Research shows that the particle size of SARS-CoV-2, the virus that causes COVID-19, is around 0.1 micrometer (µm). However, the virus generally does not travel through the air by itself. These viral particles are human-generated, so the virus is trapped in respiratory droplets and droplet nuclei (dried respiratory droplets) that are larger. Most of the respiratory droplets and particles exhaled during talking, singing, breathing, and coughing are less than 5 µm in size. By definition, a High Efficiency Particulate Air (HEPA) filter is at least 99.97% efficient at capturing particles 0.3 µm in size. This 0.3 µm particle approximates the most penetrating particle size (MPPS) through the filter. HEPA filters are even more efficient at capturing particles larger and smaller than the MPPS. Thus, HEPA filters are no less than 99.97% efficient at capturing human-generated viral particles associated with SARS-CoV-2.
Portable HEPA filtration units that combine a HEPA filter with a powered fan system are a great option for auxiliary air cleaning, especially in higher risk settings such as health clinics, medical testing locations, workout rooms, or public waiting areas. Other settings that could benefit from portable HEPA filtration can be identified using typical risk assessment parameters, such as community incidence rates, facemask compliance expectations and room occupant density. In choosing a portable HEPA unit, you want to select a system that is appropriately sized for the area in which it is installed. One way to do this for room air cleaners is to select a HEPA fan system with a Clean Air Delivery Rate (CADR) [See EPA’s Guide To Air Cleaners In The Homepdf iconexternal icon] that meets or exceeds the square footage of the room in which it will be used. The larger the CADR, the faster it will clean the room air. If the room in which the air cleaner will be used is taller than 8 feet, choose an air cleaner with a proportionally higher CADR than that based simply on square footage. While these systems do not bring in outdoor dilution air, they are very effective at cleaning air within spaces to reduce the concentration of airborne particulates, including SARS-CoV-2 viral particles. Thus, they give effective air exchanges without the need for conditioning outdoor air.
HEPA fan systems can be used as stand-alone units, or many larger units allow flexible ductwork to be attached to the air inlet and/or outlet (note that larger ducted units don’t fall under the “room air cleaner” description and may not have a CADR rating). Using ductwork and placing the HEPA system strategically in the space can help provide desired clean-to-less-clean airflow patterns where needed. Ducted HEPA systems can also be used to establish direct source capture interventions for patient treatment and /or testing scenarios (See CDC/NIOSH discussion on Ventilated Headboard). Depending on the size of the HEPA fan/filter units and how the facility in which they are being used is configured, multiple small portable HEPA units deployed to high risk areas may be more useful than one large HEPA unit serving a combined space.
Example 2: The room described in Example 1 is now augmented with a portable HEPA air cleaning device with a CADR of 145 cfm (Qhepa = 145 cfm). The added air movement within the room improves overall mixing, so assign k = 3. How much time is saved to achieve the same 99% reduction in airborne contaminants by adding the portable HEPA device to the room?
The addition of the HEPA filter device provides additional clean air the room. Here, the clean volumetric air flow rate (Q) is: Q = Qe + Qhepa = 72 cfm + 145 cfm = 217 cfm.
ACH = [Q x 60] / (room volume) = (217 cfm x 60) / (12’ x 10’ x 9’) = 13,020/1080 = 12.06 ACH (round down to 12).
Using Table B.1, the perfect mixing wait time based on 12 ACH and a 99% reduction of airborne particles is 23 minutes.
Using the mixing factor of 3, the estimated wait time for 99% reduction of airborne contaminants in the room is 3 x 23 = 69 minutes. Thus, the increased ACH and lower k value associated with the portable HEPA filtration unit reduced the wait time from the original 5 hours and 45 minutes to only 1 hour and 9 minutes, saving a total of 4 hours and 36 minutes before the room can be safely reoccupied.
In conclusion, adding the portable HEPA unit increased the effective ventilation rate and improved room air mixing, resulting in an 80% reduction in time for the room to be cleared of potentially-infectious airborne particles.