Efficacy of Portable Air Cleaners and Masking for Reducing Indoor Exposure to Simulated Exhaled SARS-CoV-2 Aerosols

United States, 2021

William G. Lindsley, PhD; Raymond C. Derk, MS; Jayme P. Coyle, PhD; Stephen B. Martin, Jr., PhD; Kenneth R. Mead, PhD; Francoise M. Blachere, MS; Donald H. Beezhold, PhD; John T. Brooks, MD; Theresa Boots, MS; John D. Noti, PhD

Disclosures

Morbidity and Mortality Weekly Report. 2021;70(27):972-976. 

In This Article

Abstract and Introduction

Introduction

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 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).

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.

*HEPA air cleaners consist of a filter capable of removing ≥99.97% of particles from the air and a fan or blower to draw air through the filter. HEPA air cleaners are commercially available, relatively inexpensive, and easy to use.

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