When examining the process of air filtration, many factors can influence desired air quality. Such factors include type of environment, size of building, number of units, and volume of foot traffic.

In 2009, ANSI/ASHRAE 52.1 “Dust Spot” efficiency testing was removed from the standards while two other parts of the 52.1 Standard were adopted into the 52.2 Standard, allowing for 52.1 to be retired. The two parts of 52.1 adopted by 52.2 are “Arrestance” for determining MERV 1-4 and “Dust Holding Capacity.”

By definition, true HEPA filters are at least 99.97% efficient at filtering 0.3 μm mass median diameter (MMD) particles in standard tests.