldehyde from solid materials is limited by source-phase
mass transfer from the interior of the source to the surface and can be described
by eq 22.
Here, m
p is the mass of formaldehyde in the material (source), m
i
is the mass of formaldehyde at the surface, and k
s is the source-phase mass transfer coefficient. Equation 22 can be used for homogeneous
materials but not for products which are multilayered or of complex construction.
Zhang et al.(240) as well as Deng et al.(241) have proposed a more sophisticated model
based on the diffusion coefficient, the partition coefficient, and the initial concentration
in a dry building material in order to calculate the influence of temperature on formaldehyde
emission. Xiong et al. describe an extraction method, where determination of the initial
concentration in the material C
m,0 is based on Henry’s linear adsorption isotherm.(242) The material is placed in
an airtight environmental chamber under static conditions, and the equilibrium concentration
C
i
is measured involving multiple flush/equilibrium cycles (see eq 23).
K
H is the partition coefficient (dimensionless), V is the chamber volume, V
m is the volume of the material, and i is the number of flush/equilibrium cycles.
C
m,o and K
H are obtained from regression analysis.
If the emission rate is constant for a given temperature and humidity and not dependent
on the air exchange rate, eq 15 can be used to calculate the concentration in the
steady state. A long-term study of formaldehyde emission decay from particle board
has been carried out by Zinn et al.(66) For products manufactured in 1986 and 1987,
the overall three-quarter (75% of initial concentration) and half-lives were 38 and
216 days, respectively.
6.2
Emission from Wood-Based Materials
It is obvious from Table 6 that UF-bonded wood products constitute a group of comparatively
strong formaldehyde emitters. The steady-state chamber value of C
eq = 0.7 ppm measured for chipboard by Andersen et al.(230) at T = 22 °C and r.h.
= 35% can be converted to an area-specific emission rate of SERA = 255 μg m−2 h−1.
The effect of temperature and humidity on UF-bonded particle board has been studied
by Berge et al.,(234) who observed a strong increase in the steady-state concentration
from 0.40 ppm to 1.70 ppm at n = 0.2 h−1 and L = 1.6 m2 m−3 when temperature and relative
humidity were increased from 22 to 27 °C and from 30% to 60%, respectively. Kelly
et al.(73) have compared different types of wood products. Uncoated UF products covered
a range from 8.6 μg m−2 h−1 to 1560 μg m−2 h−1 with a median of 164 μg m−2 h−1. For
coated UF wood products, the range was in the same order of magnitude but the median
of 15 μg m−2 h−1 was distinctly lower. In contrast, the range of formaldehyde concentrations
for uncoated PF wood products was between 9.5 μg m−2 h−1 and 13 μg m−2 h−1. Hodgson
et al.(243) have studied cabinetry materials as sources of formaldehyde in a newly
built house. The authors state that formaldehyde emission factors for five of the
indoor sources were generally consistent with the results presented by Kelly et al.(73)
Mølhave et al.(244) have measured formaldehyde emission from furniture and home equipment
in a large-scale chamber. From their data they have estimated the long-term exposure
of occupants.
Over the years, the formaldehyde emission potential of wood based products has continually
decreased, a circumstance due to a varitey of regulations. In Germany, the so-called
ETB guideline (ETB = Committee on Harmonized Prescriptions for Construction) was created
in 1981 and classified particle board into three emission categories, E1−E3. The German
Chemicals Act, which was revised in 1993, regulates the release of formaldehyde as
follows: “coated and uncoated wood-based products (particle board, fiber board, plywood)
must not be put onto the market if they exceed an equilibrium formaldehyde concentration
of 0.1 ppm (0.12 mg m
−3
). The equilibrium concentration is to be determined in a large chamber using a test
method in line with the scientific and technical states-of-the art”. Some voluntary
quality certification systems such as the German “Blue Angel” label specify a steady-state
concentration of 0.05 ppm.(245) In Germany, almost all wood-based materials produced
meet the requirements of the Chemicals Act. This has recently been demonstrated for
oriented-strand board (OSB). Ludewig et al.(246) determined the emission of formaldehyde
from 31 samples. Steady-state concentrations ranged from 0.01 ppm to 0.09 ppm with
a median of 0.04 ppm. Other counties have established similar test procedures, legislation,
and labeling systems for regulating formaldehyde emissions from building products.
Wood-based composites such as laminate are also known to be low-emitting materials.
Kim et al.(247) have measured emission rates of 0.03 mg m−2 h−1. Wiglusz et al.(248)
were not able to detect any emissions of formaldehyde from laminate flooring. Emission
rates for some other materials such as mineral wool,(249) PVC, parquet, ceilings,
walls,(250) solid wood,(43) wall coverings,(70) carpeting,(251) fiberglass products,(73)
etc. are also presented in Table 5.
6.3
Emission from Paints
In the case of wet products such as paint, it is necessary to take into account gas-phase
mass transfer, which is based on molecular diffusion across a laminar boundary layer
as described in eq 24.(252)
D is the diffusion coefficient, δ is the thickness of the boundary layer, C
S is the concentration of formaldehyde at the source surface, C
air is the concentration of formaldehyde in the air, and k
g is the gas-phase mass transfer coefficient. If the source phase mass transfer rate
k
s (see eq 22), which describes the diffusion from the interior of the source to the
surface, is taken into account, three different scenarios must be considered:(173)
(a) When k
g ≫ k
s, emission is controlled by the external diffusion process. This applies to most
wet-applied or liquid products during drying or curing. (b) When k
g ≪ k
s, emission is controlled by the internal diffusion process and the influence of air
flow conditions in the test facility should be negligible. This applies to most materials
manufactured in the solid phase and also to wet-applied or liquid products after drying
or curing. (c) A more difficult situation arises when k
g ≈ k
s or if the ratio k
s/k
g changes over time. For an aging product, k
s/k
g will normally increase and reach infinity for t → ∞. Chang et al.(75) have developed
a semiempirical model assuming that the formaldehyde in the applied paint is distributed
in a top layer (thin surface-coating section) and a bottom layer (paint embedded in
the substrate). The analytical solution of the mass balance equations(75) is a triple
exponential function (eq 25).
C(t) (mg m−3) is the formaldehyde concentration in the chamber, k
m (h−1) is the rate constant for emission from the top layer, k
b (h−1) is the rate constant for emission from the bottom layer, and n is the air
exchange rate. Figure 11 shows a modeled decay of formaldehyde from latex paint. The
parameters were taken from Chang et al.(75) and are given in the figure captions.
The curve shape is typical for the emission from freshly applied products. For k
m > k
b, the first phase (<20 h) is controlled by evaporation of formaldehyde from the top
layer, where the term exp(−k
m
t) dominates. With increasing t, the term exp(−k
b
t) more and more affects the progression of the curve.
Figure 11
Emission of formaldehyde from latex paint, measured in a 0.053 m3 chamber with n =
0.5 h−1, T = 23 °C, r.h. = 50%, and loading L = 0.48 m2 m−3. The curve was obtained
from a triple exponential fit of the experimental data (see eq 24). The parameters
A
1 = 1.926 mg m−3, n = 0.5 h−1, A
2 = 0.513 mg m−3, k
m = 0.0745 h−1, A
3 = 0.274 mg m−3, and k
b = 0.0042 h−1 were taken from Chang et al.(75)
6.4
Other Products
In 2005, Bruinen de Bruin et al.(253) released a report on the characterization of
indoor sources for formaldehyde and other pollutants. The authors examined wood and
wood-based products, paper products, wall coverings, rubber, paint, glue, adhesives,
lubricants, cosmetics, electronic equipment, and combustion. Emission factor tables
are provided which derive from numerous sources. Kelly et al.(73) compared formaldehyde
emissions from new and washed permanent-press shirts and found that normal laundering
reduced emissions by about 60%.
