Tom Avril checks out the inside of a box made from furnace filters.

The air-quality industry uses a rating system called MERV — minimum efficiency reporting value. To capture viruses, guides from the industry and Harvard’s T.H. Chan School of Public Health both recommend air filters with a rating of at least MERV-13.

That means in a single pass, they capture 85% of particles that measure between 1 to 3 microns (millionths of a meter) and at least half of the particles that measure between 0.3 to 1 microns. Individual viruses are much smaller than that, but they are carried in larger particles of moisture and dust — most of them in the size range for which these filters are designed. (HEPA filters trap even more.)