1/ There have been lots of questions about filtration and SARS-CoV-2. To understand how effective a filter is going to be, you need to know two things: the size distribution of the particles/droplets that contain the virus and the installed efficiency of the filter.

2/The challenge is that both of these things are environment-dependent and generally not known well for a specific building.

3/The size distribution of particles and droplets presented to the filter depends on the infected individuals in the space and emission modes (e.g., breathing vs. talking vs. coughing) and the transport dynamics in the space (air flow, deposition, and humidity).

4/There is no one size distribution that we are targeting.

5/The filter is generally tested in a laboratory (e.g., according to ASHRAE Standard 52.2, EN 779, etc.) and given a designation that indicates the efficiency curve at a particular flow rate and for a particular challenge aerosol.

6/That designation (for example the MERV from Standard 52.2) is NOT an indication of installed performance. In a building, the filter is used for a different challenge aerosol, at a different velocity, and for the specific installation conditions.

7/One of the more common differences between a lab test and installed performance is bypass (gaps around the filter). This will lead to diminished performance. The bigger the gap and the higher pressure drop of the filter, the more that bypass will diminish efficiency.

8/Filtration efficiency is also dynamic rather than static. Many filters decline in performance as they age, sometimes substantially. The amount of decline depends on what and how much the filter is loaded with and is generally different in every building and for every filter.

9/Remember that SARS-CoV-2 is a tiny fraction of what is on a typical building filter. All of the dust and other particles will serve to increase pressure drop which can cause flow to drop and bypass flow to increase, further diminishing efficiency.

10/This is the controversial part of this thread. It is irresponsible to tell people to use a specific level of a filter (e.g., a particular MERV) because we don’t know what its installed performance will be.

11/In many buildings, you will not get the lab performance and even if you do when the filter is new, you may not as the filter ages.

12/What should we do #1
Make sure that your filter installation is good. Install gaskets or use gasketed filters. In some systems this is very hard to do and you may need to invest in improving your filter rack.

13/What should we do #2
Make sure your fan can supply adequate air flow through the filter. In some cases, you may need to upgrade your fan.

14/ What should we do #3.
Use a good filter. “Good” will mean different things depending on your system/building and procurement issues. In general, I recommend MERV 13-15, but I would much rather that someone uses a MERV 11 or 12 filter well than a MERV 15 filter poorly.

15/ What should we do #4
Maintain your filter. You can change it based on manufacturers recommendations, pressure drop measurements, or on a schedule. Just make sure that you are changing it (carefully, to avoid exposure while changing).

16/ Perhaps most importantly, a filter is not a substitute for other measures. Mask wearing, physical distancing, considering ventilation and air flow, surface cleaning, managing people flows in the space, and local air cleaning and ventilation are all important.