6.5
Formaldehyde from Indoor VOC/Ozone Reactions
The formation of carbonyl compounds from the reactions of ozone with unsaturated VOCs
has been known for decades, but the possible impact on indoor air quality was only
recognized in the 1990s when Zhang et al.(254) conducted a field study of six residential
houses during the summer of 1992. Based on these and other findings,(78) Zhang et
al.(255) have investigated the indoor chemistry of ozone with styrene, limonene, and
4-vinylcyclohexene (VCH) in a 4.3 m3 Teflon chamber at temperatures of 21−24 °C and
humidities of 17−56%, respectively. The styrene/ozone and limonene/ozone systems gave
formaldehyde concentrations between 100 μg m−3 and 500 μg m−3. In the case of VCH,
formaldehyde concentrations between 25 μg m−3 and 36 μg m−3 were measured. Although
the authors state that the conditions and the chemicals employed in the reactions
were similar to those for indoor environments, the concentrations of unsaturated hydrocarbons
were higher than normally found in indoor air. A more realistic scenario was chosen
by Morrison and Nazaroff,(85) who measured formaldehyde emission rates of 4−30 μg
m−2 h−1 when carpet was exposed to 100 ppb ozone. The effect of ozone, infrared, sunlight,
UV-A, and UV-B on the emission of formaldehyde from wood-based flooring materials
was studied by Kagi et al.(256) Typical indoor/outdoor ratios of ozone concentrations
range from 0.2 to 0.6, depending on the ventilation rate.
79,257
Indoor concentrations of 10 ppb and higher are only possible during summer ozone episodes
or in the case of strong indoor sources such as air- and water-cleaning devices. The
study by Singer et al.(83) showed elevated formaldehyde indoor levels of the order
of 10 ppb when household products were used with ca. 60 ppb ozone present. Nicolas
et al.(71) exposed building products in a 17 L test chamber to 100−160 ppb of ozone
at 23 °C and 30−50% r.h. and n = 12 h−1 for a period of 48 h. Increased formaldehyde
emissions of the order of 5−40 μg m−2 h−1 associated with reduced emission rates of
unsaturated compounds were observed as compared with the reference values (no ozone)
of 0.5−25 μg m−2 h−1. Reiss et al.(88) reported an effect of heterogeneous ozone chemistry
on latex paint. Waring et al.(258) have studied the efficiency of portable air cleaners.
During an air cleaning period and in the presence of an air freshener, the formaldehyde
concentration increased from 17−19 μg m−3 to 45−49 μg m−3. Fiedler et al.(259) have
also shown that formaldehyde is formed from VOC/ozone mixtures. The authors exposed
healthy women to a mixture of 23 volatile organic compounds (VOCs) with and without
ozone. At a total VOC concentration of 26 mg m−3, the formaldehyde concentration increased
from 13 μg m−3 (no ozone) to 40 μg m−3 in the presence of 40 ppb ozone.
6.6
Formaldehyde from Combustion
The formation of formaldehyde from combustion processes is well-known. Here the emission
rate is often presented in the unit μg g−1 (mass emitted per mass of material burnt).
Lee and Wang(260) measured formaldehyde concentrations up to 300 μg m−3 in a 18.26
m3 test chamber when burning incense sticks. The emission rate was 300−1700 μg per
g stick. Wood combustion and cigarette-smoking have already been treated in section
. Baek and Jenkins(98) measured an average formaldehyde concentration of 234 μg m−3
when six cigarettes were smoked in a 30 m3 chamber under almost static conditions.
A study by Baker(261) with 13 experimental cigarettes (saccharides added) gave emission
rates between 30 μg g−1 and 57 μg per cigarette smoked. In the study by Singer et
al.,(262) distinctly higher formaldehyde emission rates of 950−1310 μg per cigarette
smoked were measured. Maroni et al.(263) report 70−100 μg formaldehyde in the undiluted
mainstream smoke of nonfilter cigarettes and 0.2 mg per cigarette in sidestream smoke.
According to Baker et al.,
261,264
formaldehyde is mainly generated from the pyrolysis of saccharides used as tobacco
ingredients.
7
Formaldehyde Concentrations Indoors
7.1
Evaluation of Data
Residential and occupational indoor formaldehyde concentrations have been measured
in numerous studies. However, it must be pointed out that different studies provide
data of different quality and care has to be taken when comparing the studies presented
in Table 7. Two strategies are commonly applied: (a) the collection of samples from
a large number of randomly selected homes in order to estimate the exposure of the
general population and (b) the monitoring of a limited number of homes to measure
the time vs concentration behavior and to see the influence of aging. In general,
concentrations of indoor pollutants are log−normally distributed.(265) This means
that the geometric mean (GM) rather than the arithmetic mean (AM) should be used for
the statistical evaluation. Moreover, a nonparametrical classification, e.g. 50-P
(median) and 95-P, is convenient. In many cases, the range of measured concentrations
(min−max) is also given. The critical evaluation of published data and comparison
of different studies always require additional information, e.g. the criteria used
in the selection of homes (at random or complaint cases), the type of sampling (active
or passive), sampling time, and so on. Strategies for the measurement of formaldehyde
and other indoor pollutants are available as ISO standards.
184,266
Table 7
Comparison of Formaldehyde Levels in Indoor Air as Determined in Different International
Studies (P = Percentile, 50-P = Median, GM = Geometric Mean, AM = Arithmetic Mean)
on the Basis of Regiona
continent/country
location
C
indoor
comments
ref
Europe
Germany (1991)
327 residences
55 μg m−3
50-P
(269)
106 μg m−3
95-P
(269)
Germany (2008)
586 residences
23.5 μg m−3
50-P (survey for children)
(270)
47.7 μg m−3
95-P (survey for children)
(270)
Germany (2003)
14 office buildings, 1386 measurements
6.0 μg m−3
50-P
(273)
Germany (2001)
180 Berlin residences
38 μg m−3
50-P
(271)
98 μg m−3
95-P
(271)
Germany (1995)
252 residences
12−649 μg m−3
range (complaint cases)
(64)
Germany (1993)
190 residences
62 ppb
50-P
(461)
Austria (2002)
160 homes
8.8−115 μg m−3
range
(462)
25 μg m−3
50-P
(462)
Switzerland (1992)
private residences
46 μg m−3
50-P (recently renovated)
(463)
468 μg m−3
95-P (recently renovated)
(463)
Denmark (1987)
14 Danish town halls
40 μg m−3
mean
(371)
0−80 μg m−3
range
(371)
Denmark (1991)
2 new twin apartments
63−384 μg m−3
range (vacant)
(295)
14−276 μg m−3
range (occupied)
(295)
Denmark (1992)
36 apartments
37 μg m−3
50-P
(296)
