There are five particles that determine air quality: carbon monoxide, sulfur dioxide, nitrogen dioxide and ozone. These provide an air quality index.
Air quality is measured by a numbered scale which begins at zero and goes to 500. Anything above 150 to 200 is considered terrible air quality, even though the scale goes to 500.
Source: The Outdoor Wear, August 12, 2021. Link. HEPA is a pleated mechanical filter for the air, and everyone should have one of these installed indoors. It has been proven to remove up to 99.97 percent of the contaminants and airborne particulate matter measuring 0.3 microns.