SARS-CoV-2, the virus that causes COVID-19, can be spread by exposure to droplets
and aerosols of respiratory fluids that are released by infected persons when they
cough, sing, talk, or exhale. To reduce indoor transmission of SARS-CoV-2 between
persons, CDC recommends measures including physical distancing, universal masking
(the use of face masks in public places by everyone who is not fully vaccinated),
and increased room ventilation (
1
). Ventilation systems can be supplemented with portable high efficiency particulate
air (HEPA) cleaners* to reduce the number of infectious particles in the air and provide
enhanced protection from transmission between persons (
2
); two recent reports found that HEPA air cleaners in classrooms could reduce overall
aerosol particle concentrations by ≥80% within 30 minutes (
3
,
4
). To investigate the effectiveness of portable HEPA air cleaners and universal masking
at reducing exposure to exhaled aerosol particles, the investigation team used respiratory
simulators to mimic a person with COVID-19 and other, uninfected persons in a conference
room. The addition of two HEPA air cleaners that met the Environmental Protection
Agency (EPA)–recommended clean air delivery rate (CADR) (
5
) reduced overall exposure to simulated exhaled aerosol particles by up to 65% without
universal masking. Without the HEPA air cleaners, universal masking reduced the combined
mean aerosol concentration by 72%. The combination of the two HEPA air cleaners and
universal masking reduced overall exposure by up to 90%. The HEPA air cleaners were
most effective when they were close to the aerosol source. These findings suggest
that portable HEPA air cleaners can reduce exposure to SARS-CoV-2 aerosols in indoor
environments, with greater reductions in exposure occurring when used in combination
with universal masking.
A breathing aerosol source simulator was used to mimic a meeting participant exhaling
infectious particles (source), and three breathing simulators were used to mimic a
speaker and two participants exposed to these aerosol particles (receivers) (Figure
1). The methods used were similar to those used in previous studies of aerosol dispersion
and transport in indoor spaces (
3
,
4
,
6
). The simulators were placed in a 584–ft2 (54–m2) conference room with a heating,
ventilation, and air conditioning (HVAC) system that provided 0.1 m3 per second of
air flow (202 ft3 per minute; two air changes per hour) with no air recirculation.
Two HEPA air cleaners (Honeywell 50250-S, Kaz Inc.) were used, each rated to provide
250 ft3 per minute (0.12 m3 per second) of air filtration for a combined total of
5.2 air changes per hour. The two air cleaners were used in four different locations:
1) center of the room on the floor behind the source simulator; 2) left and right
sides of the room on the floor; 3) left and right sides of the room and elevated 32
in (0.8 m); and 4) front and back of the room on the floor. Control experiments used
no air cleaners.
FIGURE 1
Representation of conference room* containing a breathing aerosol source simulator†
used to mimic a meeting participant exhaling infectious particles (source),§ and three
breathing simulators used to mimic a speaker and two participants exposed to these
aerosol particles (receivers) — United States, 2021¶
Abbreviation: HEPA = high efficiency particulate air.
* The room is 21 ft (6.3 m) x 31 ft (9.3 m) x 10 ft (3 m).
† The mouths of the participant source and participant receiver simulators were 40
in (1 m) above the floor, simulating persons sitting in a meeting or classroom. The
mouth of the speaker receiver was 5 ft (1.5 m) above the floor, simulating a speaker
standing in the front of the room. The air cleaners were placed either side-by-side
in the center of the room on the floor, in the front and back of the room on the floor,
on the left and right sides of the room on the floor, or on the left and right sides
of the room and elevated 30 in (0.8 m). The room ventilation system air inlets and
outlets were located in the ceiling as part of the light fixtures.
§ The source simulator breathed continuously at 15 liters per minute, and the aerosol
generator was repeatedly cycled on for 20 seconds and off for 40 seconds to avoid
exceeding the range of the aerosol instruments.
¶ Two participant breathing simulators (participant receivers) had a design based
on the respiratory aerosol source simulator and breathed continuously at 15 liters
per minute. The speaker breathing simulator (speaker receiver) was a commercial simulator
that breathed at 28 liters per minute.
The figure is a diagram of a conference room containing a breathing aerosol source
simulator used to mimic a person exhaling infectious particles and three breathing
simulators used to mimic persons exposed to these aerosol particles.
The source simulator (
6
) breathed continuously at 15 L/min. Two participant simulators (participant receivers)
similar in design to the respiratory aerosol source simulator breathed continuously
at 15 L/min. The speaker simulator (speaker receiver) was a commercial simulator (Warwick
Technologies Ltd.) that breathed at 28 L/min. To mimic human heads, all simulators
had headforms with elastomeric skin (source simulator headform, Hanson Robotics; receiver
simulator headforms, Respirator Testing Head Form 1–Static, Crawley Creatures Ltd.).
