July 15, 2021

Study Finds Portable Air Cleaners Effective at Reducing Exposure to Simulated SARS-CoV-2 Aerosols

The use of portable high-efficiency particulate air (HEPA) cleaners within a room significantly reduced occupants’ exposure to simulated SARS-CoV-2 aerosols, according to a study discussed in CDC’s Morbidity and Mortality Weekly Report (MMWR) on July 9. While supplementing building ventilation systems with HEPA units was already known to help reduce the number of airborne infectious particles indoors, this study found that using HEPA air cleaners close to an aerosol source—in this case, a simulated SARS-CoV-2-infected meeting participant—reduced the exposure of simulated uninfected participants in the room by up to 65 percent. Researchers found that the reduction in exposure varied based on the placement of HEPA units within the simulated meeting room, and exposure could be further reduced by up to 90 percent by universal masking of meeting participants.

The study involved four manikins and two HEPA units placed within a meeting room. One of the manikins was designed to emit aerosols, mimicking an infected meeting participant exhaling SARS-CoV-2 particles into the room. The other three mimicked uninfected meeting participants inhaling the potentially infectious aerosols. Two HEPA units meeting EPA’s recommended clean air delivery rate were placed at various locations in the room, and researchers measured the units’ effectiveness at removing exhaled aerosols for each location. Depending on the location, the HEPA units reduced participants’ exposure by 49 to 65 percent. The most effective arrangement was found to be placing both units on the floor at the center of the room, as opposed to the units being elevated, placed at the front or back of the room, or placed to participants’ left or right when facing the front of the room.

Additionally, the study found that the use of masks on all the manikins, without air cleaners, reduced uninfected participants’ exposure by 72 percent. The combination of masking with the use of HEPA units reduced exposure by up to 90 percent. These findings suggest that both masking and the use of air cleaners can significantly reduce SARS-CoV-2 exposures, with the greatest reduction in exposure occurring when both measures are used together.

The MMWR report further discusses several limitations of the study. First, the air in the room used for the study was well mixed, improving the transfer of aerosol particles to the HEPA units by air currents. The report explains that aerosol dispersion in any room depends upon air currents unique to each setting, varying by room dimensions and ventilation rates, so air cleaners may be less effective at reducing exposures in rooms with poor air mixing and stagnation zones. Next, the study did not reflect every potential configuration of infected and uninfected occupants in real-world settings. In this study, one simulated infectious aerosol-emitting manikin remained in one location, but an infected individual may move around the room, or multiple infected people may be present. Finally, the study only assessed aerosol exposure—not disease transmission.

The authors conclude that while the study provides useful information on the effectiveness of HEPA air cleaners, “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.”

For more information, refer to the full report.