Indoor Environment Control Strategies: Aerosol Filtration
Filtration systems can reduce pathogenic airborne particle concentrations, reducing exposure and potentially reducing infection risk and/or severity of illness. Potentially infectious aerosols generated from the respiratory tract are typically 1-10 µm, and generally have a lower bound of 0.5 µm . An ASHRAE position document published in 2020 states that filter efficiency of MERV13 or better can be used to provide useful air cleaning against airborne transmitted pathogens . MERV13 filters are rated to capture half of 0.3-1 µm, 85% of 1-3 µm, and 90% of 3-10 µm particles. Filters with lower MERV ratings are not designed for capture of submicron particles. MERV13 filters are expected to capture most infectious aerosols with the advantage of working at lower resistance than higher rated filters, enabling the cleaning of more air volume per unit of time and with less energy demand on the fan. They are also less costly than higher rated filters. However, higher rated MERV filters and HEPA should be considered where they are possible to implement.
Filters can clean air that is being recirculated within an indoor environment, but they can also be used to increase equivalent air exchange rates by delivering a flow of clean air to an occupied indoor setting. A study of aerosol particle removal in an active classroom by portable HEPA cleaners showed a 95% reduction in particles 10 nm-10 µm after 37 minutes of use . Four air cleaners generated a total 1026.4 m3/hr of clean air resulting in an air equivalent exchange rate of 5.5/hr. The authors estimated that increasing air exchange to 5.7 could reduce risk of SARS-CoV-2 infection by 6.3-fold given 2 hours of exposure. Teachers and students generally indicated little disturbance by the noise of the specific model of air cleaner used in this classroom setting. Indoor environmental quality experts encourage use of portable air cleaners for schools and have included a tool to help users achieve at least 5 ACH from using the cleaners (https://tinyurl.com/portableaircleanertool) .
Several studies reiterate the efficacy of filtration-based air cleaners for removing particulate matter down to the size of ultrafine particles (≤0.1 µm) in occupied classrooms or offices [5-8]. A review of studies on particle filtration in homes and commercial buildings showed an easing of allergic rhinitis, asthma symptoms, and potential reductions in mortality . A more recent randomized intervention study over 2 years showed reduction of PM2.5 and VOCs, with associated cardiovascular health benefits of reduced systemic inflammation and blood pressure . Respiratory and cardiovascular health benefits of air cleaners are also summarized by an ASHRAE technical report  and an EPA technical summary . The efficacy of air filtration to reduce exposure to particles <10 µm resulting in measurable health benefits suggests that they could reduce exposure to infectious aerosols, which tend to predominate in the same size range.
Modeling studies in non-healthcare settings that have assumed some level of inhalation infectious dose have shown that, compared with the absence of filters, their use in heating, ventilating, and air conditioning (HVAC) systems could significantly decrease the portion of disease transmission caused by infectious aerosols [13, 14]. In the case of SARS-CoV-2, use of appropriate filters in HVAC systems could reduce viral exposure when air is being recirculated [4, 15]. On the other hand, between-room transmission via air ducts may not represent a substantial concern given over a year of comprehensive contact investigation and few, if any, reports of transmission by this means. It is clear that air filtration can reduce airborne particles that could contain infectious pathogens, thus reducing inhalation exposure to occupants in environments where air contamination can build up. Despite this, the little research has been done to evaluate the performance of filtration systems on actual infection outcomes. Despite the logistical challenges of doing such research (e.g., prospective monitoring of populations for infection while ensuring indoor exposure with various levels of air filtration while controlling for ventilation and air flow), studies of this nature hold the potential to demonstrate air filtration effectiveness for population-level infection control.
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