HEPA is a type of pleated mechanical air filter. It is an acronym for “high efficiency particulate air [filter]” (as officially defined by the U.S. Dept. of Energy). This type of air filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns (µm).
The diameter specification of 0.3 microns responds to the worst case; the most penetrating particle size (MPPS). Particles that are larger or smaller are trapped with even higher efficiency. Using the worst-case particle size results in the worst-case efficiency rating (i.e., 99.97% or better for all particle sizes). To meet the HEPA standard, the filter must remove 99.97% or more of all particles which are 0.3 microns (micrometers) in diameter.
In other words, for every 10,000 particles that are 0.3 microns in diameter, only three of them pass through. The smallest virus particles or droplets, including COVID-19, average 0.7 microns.
Look for filters with an H13 rating, also a minimum Merv 8 prefilter to increase the life of the HEPA.
We measure how air completely cycled in a space with the term Air changes per hour, abbreviated ACPH or ACH.
The CDC recommends a minimum of 2 ACH, we feel a better target is 4 ACH based on this CDC guideline, “If you were in close contact with someone who tested positive for COVID-19 (within 6 feet for more than 15 minutes total over 24 hours).
We also sell units capable of higher ACH.