Ambient Air: To test for 99.97% efficiency, a reading must first be done to test the number of 0.3 micron sized particles in the air before filtration.
A MetOne laser particle counter is used. It is calibrated to count 0.3 micron sized particles per cubic foot of air.
Tested Ambient Air: 613,300 particles / cubic foot
HEPA efficiency in this case, is a filtered result of 184 particles / cubic foot or less.