Finland (2006)
8 buildings
19, 21, 26 μg m−3
mean (0, 6, 12 months)
(313)
Finland (2009)
23 office buildings
11 μg m−3
GM (suspected problems)
(464)
Sweden (2004)
27 Uppsala dwellings
8.3 μg m−3
GM
(465)
Sweden (2005)
64 bedrooms
23 μg m−3
50-P (24 h)
(212)
29 μg m−3
50-P (6 days)
(212)
Sweden (2001)
181 classrooms
3 μg m−3
GM
(466)
<3−72 μg m−3
range
(466)
France (2006)
Strasbourg libraries
20 μg m−3
50-P
(108)
95 μg m−3
95-P
(108)
France (2006)
Strasbourg locations
5.3−73.8 μg m−3
range (public spaces)
(267)
France (2008)
Strasbourg homes
26.7 μg m−3
50-P (living rooms)
(467)
France (2003)
61 Paris dwellings
34.4 μg m−3
GM (living room)
(468)
≈ 78 μg m−3
95-P (living room)
(468)
France (2009)
157−187 babies’ homes
17.7−19.4 μg m−3
GM
(277)
Italy (2009)
20 homes
12.9−9.3 μg m−3
50-P (January−June)
(469)
Poland (2005)
5 office buildings
2.3−32.3 μg m−3
range
(470)
Turkey (2003)
399 kitchens in Ankara
0−2086 μg m−3
range
(471)
79.9 μg m−3
AM
(471)
Turkey (2006)
25 Ankara dwellings
2.3−866.2 μg m−3
range
(472)
67.1 μg m−3
GM
(472)
US/Canada
USA (1995)
14 residences
11.1 ppb
mean (winter)
(473)
26 residences
16.1 ppb
mean (summer)
(473)
USA (2000)
4 manufactured houses
21−47 ppb
range (new buildings)
(474)
7 site-built houses
14−58 ppb
range (new buildings)
(474)
USA (2006)
different locations
19.6 μg m−3
GM (stores)
(475)
14.3 μg m−3
GM (dining)
(475)
USA (2007)
234 homes
20.1 μg m−3
50-P (RIOPA—indoor)
(275,282)
32.5 μg m−3
95-P (RIOPA—indoor)
(275,282)
20.5 μg m−3
50-P (RIOPA—personal)
(275,282)
34 μg m−3
95-P (RIOPA—personal)
(275,282)
USA (1989)
470 mobile homes
70 ppb
AM
(302,475)
<30 to >300 ppb
range (built 1966−1984)
(302,475)
USA (2008)
360 travel trailers
81 ppb
GM (occupied)
(306)
90 park models
44 ppb
GM (occupied)
(306)
69 mobile homes
57 ppb
GM (occupied)
(306)
Canada (2003)
151 homes (summary of 5 studies)
29.8 μg m−3
50-P
(476)
Canada (2005)
59 residences
29.6 μg m−3
50-P
(417)
Canada (2008)
96 Quebec homes
9.6−90 μg m−3
range
(310)
Latin America
Brazil (2006)
academic institute
<1−82 μg m−3
range (11 laboratories)
(358)
7−8 μg m−3
range (5 libraries)
(358)
5−9 μg m−3
range (3 classrooms)
(358)
Mexico (2003)
different locations
4−122 μg m−3
range
(321)
Asia
Korea (2008)
52 classrooms summer
70 ppb
GM
(300)
48 classrooms autumn
40 ppb
GM
(300)
46 classrooms winter
60 ppb
GM
(300)
Korea (2008)
6 apartments
209−457 μg m−3
range (new or renovated)
(298)
Korea (2009)
50 school buildings
150 ppb
GM (summer)
(477)
45 school buildings
100 ppb
GM (winter)
(477)
Japan/Korea (2006)
292 new homes
134 μg m−3
mean, first year
(299)
60 new homes
86 μg m−3
mean, third year
(299)
Japan (2006)
25 Shimizu residences
71.5 μg m−3
90-P (summer)
(268)
21 Shimizu residences
25.9 μg m−3
90-P (winter)
(268)
Japan (2004)
37 Nagoya residences
17.6 μg m−3
GM
(465)
Hong Kong (2002)
6 residential homes
11−24 μg m−3
range
(478)
Hong Kong (2006)
422 offices
32 μg m−3
GM (air-conditioned)
(315)
Hong Kong (2009)
100 homes
85.7 μg m−3
50-P
(301)
China (2004)
28 hotel ballrooms
29.7 μg m−3
GM
(479)
China (2007)
public vehicles
13−94 μg m−3
range (taxi, bus, subway)
(285)
Bangladesh (2007)
91 kitchens, impact on children
26.2 μg m−3
GM (biomass burning)
(293)
36.9 μg m−3
GM (fossil burning)
(293)
Africa
Egypt (2000)
294 Cairo residences
96.6 μg m−3
AM (winter and summer)
(480)
Australia/New Zealand
Australia (2002)
185 homes in Perth
1−166 ppb
range (depends on room)
(278)
20.4−23.8 ppb
GM (depends on room)
(278)
Australia (2006)
4 schools
3−38 μg m−3
GM (depends on season)
(481)
Australia (2000)
192 caravans
29 ppb
GM (60 occupied)
(304)
100 ppb
GM (132 unoccupied)
(304)
Other
aircraft (simulated)
occupied cabin
8−10 ppb
range (ozone initiated)
(89,90)
submarine (2006)
submerged operation
<10 μg m−3
(1 week)
(482)
a
The concentrations are shown in ppb or μg m−3 as they appear in the references.
7.2
Formaldehyde in Conventional Homes
Maroni et al.(263) have summarized a number of investigations carried out before 1990.
A compilation of formaldehyde indoor concentrations in different European countries
before 1990 may be found in an EU report.(207) References to more recent studies are
available in other publications.
17,267,268
The WHO(27) based an evaluation on 151 sets of figures from five Canadian studies
conducted between 1989 and 1995. In indoor air, the 50-P and the 95-P were 29.8 μg
m−3 and 84.6 μg m−3, respectively. In Germany, environmental surveys of indoor air
have been performed by passive sampling since 1985. In the first survey, in 1985/86,(269)
with 327 data, a 50-P and a 90-P of 55 μg m−3 and 106 μg m−3 were calculated. The
recent survey, from 2003 to 2006,(270) (555 residences) showed reduced 50-P and 95-P
values of 23.5 μg m−3 and 47.7 μg m−3. The results of both studies are directly compared
in Figure 12. It should, however, be noted that the 1985/86 survey was carried out
before German reunion and that the 2003−2006 survey focused on children. Schleibinger
et al.(271) measured formaldehyde in 180 apartments in Berlin between 1988 and 1999
by passive sampling. It is interesting that their results of 38 μg m−3 (50-P) and
98 μg m−3 (95-P) fall between the data from the environmental survey but direct comparison
is not possible since the sampling strategies differ. The Federal Institute for Risk
Assessment (BfR)(272) has compared different German studies and comes to the conclusion
that only the results of the German Environmental Survey can be regarded as representative
for the German population. Salthammer et al.(64) measured formaldehyde concentrations
in German dwellings between 1986 and 1993 at the request of persons complaining about
irritation symptoms. The range was 12−649 μg m−3 with an arithmetic mean of 119 μg
m−3. However, the annual specific arithmetic mean values decreased from 136 μg m−3
in 1986 to 91 μg m−3 in 1993. An interesting study in 14 office building with 1386
formaldehyde measurements was carried out by Bischof et al.(273) The overall median
concentration was 6.0 μg m−3 while the highest median in one building was 26.5 μg
m−3 (42 measurements). Moreover, there was a tendency for higher formaldehyde concentrations
to be found in buildings with more than 600 employees and in open-plan offices.
Figure 12
Comparison of two environmental surveys carried out by passive sampling in Germany
in 1985/86 and 2003−2006. The data were taken from http://www.umweltbundesamt.de.
According to the Federal Institute for Risk Assessment (BfR),(272) the results of
the German Environmental Survey (GerES) can be regarded as representative for the
German population.
