I realized I hadn't properly discussed the definition of a "fit factor" in the last post, but since this is (IMO) important, basic mask information everyone needs to know, and since it is a prerequisite for understanding fit test exercises, I'm going to discuss it in a separate post without added jargon and confusion.

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, 0.05*100 = 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

Fit Factor Assumptions and Qualitative Fit Tests

Okay, all the important equation parts are done, so if you made it this far, good for you.

But there are still a few odd details you may wonder about, like OSHA's requirement for 100 fit factor for half-face, 500 for full face. What's up with that? And what about the fit factors for qualitative fit tests?

OSHA assumes that if you pass a qualitative fit test, your fit factor is at least 100, and vice versa. Is it true? Well, you'll need to read the previous post to find out, but spoiler: maybe not. OSHA is fine with you making this assumption, so make of that as you will.

OSHA also wants you to have mask leakage of <1% due to face seal leakage, or a fit factor of 100 or more, regardless of if you wear an N95 or better. Only filter penetration should be 5% or less. Hence why (some) organizations spent boatloads of cash for TSI N95-Companions, an accessory for the PortaCount CPC machine.

But if you buy a decent N95 mask, it'll probably already have a fit factor well in excess of 100, making the TSI N95-Companion completely unnecessary. All a mask company is really demonstrating when a mask only passes on an TSI N95-Companion is that cheaped out on the filter.

What about CNP machines?

CNP machines only estimate aerosol fit factor, and for CNP fit factor, they use a different formula, which we won't cover here.