In CPC parlance, fit factors are how much cleaner a mask is inside, compared to outside. Hence the equation:

Fit Factor = Ambient Concentration / Mask Concentration

It seems cryptic, but it really isn't that hard if you imagine aerosol particles like apples inside the bucket vs outside the bucket.

How do we determine the number of "apples?" Well, that's the job of the CPC, in the form of particles per cubic centimeter of air.

Technically, we could use any particle detector, but CPCs are usually used due to their ability to detect a wide variety of indoor air particles, as well very small particles, well beyond what a laser can detect normally in dry, alcohol free air.

The number you get is essentially how much cleaner the air is inside the mask than it is outside. For example:

• A fit factor of 10 means the air inside is 10 times cleaner than outside.

Total Inward Leakage

A fit factor of 10 also means that there's a Total Inward Leakage of 10%. Which makes sense: if you have 10% of the count of apples outside the bucket equals the count of apples inside the bucket, then there are 10 times as many apples outside than inside. Here's the equation:

Total Inward Leakage Percentage = 1 / Fit Factor * 100

For example:

• For a fit factor of 20, 1/20 = 0.05, 0.05100 = 5% total inward leakage.

Hey, that's the allowable filter penetration limit of N95 masks!

Assigned Protection Factor

There's a lot of smoke and mirrors on how assigned protection factors came to be, but if you ever come across this term in NIOSH or OSHA literature, just remember this formula:

Assigned Protection Factor = Fit Factor / 10