For conventional homes, the WHO(274) cites average exposure concentrations to formaldehyde
between 30 μg m−3 and 60 μg m−3. However, this statement is based on measurements
carried out before the year 2000. In the 234 RIOPA homes (RIOPA = relationships of
indoor, outdoor, and personal air), a median of 20.1 μg m−3 and a 95-P of 32.5 μg
m−3 were measured.(275) Nevertheless, the authors assert that formaldehyde should
still be regarded as belonging to the group of strongest indoor pollutant sources
with an estimated median source strength of 3.9 mg h−1. In the BEAM study (BEAM =
Boston Exposure Assessment in Microenvironments), geometric means of 19.6 μg m−3 and
14.3 μg m−3 were found in stores and restaurants, respectively.(276) A French study
reports formaldehyde concentrations between 19 μg m−3 and 20 μg m−3 in Paris newborn
babies’ homes.(277) When comparing these data with the results of the German Environmental
Survey of 2003−2006 (the results from all German indoor surveys are available from
http://www.umweltbundesamt.de) and other results from “normal” living spaces (see
Table 7), average exposure concentrations between 20 μg m−3 and 40 μg m−3 seem to
be more realistic. Although statistical parameters are valuable tools, a detailed
risk assessment requires additional information such as the distribution of formaldehyde
concentrations in histogram form and the number or percentage of data exceeding a
defined value. Unfortunately, such information is only rarely found in the scientific
literature. Dingle and Franklin(278) provide a distribution of mean formaldehyde concentrations
in 185 homes in Perth, Western Australia. The geometric mean was 22.9 ppb, and for
86% of the homes, the level was <50 ppb. Seifert and Salthammer(279) have published
the results of 595 formaldehyde measurements carried out by the Bremer Umweltinstitut
before 1995. More than 60% of the indoor concentrations were below 0.05 ppm. On the
other hand, about 10% of the concentrations exceeded 0.1 ppm. However, it is not certain
that all of the dwellings included in this study were randomly selected.
Measuring personal exposure to formaldehyde and other airborne pollutants in the indoor
environment is also of great interest.(280) This is usually done by placing passive
samplers close to the breathing zone of test persons. A number of studies on this
topic is known,
212,281−283
and in most cases the personal exposure values were not significantly different from
indoor concentrations.
7.3
Formaldehyde in New and Renovated Homes
Increased formaldehyde concentrations are often correlated with renovation work, new
materials, or special conditions, e.g. private and public transport,
284−286
medical laboratories,
287−290
museums,
291,292
combustion,(293) etc. Broder et al.(294) made comparisons of the health and house
characteristics of the occupants of 231 control homes and 571 houses containing urea−formaldehyde
foam insulation. Godish(237) compared measurements, carried out between 1978 and 1989,
in different types of U.S. houses (houses insulated with urea−formaldehyde foam, prefabricated
houses, and conventional houses). The WKI carried out 367 formaldehyde measurements
in new prefabricated houses between 1996 and 2006. This type of housing is commonly
made with wood-based materials such as particle board and OSB. The distribution of
concentrations is shown in Figure 13 and is obviously log−normal (see solid curve
in Figure 13). The median was 0.04 ppm, and the German guideline value of 0.1 ppm
was exceeded by 14% of the data. The influence of modern building products, especially
insulation materials made of mineral wool, on indoor formaldehyde levels was investigated
in the EURIMA-WKI test house study.(154) The results of one experiment are shown in
Figure 14 (for more details, see also the Supporting Information of ref (154)). Following
installation of the test house (n = 0.3 h−1, no mineral wool, carpeting, adhesives,
or furniture) in a 48 m3 stainless steel chamber (n = 2.0 h−1), the formaldehyde concentration
inside the house increased to approximately 22 μg m−3. Although low-emitting and certificated
materials were used, it was not possible to obtain formaldehyde levels below 20 μg
m−3, since most common building products (e.g., wallpaper) release low amounts of
formaldehyde. This indicates that low concentrations of formaldehyde can hardly be
avoided in a new living space. It is interesting that the presence of mineral wool
had no influence on the formaldehyde level in the house, but when the test house contained
carpeting, carpet adhesive, and a sideboard made of lacquered particle board, an increase
of the formaldehyde concentration up to 69 μg m−3 could be observed.
Figure 13
Distribution of formaldehyde concentrations in new prefabricated houses between 1996
and 2006. The log−normal fit curve was obtained from nonlinear regression analysis.
The 50-P value was 0.04 ppm, and 14% of the data exceeded the German guideline value
of 0.1 ppm.
Figure 14
Influence of building products on the formaldehyde concentration inside a test house
(n = 0.3 h−1) and in the 48 m3 stainless steel chamber (n = 2.0 h−1). Measurements
were performed between Oct 15 and Nov 29, 2007 (from Salthammer and Mentese(154)).
In the Danish twin apartment study,(295) formaldehyde was monitored in two apartments
as a function of occupancy and season. After completion of the construction work,
the formaldehyde concentration fell from 200−300 μg m−3 to 80 μg m−3. Subsequently,
it rose to above 400 μg m−3 when the apartment was vacant during the spring and summer.
In the occupied apartment, the formaldehyde concentration was 10 μg m−3 in the summer
and 100 μg m−3 in the fall. These differences probably result from insufficient ventilation
in the vacant apartment in the summer and from less ventilation of the occupied apartment
in the fall. Harving(296) et al. calculated a median of 37 μg m−3 from measurements
taken in 36 Danish apartments. They conclude that formaldehyde is of minor importance
as an irritant in the indoor environment, but they also state that natural ventilation
in a great number of Danish dwellings is too low from a health point of view. Brown(297)
has studied indoor formaldehyde levels in Australian dwellings after construction.
Within 246 days, the concentrations in living rooms decreased from 120 μg m−3 to 46
μg m−3. Surprisingly high formaldehyde concentrations between 209 μg m−3 and 457 μg
m−3 were measured in new apartment buildings in Korea.(298) Park and Ikeda(299) measured
formaldehyde levels over a period of three years in new and older homes. In the new
homes, there was a falling trend from 134 μg m−3 (mean, first year) to 86 μg m−3 (mean,
third year), while in the older homes the mean concentration of 88−90 μg m−3 became
stable within three years. Sohn et al.(300) studied indoor and outdoor concentrations
of formaldehyde as a function of the season. Geometric means ranged from 0.04 ppm
(fall) to 0.09 ppm (winter). The indoor/outdoor ratios were about 5. Formaldehyde
concentrations in Hong Kong homes (50-P = 85.7 μg m−3 for 100 samples) and the effect
of aging were investigated by Guo et al.(301) A comparison with indoor formaldehyde
concentrations in China, Taiwan, Korea, and Japan is presented in the same study.
7.4
Formaldehyde in Mobile Homes
Relatively high formaldehyde concentrations can be measured in mobile homes. This
has already been stated by Sexton et al.,(302) who investigated 470 mobile homes manufactured
between 1966 and 1984 and in 147 cases (31%) found concentrations > 0.1 ppm. Hanrahan
et al.(303) have measured formaldehyde levels up to 2.8 ppm. The data by Dingle et
al.(304) based on 192 caravans are consistent with other studies, but these authors
have also found distinct differences between occupied and unoccupied caravans. Furthermore,
Dingle et al. point out that the increased formaldehyde concentrations in mobile homes
result from higher loading rates with wood-based materials of approximately 1.4 m2
m−3 and lower air exchange rates compared to conventional buildings. Main and Hogan(305)
exposed 21 test persons to formaldehyde concentrations between 0.12 ppm and 1.6 ppm
in two mobile trailers. Symptoms such as eye and throat irritation, headache, and
fatigue were observed. In the United States, discussion about formaldehyde in mobile
homes returned to public attention when survivors of hurricane Katrina, who live in
trailers provided by the U.S. Department of Homeland Security (FEMA), complained about
strange odors and adverse health effects. The U.S. Centers for Disease Control and
Prevention (CDC) randomly selected 519 out of 120,000 trailers and mobile homes. The
range of concentrations was between 3 and 590 ppb with a geometric mean of 77 ppb.