The face masks used on the headforms were three-ply cotton cloth face masks with ear
loops (Defender, HanesBrands Inc.). Experiments were conducted either with all simulators
unmasked or all simulators masked (universal masking).
The concentrations of 0.3 μm to 3 μm aerosol particles were measured at the mouth
of each receiver using optical particle counters (Model 1.108, Grimm Technologies,
Inc.) to determine the exposure of each receiver simulator to aerosol particles. When
the simulators were masked, the particle counters collected aerosol samples from inside
the masks (i.e., the particle counter measured the concentration of the aerosol being
inhaled by the receiver simulator). For each optical particle counter, the total aerosol
mass concentration was averaged over 60 minutes to determine the mean aerosol mass
concentration (mean aerosol exposure) to which each receiver was exposed. Each experiment
was repeated four times for a total of 20 tests. All data were analyzed using the
Kruskal Wallis test to assess overall significance, followed by a Wilcoxon Rank Sum
pairwise comparison with a Benjamini and Hochberg adjusted p-value for multiple comparisons.
R software (version 3.6.0; R Foundation) was used to conduct all analyses.
The mean aerosol concentrations for the two participant receivers and the speaker
receiver were generally similar during each experiment, indicating that the air in
the room was well mixed over the 60-minute test period (Table). For all assessed scenarios,
use of the HEPA air cleaners significantly reduced the aerosol exposures for the two
participant receivers and speaker receiver (p = 0.001) (Figure 2). Without masks,
the combined mean aerosol concentrations for the two participant receivers and speaker
receiver were reduced by 49% with the air cleaners in the left and right elevated
positions, 52% in the left and right floor positions, 55% in the front and back floor
positions, and 65% in the center floor positions. The reductions with the air cleaners
in the center floor position were higher than those with the air cleaners in the left/right
or front/back positions (p<0.01). The aerosol concentrations when the air cleaners
were in the left and right floor, left and right elevated, and front and back floor
position results did not differ significantly from one another. Without the HEPA air
cleaners, universal masking reduced the combined mean aerosol concentration by 72%
(p<0.001). When both universal masking and the HEPA air cleaners were used, the combined
mean concentrations for the two participant receivers and the speaker decreased by
as much as 90% (p<0.001) (Table).
TABLE
Mean aerosol concentrations and standard deviations measured at the mouth of each
simulator over 60 minutes at varying HEPA air cleaner locations, by masking status
— United States, 2021
Simulator/Masking status
Mean aerosol concentrations at four HEPA air cleaner locations, % (SD)
No air cleaner
Left and right (elevated)
Left and right (floor)
Front and back (floor)
Center of room (floor)
No masks
Participant A
99.8 (28.3)
62.1 (8.2)
61.0 (2.9)
40.7 (8.4)
33.3 (1.5)
Participant B
105.8 (7.7)
45.2 (1.7)
48.6 (1.9)
43.8 (1.2)
41.9 (1.4)
Speaker
94.4 (12.6)
44.7 (0.9)
33.4 (1.8)
50.0 (10.5)
30.8 (1.1)
Participants and speaker combined*
100.0 (12.1)
50.7 (3.3)
47.7 (1.6)
44.8 (5.7)
35.3 (1.3)
Universal masking
Participant A
31.2 (2.4)
22.5 (9.2)
33.1 (4.0)
12.2 (3.6)
10.9 (2.3)
Participant B
32.7 (3.9)
13.7 (3.5)
11.4 (0.9)
13.4 (4.5)
12.8 (2.7)
Speaker
21.7 (2.2)
7.3 (0.7)
6.8 (0.7)
8.1 (2.7)
5.1 (1.2)
Participants and speaker combined*
28.5 (2.8)
14.5 (4.3)
17.1 (1.7)
11.2 (3.6)
9.6 (2.1)
Abbreviations: HEPA = high efficiency particulate air; SD = standard deviation.
* The values for participants and speaker combined represent the average of the results
for the two participant receivers and the speaker receiver.
FIGURE 2
Concentrations* of aerosol particles at mouths of two participants and speaker relative
to the combined average concentration measured for participants and speaker when high
efficiency particulate air cleaners were not used and masks were not worn† — United
States, 2021
Abbreviation: HEPA = high efficiency particulate air.
* The aerosol concentrations were measured at the mouths of two simulated participant
receivers and simulated speaker receiver for 60 minutes while the simulated infected
participant source exhaled aerosols into the room.