In their study, CDC stated that the indoor temperature was a significant factor for
formaldehyde levels irrespective of trailer make or model.(306) Maddalena et al.
307,308
studied four unoccupied FEMA temporary housing units, each produced by a different
manufacturer, to assess their indoor emissions. Steady-state indoor formaldehyde concentrations
ranged from 378 μg m−3 (0.31 ppm) to 632 μg m−3 (0.52 ppm) in the morning and from
433 μg m−3 (0.35 ppm) to 926 μg m−3 (0.78 ppm) in the afternoon. Air exchange rates
ranged from 0.15 h−1 to 0.39 h−1. Wolkoff and Kjaergaard(309) state that formaldehyde
emission from wood-based materials is proportional to relative humidity at a given
temperature. This might also play a role in the hot and humid climate of the southern
U.S. states.
7.5
Influence of Climatic Parameters on Formaldehyde Levels
The effect of climatic parameters on indoor formaldehyde concentrations has been considered
in several studies. Salthammer et al.(64) found a negative correlation between the
formaldehyde concentration and the air exchange rate. A similar observation was made
by Gilbert et al.(310) Ninety-six homes were studied in Quebec, and 80% of those homes
had air exchange rates < 0.23 h−1. Based on the entire sample, the air exchange rate
which ensured a formaldehyde concentration below Health Canada’s long-term exposure
limit of 50 μg m−3 in 95% of the homes was 0.26 h−1. Gilbert et al. recommend that
an air exchange rate meeting ASHRAE’s recommendation of 0.35 h−1 appears sufficient
to ensure a concentration within the Canadian guideline in most homes. CDC has found
slight associations of indoor temperature and relative humidity and logarithmic formaldehyde
concentrations in occupied FEMA trailers.(306) A temperature effect was also reported
by Jo and Sohn.(311) Tuomainen et al.(312) found that ventilation and occupancy had
only a small effect on the formaldehyde concentration in one case and one control
building. In this study, the formaldehyde concentrations were low (1−27 μg m−3) and
the number of samples was 12 or less. Dingle and Franklin(278) investigated indoor
formaldehyde concentrations as a function of the season and the age of the house.
Higher levels were found in newer homes and in homes monitored in summer. Jaernstroem
et al.(313) also report higher formaldehyde concentrations in the summer. Furthermore,
these authors applied the method of principal component analysis (PCA)(314) to find
out which variables affected indoor air quality most. These variables, which are not
necessarily independent, were season, relative humidity, temperature, air exchange,
floor coverings, ceilings, wall coverings, and occupancy. Wong et al.(315) attempted
to relate indoor formaldehyde concentrations to other pollutants and climatic parameters
but found only weak correlations. The data for prefabricated houses shown in Figure
15 were grouped by year. The results for 1996 and 2006 are shown in Figure 15A as
Box−Whisker plots. At first sight, it seems surprising that both samples give the
same median of 0.4 ppm. On closer inspection, however, the distribution is much smaller
for the data from 2006 and, even more important, the air exchange rates (see Figure
15B) decreased from 1996 to 2006. This leads to the conclusion that formaldehyde emission
rates in prefabricated houses will have fallen during the last 10 years, but this
positive effect has been canceled out by decreasing air exchange rates.
Figure 15
Box-and-whisker plots (min, max, mean, 25-P, 50-P, 75-P) of formaldehyde concentrations
(A) and air exchange rates (B) for the years 1996, 2001, and 2006 (data from Figure
13).
8
Exposure and Risk Assessment
8.1
Calculation of the Daily Intake
Although outdoor air contributes adversely to indoor air quality,(268) it was believed
that human exposure to formaldehyde mainly originates in the indoor environment rather
than the ambient environment.(316) Two parameters are of major importance in the evaluation
of indoor exposure to formaldehyde: (a) the concentration in air and (b) the time
spent indoors. The latter depends on the age group and on the daily activities. Brasche
and Bischof(317) have investigated the time spent indoors in German homes with regard
to age, gender, characteristics, and location of the home. The overall mean time of
15.7 h per day is in line with results from the United States and Canada. Approximations
made by the U.S. EPA(318) have been widely applied to calculate the daily intake of
formaldehyde. The EPA indicated an inhalation rate of 0.63 m3 h−1 and 10 h per day
for residential exposure. Stubenrauch et al.(319) estimate inhalation rates of 0.8
m3 h−1 for adults and 0.25 m3 h−1 for young children over 21 h per day. The total
daily intake (DI) for an individual moving in n different environments can be computed
from eq 26, where C
j
, IR
j
, and t
j
represent the concentration, inhalation rate, and time spent in the environment j.
The WHO(27) has calculated probabilistic estimates of 24 h time-weighted average concentrations
of formaldehyde in the air on the basis of five Canadian studies. The results indicate
that one in two persons would be exposed to concentrations of 24−29 μg m−3, while
1 in 20 persons would be exposed to 80−94 μg m−3. Baez et al.(320) applied the EPA
exposure scenario to Mexican homes and calculated a daily uptake of 173−600 μg day−1
on the 95-P level. It should, however, be pointed out that, for a respiratory tract
irritant, the time-weighted average concentration and especially the peak exposure
are more meaningful than the daily intake.
Acute, chronic (noncancer), and potential carcinogenic effects on humans have been
reported for formaldehyde. Numerous case-control studies, cohort studies, and reviews
have been published. The most relevant and frequently cited review papers, representing
different views and perspectives, are summarized in Table 8. Results are also available
for specific subgroups, such as children,
321−327
special indoor environments, such as medical laboratories,
288,328−331
workplaces,
200,332,333
exposure studies,
334−336
and others.(7) Animal studies are not included in this review. It should only be mentioned
that Lu et al.(337) have recently reported an effect of inhaled formaldehyde on the
learning and memory of mice due to oxidative stress.
Table 8
Reviews of Studies of the Acute and Chronic Adverse Health Effects of Formaldehyde
topic
ref
the INDEX report
Kotzias et al. (2005)(17)
World Health Organization—report on formaldehyde
WHO (1989, 2002)
10,27
classification of formaldehyde as human carcinogen
IARC (2006)(483)
sensory irritation in relation to carcinogenicity
Arts et al. (2006)(338)
evaluation of data on carcinogenicity
Appel et al. (2006)(348)
risk assessment for the population in Japan
Naya (2005)(357)
risk factors for respiratory and allergic effects in children
Mendell (2007)(229)
evaluation of epidemiological studies (1994−2006)
Duhayon et al. (2008)(200)
occupational exposure limit based on irritation
Paustenbach et al. (1997)(201)
hazard characterization and exposure−response relationship
Liteplo and Meek (2003)(476)
mode of action for carcinogenicity of formaldehyde
McGegor et al. (2006)(16)
evaluation of epidemiological studies
Bosetti et al. (2008)(351)
evaluation of literature related to effects with indoor air exposure
Arts et al. (2008)(349)
formaldehyde exposure and leukemia
Zhang et al. (2009)(202)
8.2
Short-Term and Long-Term Exposure (Noncancer)
Acute (short-term) human exposure to formaldehyde causes discomfort; irritation of
the eyes, nose, and throat; lachrymation; sneezing; coughing; nausea; and finally
death.(27) Table 9 has been adopted from WHO(27) and provides an overview of acute
human health effects at differrent levels of exposure. In the INDEX report,(17) studies
of humans under controlled conditions are summarized as follows: “acute exposures
to air concentrations ranging from 0.5 mg m−3 to 3.7 mg m−3 induce reversible eye,
nose, and throat irritation, produce changes in nasal lavage fluid contents (indicative
of irritation of the nasal epithelium), do not consistently or markedly affect pulmonary
function variables in most individuals.” No-observed-(adverse)-effect levels (NO(A)EL)
have been published on the basis of different criteria. The NOAEL of 0.03 mg m−3 mentioned
in the INDEX report corresponds to the lowest odor threshold reported.(17) Studies
of irritation in humans have been reviewed by Paustenbach et al.(201) and Arts et
al.(338) These authors concluded that irritation starts at levels around 1 ppm. In
a recent study designed on the basis of current standards, Lang et al.(24) examined
the possible occurrence of sensory irritation and subjective symptoms in human volunteers
exposed to formaldehyde concentrations relevant to the workplace. They concluded that
sensory eye irritation is the most sensitive parameter and obtained a NOAEL of 0.5
ppm. This is in contrast to the WHO data in Table 9, which gives a lower threshold
for throat and nose irritation of 0.1 mg m−3. However, in the case of the WHO publication
cited, it is difficult to trace the original source, which makes a critical evaluation
difficult. Wolkoff et al.(339) have also reviewed human odor and sensory irritation
threshold values for formaldehyde and other compounds. They conclude that many sensory
irritants are formed from alkene oxidation reactions, where formaldehyde is a major
product.
Table 9
Effects of Formaldehyde in Humans after Short-Term Exposure(274)
conc range or avg (mg m−3)
time range or avg
health effects in general population
0.03
repeated exposure
odor detection threshold (10-P)a
0.18
repeated exposure
odor detection threshold (50-P)a
0.6
repeated exposure
odor detection threshold (90-P)a
0.1−3.1
single and repeated exposure
throat and nose irritation threshold
0.6−1.2
single and repeated exposure
eye irritation threshold
0.5−2.0
3−5 h
decreased nasal mucus flow rate
2.4
40 min on 2 successive days with 10 min of moderate exercise on second day
postexposure (up to 24 h) headache
2.5−3.7
b
biting sensation in eyes and nose
3.7
single and repeated exposure
decreased pulmonary function only during heavy exercise
5−6.2
30 min
tolerable for 30 min with lachrymation
12−25
b
strong lachrymation, lasting for 1 h
37−60
b
pulmonary edema, pneumonia, danger to life
60−125
b
death
a
Frequency of effect in population.
b
Time range or average unspecified.
Formaldehyde is a well-known skin sensitizer, but its interrelationship with asthma
has been debated for many years. Publications of different design and quality are
available on this subject,
323,324,335,340−346
but considering the widespread exposure to formaldehyde, reports on respiratory sensitization
are few. In a recent review focusing on the effects of residential formaldehyde levels,
Arts et al.(347) evaluated the data available, especially for children. They concluded
that the question is still open whether there is a causal relationship between formaldehyde
and allergic asthma or whether formaldehyde induces airway irritation resembling asthmatic
reactions. Similarly, Appel et al.(348) were not able to adduce clear evidence for
asthma induced by formaldehyde.
In occupational and residential environments, long-term exposure (chronic, noncancer)
to increased levels of formaldehyde results in irritation of the upper and lower airways
and eyes. Most studies related to chronic effects refer to the working environment,
where formaldehyde is frequently used. The predominant effect of formaldehyde is sensory
irritation at low concentrations, which will progress to cytotoxic irritation with
cell destruction at higher concentrations. These effects are concentration- and not
time-dependent. The threshold concentrations for sensory and cytotoxic irritation
are therefore very similar for acute and chronic exposures. Concentrations not leading
to sensory irritation after acute exposures are not expected to result in adverse
effects after prolonged exposures.
8.3
Formaldehyde as a Human Carcinogen
The classification of formaldehyde as a known human carcinogen by IARC is based on
cohort mortality studies of workers exposed to formaldehyde with an increased incidence
of nasopharyngeal cancer.(332) However, the evaluation by IARC has been questioned
by several authors. Marsh and Youk(349) pointed out that their reanalysis of the available
data provided little evidence for a causal association between formaldehyde exposure
and mortality and they recently provided evidence that the increased incidence of
nasopharyngeal cancer might be related to exposures to several suspected risk factors
for upper respiratory system cancer (e.g., sulfuric acid mists, mineral acid, metal
dusts, and heat) in the metal industries of that area.(333) Moreover, Marsh et al.(350)
criticized that the nasopharyngeal cancer risk models developed by Hauptmann et al.(332)
and used by IARC to justify their classification of formaldehyde as a human carcinogen
were mis-specified and nonrobust. Thus, the authors claimed that the decision of IARC
should be reconsidered. Duhayon et al.(200) and Bosetti et al.(351) have reviewed
epidemiological studies published after 1994. The authors state that the evidence
for an association between formaldehyde exposure and nasopharyngeal cancer appears
debatable and also suggest a need to reconsider the current carcinogenic classification
by IARC. The German Federal Institute for Risk Assessment (BfR) and other institutions
have stated that results from recent epidemiological studies support a possible causal
relationship between inhalation exposure to formaldehyde and nasopharyngeal cancer.
16,348
In their publication, the BfR points out that a slight sensory irritation response
can be observed at concentrations of 0.2−0.3 ppm and that ocular and upper respiratory
tract sensory irritation is not present below 0.1 ppm. A formaldehyde concentration
of 0.1 ppm is therefore proposed as a safe level.
According to the evaluation by IARC, the epidemiologic evidence was strong but not
sufficient to conclude that formaldehyde exposure causes leukemia in humans. Moreover,
a plausible mechanism has not been identified how leukemia may be induced after formaldehyde
inhalation. Two large cohort studies
352,353
pointed at possible excesses while Coggon et al.(354) could detect no link between
formaldehyde exposure and leukemia risk. Since the discussion is mainly driven by
the National Cancer Institute (NCI) study,(352) the observation of Marsh and Youk(355)
is relevant that the upward trend noted in the NCI study is produced by a deficit
of leukemia cases among the unexposed and low exposed workers. Their reanalysis provided
little evidence to support NCI’s suggestion of a causal association between formaldehyde
exposure and mortality from leukemia. In the recent follow-up of the NCI study, Beane
Freeman et al.(356) admitted that Hauptmann et al. had not included 1006 deaths in
their previous analyses. The authors also showed that in some cases the original cause
of death had to be changed. Bean Freeman et al.(356) summarize that the overall leukemia
risk trends have decreased in comparison to their previous publication but still remain
somewhat elevated and that further studies are needed to evaluate the risk of leukemia
in other formaldehyde exposed poulations.
Some recent studies deal with the risk of cancer in relation to formaldehyde in air.
Naya and Nakanishi(357) have evaluated case-control as well as cohort studies in humans
and recommend a reference value of 0.01 ppm in outdoor air for the general population
in Japan. The U.S. EPA provides an Integrated Risk Information system (IRIS) (http://www.epa.gov/iris)
for calculating the cancer risk for formaldehyde and other chemicals.(358) Loh et
al.(359) ranked the cancer risks of organic hazardous air pollutants in the United
States and applied the unit risk model, where the chronic daily intake is multiplied
by a cancer potency factor.(360) The Scientific Committee on Occupational Exposure
Limits (SCOEL) recommends that regulations and health-based exposure limits for formaldehyde
should be based on an established NOAEL.(361) Irigaray et al.(362) have identified
formaldehyde as one of the compounds of major concern when evaluating lifestyle-related
factors and environmental agents causing cancer.