† The legend indicates the locations of the HEPA air cleaners in the room. Each bar
is the mean of four experiments. Error bars show the standard deviations.
The figure is a bar chart showing concentrations of aerosol particles at mouths of
three persons relative to the combined average concentration measured for those persons
when HEPA air cleaners were not used and masks were not worn.
Discussion
In this study, the use of HEPA air cleaners in a conference room significantly reduced
the exposure of nearby participants and a speaker to airborne particles produced by
a simulated infected participant. The air cleaners were most effective when they were
located in the center of the room close to the aerosol source. Moreover, the combination
of HEPA air cleaners and universal masking was more effective than was either intervention
alone. The use of masks without air cleaners reduced the aerosol exposure of the receivers
by 72%, and the use of air cleaners without masks reduced the exposure by up to 65%.
When used together, the HEPA air cleaners and masks reduced exposure to respiratory
aerosols by up to 90%. These findings suggest that the use of portable HEPA air cleaners
and universal masking can each reduce exposure to simulated SARS-CoV-2 aerosols in
indoor environments, with larger reductions occurring when air cleaners and masking
are used together.
Ventilation is a well-established method for reducing potential exposures to infectious
aerosols (
7
). By removing airborne particles from a room, ventilation systems can reduce exposures
that occur by inhalation of infectious aerosols, deposition on susceptible mucous
membranes, or conveyance to mucous membranes by contaminated hands. However, in most
nonclinical settings, ventilation systems are designed only with sufficient airflow
to provide fresh air while maintaining comfortable temperature and humidity levels;
these systems typically are not designed to have the much higher airflow rates that
are needed to reduce disease transmission (
8
). During the ongoing pandemic, public health and professional organizations have
provided guidance for increasing ventilation and air filtration to decrease the spread
of SARS-CoV-2 (
2
,
9
,
10
). One recommended option, especially when existing HVAC systems might be insufficient,
is adding portable HEPA air cleaners to rooms (
2
). The results of this study support the use of portable HEPA air cleaners to reduce
exposure to airborne particles.
The findings in this report are subject to at least five limitations. First, the dispersion
of aerosols in a room depends upon air currents, which are unique to each setting.
In this study, the conference room air was well mixed, which helped transport aerosols
to the air cleaners. In rooms with poor air mixing and potential stagnation zones,
air cleaners might be less effective. Airflow patterns in real-world settings such
as classrooms will vary among buildings and rooms, and rooms of different dimensions
and with different ventilation rates will also have different airflow patterns. Second,
the aerosol source manikin in this study was kept in one fixed location. In reality,
potentially infectious occupants could be anywhere in the room and might move around
the room occasionally. Third, this study only used one source manikin and three receiver
manikins; additional sources and receivers could change the dynamics of aerosol dispersion
within a room. Fourth, the study was limited to aerosol particles of 0.3 μm to 3 μm
in size, which are small enough to remain airborne for an extended time but large
enough to carry pathogens. However, particles outside this size range would behave
differently. Finally, the study only assessed aerosol exposure; it did not directly
examine disease transmission. Although the study provides useful information about
the dynamics of respiratory aerosol particles and the effects of HEPA air cleaners
and universal masking, many other factors are also important for disease transmission,
including the amount of virus in the particles, how long the virus survives in air,
and the vaccination status of the room occupants.
Portable HEPA air cleaners offer a simple means to increase the filtration of aerosol
particles from a room without modifying the existing building ventilation system (
2
). The optimal location for HEPA air cleaners will depend upon the unique conditions
in each room, but they are likely to be most effective when they are placed as close
to the occupants as is practicable. Larger reductions in exposure occur when air cleaners
are used in combination with universal masking. These findings support the utility
of portable HEPA air cleaners and universal masking for reducing exposure to indoor
aerosols containing SARS-CoV-2. Efforts to reduce SARS-CoV-2 aerosol exposure could
help limit transmission of the virus and decrease incidences of COVID-19 illness and
death.
Summary
What is already known about this topic?
Ventilation systems can be supplemented with portable high efficiency particulate
air (HEPA) cleaners to reduce the number of airborne infectious particles.
What is added by this report?
A simulated infected meeting participant who was exhaling aerosols was placed in a
room with two simulated uninfected participants and a simulated uninfected speaker.
Using two HEPA air cleaners close to the aerosol source reduced the aerosol exposure
of the uninfected participants and speaker by up to 65%. A combination of HEPA air
cleaners and universal masking reduced exposure by up to 90%.
What are the implications for public health practice?
Portable HEPA air cleaners can reduce exposure to simulated SARS-CoV-2 aerosols in
indoor environments, especially when combined with universal masking.