For the risk assessment approach with regard to carcinogens, the mode of action is
of pivotal importance. For genotoxic chemicals that lead to tumors by mutations of
the DNA, it is generally accepted that a threshold cannot be defined and linearized
mathematical models are used to define an exposure with acceptable risk as regards
socioeconomic considerations. A different approach is applicable to nongenotoxic carcinogens
which lead to tumors by a threshold mechanism, such as, for example, chronic irritation.
Exposures below this threshold are not expected to have a carcinogenic effect. In
2000 the German MAK commission concluded that, at low exposure, concentrations without
an increase of cell proliferation genotoxicity “play no or at most a minor part...so
that no significant contribution to human cancer risk is expected”. This assessment
has so far been annually confirmed by the commission.(363) This mode of action has
recently also been supported by Liteplo and Meek(364) and has been refined and put
into the context of the 2006 IPCS (International Programme on Chemical Safety) human
framework for the analyses of cancer’s mode of action for humans.(16) Accordingly,
concentrations not resulting in cytotoxic irritation with an increased cell proliferation
would represent a threshold for carcinogenic action upon the upper respiratory tract.
As cytotoxic irritation will only occur at concentrations clearly above those leading
to sensory irritation, a carcinogenic action is not to be expected so long as sensory
irritation is avoided. This sensory irritation is the decisive end point for all indoor
air limits proposed in the last years by regulatory bodies, and these limits should
therefore provide protection against tumor induction by formaldehyde.
8.4
Formaldehyde and SBS
The term “Sick Building Syndrome (SBS)” has been used to describe the mostly nonspecific
complaints of occupants of buildings.(365) Brightman and Moss,(366) who have evaluated
different studies, pointed out that SBS describes a constellation of symptoms which
have no clear etiology and are attributable to exposure to a particular building environment.
On the basis of a WHO report,(367) Mølhave(368) has suggested a definition for SBS.
The most common symptoms are eye, nose, and throat irritation; dry and itching skin;
nonspecific hypersensitivity; sensation of dry mucous membranes; headache, fatigue,
and dizziness; airway infections and cough, wheezing and nausea. In general SBS does
not correlate with any single factor, and only occasionally has formaldehyde been
linked to SBS. Mendell(369) has summarized reported associations between work-related
symptoms and a potential 37 factors and measurements from 33 studies conducted between
1984 and 1992. Formaldehyde was only mentioned by Skov et al.,(370) who did not find
an association. Formaldehyde was therefore classed by Mendell as a compound with a
“consistent lack of association with symptom reports”. Mainly consistent with symptom
reports were air-conditioning, job dissatisfaction, and allergies/asthma. Similar
results were obtained from the Danish town hall study
371,372
and the German ProKlimA study.(273) An interesting aspect was raised by Sundell et
al.,(373) who identified higher formaldehyde and lower TVOC (total volatile organic
compound) levels as risk indicators for SBS. This was attributed to VOCs reacting
with other air pollutants such as ozone to form irritant byproducts such as formaldehyde.
Brightman and Moss(366) have called this theory “the missing VOCs”. A link between
VOC exposures and SBS was demonstrated for the first time by TenBrinke et al.(374)
SBS should not be confused with the so-called “Sick House Syndrome”. This term is
mainly used in Japan and also takes into account the health problems of individuals
in private dwellings.
375,376
9
Reduction of Indoor Formaldehyde Pollution
Techniques for lowering the concentration of formaldehyde in the indoor environment
have been discussed by several authors and can be classified into several groups:
• avoidance of sources and prevention of emissions right from the start
• removal of the source
• surface coating
• fumigation with ammonia
• increased ventilation
• catalytic reactions
• adsorption
The California Air Resources Board points out that “.. .the most effective way to
reduce formaldehyde in indoor air is to remove or reduce sources of formaldehyde in
the home and avoid adding new sources”.(219) However, the feasibility of source removal
will sometimes depend on the circumstances. It may well be an easy matter in the case
of furniture but be much more complicated in the case of a built structure. Surface
treatment by use of reactive or diffusion resistant coatings and fumigation with ammonia
were mainly applied in the case of very high formaldehyde levels in the 1980s and
1990s. The reaction with ammonia which yields hexamethylene tetramine (see eq 1) was
in particular used in prefabricated houses. It is a drastic but very effective and
long-lasting solution.
2,377
Uchiyama et al.(378) proposed the application of natural compounds such as urea, catechin,
and vanillin to suppress formaldehyde emission from plywood. Kim(379) suggested the
use of volcanic pozzolan.
These days, formaldehyde source strengths are lower. This means that people are exposed
to lower formaldehyde concentrations in the indoor environment, and it is more convenient
to reduce pollutant levels by use of intelligent housing construction and ventilation.
380,381
Here it is the responsibility of architects and engineers to ensure adequate air exchange
with low heat loss. Sherman and Hodgson(382) suggested using measured formaldehyde
emission and authoritative exposure standards to develop minimum ventilation rates
for dwellings. Photocatalytic oxidation systems for the removal of formaldehyde have
recently become popular.(383) One method works with high-flow photoreactors, where
degradation of the pollutants is performed over a solid TiO2 catalyst and with UV
light of high intensity.
384−390
Hodgson et al.(391) as well as Mo et al.(392) have shown that such ultraviolet photocatalytic
oxidation (UVPCO) systems produce formaldehyde due to incomplete mineralization of
VOCs. The other technique uses wall paint equipped with modified TiO2(393) for the
purpose of photocatalytically removing air pollutants under indoor conditions. Although
such wall paints are frequently advertised, serious publications examining their efficiency
have not yet become available. Moreover, Salthammer and Fuhrmann,(93) Gunschera et
al.,(95) as well as Auvinen and Wirtanen(94) have shown that, under the influence
of light, photocatalytic wall paints produce undesired secondary emissions such as
formaldehyde. Quiller et al.(394) have shown that, in the presence of ozone, TiO2
surfaces promote the oxidation of styrene to form formaldehyde. From time to time,
it is reported that plants act as air cleaners, but the literature on this topic is
contradictory. The results of Godish and Guindon(395) do not support previous suggestions
that botanical air purification using only plant leaves is an effective means of reducing
residential formaldehyde levels. Giese et al.(396) demonstrated that formaldehyde
is efficiently metabolized by the spider plant (Chlorophytum comosum). Schmitz et
al.(397) conclude that the assimilation and metabolism of formaldehyde by leaves appears
unlikely to be of value for indoor air purification. Wool fiber is also used for adsorption
of formaldehyde under indoor conditions. Although there is no question that wool has
the capability to adsorb formaldehyde, recent work refers to unreasonably high indoor
concentrations of 1 ppm.
398,399
Proof of the applicability of wool at lower formaldehyde concentrations is still absent.
Matthews et al.(186) have demonstrated that gypsum board has some storage capacity
for sorbed formaldehyde but appears to result in only a minor permanent loss mechanism.
A new aspect has recently been mentioned by Mui et al.(400) These authors evaluated
the energy impact assessment for the reduction of carbon dioxide and formaldehyde
exposure risk in air-conditioned offices in Hong Kong. They conclude that the energy
impact should be an important factor in future ventilation strategies regarding indoor
air quality.
10
Other Emissions Related to Formaldehyde
The appearance of other volatile organic compounds in conjunction with formaldehyde
in indoor air can be classified by chemistry or by source. Two compounds are regarded
as “chemistry-related” if one is formed by chemical reaction from the other. Two compounds
are “source-related” if they are released from the same source but originate in different
reactions. Formic acid, which is the oxidation product of formaldehyde (see eq 2),
has been recognized as a hazard in the museum environment. Raychaudhuri and Brimblecombe(401)
have found that formaldehyde can damage lead objects in display cases via conversion
to formic acid. However, they also state that the question posed by Hatchfield and
Carpenter,(402)“formaldehyde—how great is the danger to museums collections?”, remains
unanswered. Tetrault(403) has pointed out that, with the exception of lead, very few
studies support the proposition that formaldehyde is harmful to exhibits in a museum
environment. In contrast, acetic acid seems to be a more aggressive indoor organic
compound for a larger range of objects. Schieweck et al.(292) and Salthammer et al.(404)
have measured formaldehyde, formic acid and acetic acid in different departments of
a German museum. More references on museum-related sources, levels and associated
damage to materials are available in ref (291).
Wood and wood-based materials are good examples for describing the occurrence of VOCs
related to formaldehyde, as shown in Figure 16. The volatile ingredients of softwood
mainly consist of monoterpenes, of which the most important are α-pinene, β-pinene,
3-carene, limonene, camphene, myrcene, and β-phellandrene.
405,406
As outlined in section , terpenoids and other unsaturated hydrocarbons are precursors
of formaldehyde in the presence of strong oxidants. The reaction of terpenes with
ozone leads to the formation of terpene aldehydes(407) and secondary organic aerosols.
408−410
Compounds such as methanol, acetaldehyde, and acetic acid are typically released from
hardwood.(411) In the case of wood-based materials, aldehydes and other compounds
are formed during the manufacturing process. Jiang et al.(412) as well as Makowski
et al.
413,414
have measured emissions of saturated and nonsaturated aldehydes (C3−C10) from the
hot-pressing of mixed hardwood and Scots pine, respectively. Acetic acid and furfural
result from the thermal degradation of hemicelluloses. These two compounds are most
prominent in cork products.
415,416
Acetic acid is formed from the degradation of acetyl groups, while furfural is produced
from pentoses and hexoses under elimination of water. Furthermore, phenol−formaldehyde
resins are frequently used in the manufacture of cork products. For this reason, formaldehyde
and phenol are often measured together.(415)
Figure 16
Possible VOC emissions related to formaldehyde for the case of a wood-based material.
In the case of other building materials such as carpet, latex paint, and so on, the
occurrence of formaldehyde is related to the ozone-induced formation of other saturated
and nonsaturated aldehydes (C2−C13, 2-octenal, 2-nonenal, 2,4-nonadienal, benzaldehyde,
tolualdehyde).
71,78,85,88
Test chamber experiments are often in good agreement with real-room measurements.
243,254
Combustion processes can be identified by the simultaneous appearance of formaldehyde,
acetaldehyde, and acrolein.
260,261
Gilbert et al.(417) have pointed out that smoking is a source of acetaldehyde and
acrolein in indoor air.
11
Summary and Outlook
Among the large variety of gaseous indoor pollutants, formaldehyde has always had
an exceptional position, and there is no reason to believe that this view will change
in the near future. Formaldehyde is a highly reactive aldehyde, it is an important
chemical feedstock, it is a constituent of many industrial products, and it is ubiquitous
outdoors and indoors due to natural and anthropogenic processes. As Pluschke(33) pointed
out, formaldehyde was the hazardous substance “par excellence” in the 1980s. In 1970s
Germany, the release of formaldehyde from wood-based materials and the high pollution
levels in classrooms and daycare facilities for children triggered a public debate
on indoor air pollutants. Every few years, the “formaldehyde discussion” is resuscitated—in
2004, for example, following classification of formaldehyde by IARC as a Group 1 carcinogen
and recently, in the United States, when survivors of hurricane Katrina suffered adverse
effects from the poor indoor air quality in their trailers.
Emission of formaldehyde from building products and consumer goods has been limited
by authorities and by voluntary criteria. Reliable but sophisticated tools are available
to measure formaldehyde concentrations even at low levels in rural environments. Indoor
concentrations have been systematically monitored over the years. An evaluation of
recent emission studies and indoor surveys has demonstrated that the situation has
improved due to the progress made over recent decades regarding indoor products with
reduced emissions. An examination of international studies carried out in 2005 or
after (see Table 7) indicates that the average exposure of the population to formaldehyde
seems to lie between 20 μg m−3 and 40 μg m−3 under normal living conditions. Although
this trend toward decreasing concentrations is a positive one, it should be kept in
mind that such average concentrations do not take into account the higher exposure
which may result from new buildings or special indoor conditions, peak concentrations,
and individual cases. It can be expected that advanced products and intelligent housing
designs will bring about a further fall in formaldehyde concentrations indoors. However,
the average indoor air concentrations of other compounds have also decreased and the
RIOPA study has shown that among the carbonyls the indoor source strength of formaldehyde
is still high in the USA. On the other hand, Eikmann et al.(418) have stated that
in Germany formaldehyde exposure in the private environment can be regarded as manageable
and controllable.
The previously mentioned classification of formaldehyde as a Group 1 carcinogen by
IARC has started some controversial discussions. Different authorities in different
countries and organizations as well as other epidemiologists have re-evaluated available
data and come to different conclusions. Health Canada did not take IARC’s classification
into consideration and in 2005 established a new formaldehyde indoor air guideline
value of 50 μg m−3 (8 h). The German Federal Institute for Risk Assessment was aware
of the IARC classification in 2006(419) but considered 0.1 ppm (124 μg m−3) to be
a “safe level” and decided that a revision of the German guideline value was not required.
Arts et al.(347) also concluded that 0.1 ppm can be considered a safe and appropriate
level. A European working group stated in the INDEX report that formaldehyde should
be regarded as a chemical of concern at indoor levels exceeding 1 μg m−3. The same
group pointed out that from about 30 μg m−3 mild irritation of the eyes could be experienced
by the general population and odor perceived. In other studies, odor thresholds and
sensory irritation levels were found to be about one magnitude higher. Wolkoff(420)
has stated that the guideline values for formaldehyde and other reactive VOCs appear
to be overestimated. Taking into account average formaldehyde concentrations in indoor,
rural, and urban air (see Figure 17) and the toxicological data available, interpreting
values lower than 30 μg m−3 as guideline values or as recommendations seems to be
somewhat unrealistic. Instead, established guideline values in the range of the WHO
value could be used as a basis and should be complemented by application of the ALARA
principle (ALARA = as low as reasonably achievable) and of recommendations regarding
better ventilation and keeping temperatures moderate.
Figure 17
Range of formaldehyde concentrations in different environments.
No one will reasonably doubt that formaldehyde is a relevant indoor pollutant. Regulations
are urgently required, but the lower guideline value is not necessarily the better
one. If indoor-related research is focused solely on formaldehyde and all efforts
are applied to this compound, other pollutants will be easily neglected, something
which can be counterproductive for human health. As an example, radon is also classified
by IARC as a Group 1 human carcinogen, but formaldehyde has a considerably higher
ranking in the public attention, probably as the result of a large number of previous
incidents due to the acute health effects. Moreover, it seems questionable whether
formaldehyde concentrations lower than 20 μg m−3 can be permanently achieved under
normal living conditions in urban and rural environments. In addition to building
materials and household products, we have to consider other formaldehyde sources,
such as outdoor air, indoor chemical reactions, candles, cooking, gas heaters, etc.
Although quantitative data are not available, we may conservatively estimate that
these additional sources contribute 10−50% to formaldehyde indoor concentration levels.
However, the uncertainty of this estimation indicates the need for further research
on this topic.
Today, it is possible to produce low-emitting materials, and such products are already
recommended by manufacturers of furniture and housing. On the other hand, the air
exchange rates in houses have decreased in almost the same manner. Mechanical ventilation
also consumes energy, and all the factors mentioned make for a vicious circle. The
formaldehyde story will be continued.