This is the source text for a printout to give out to people participating in community fit testing.

What is fit testing?

Fit testing is a way to see how well your masks work for you. While most high quality masks are made out of material that filters around 99% or more, the seal to the face makes the biggest difference in how protected someone is.

The kind of fit testing we do is called “qualitative fit testing.” There is another kind of fit testing, done with an expensive specialized machine, called quantitative fit testing.

What does a fit test tell me?

With the kind of test we do, passing a fit test means you’re probably getting around 99% or better when wearing your mask.

Technically, passing means you probably have less than 1% leakage of outside air into your mask. It’s a bit more complicated than this, but that’s the general idea.

Without a fit test, most people can get around 90% when wearing a N95 with headbands when clean shaven and going with their intuition. With earloop style masks (KN95, KF94) this number is lower. And many people do not necessarily get to this 90% level in even an N95.

A fit test tells us you are very likely in the 99% or higher range.

What are the limitations of this testing?

The biggest limitation is that you may put on your mask in a slightly different way next time you wear it. Because we only test once, this means you could have a subtle difference that affects fit.

A good example of this is facial hair. If you have facial hair and allow it to grow out for more than a day, you will likely not get as good of a fit.

We working to make materials for home fit testing more available. If you have a home testing setup you can test a few times with each mask you wear to have a good idea of how consistent your fit is.

Does passing the fit test mean I can’t get COVID when wearing my mask?

No. Fit testing is a risk reduction measure and is not a guarantee.

I have a mask at home that’s really cool and has P100 filters but I didn’t fit test it, is it better than the masks I fit tested?

Your fit tested masks are likely more protective than your non-fit tested masks, even if your non-fit tested masks have higher filtration or are elastomeric respirators.

Is the test we did an official OSHA fit test?

No, we are doing a slightly modified version of the official OSHA fit test. There are some differences. For instance, we use continuous stream nebulizers instead of the OSHA specified nebulizers. We believe the test as performed is likely more sensitive than the official OSHA test, meaning that people who pass our test would pass an OSHA test, but not everyone who passes the OSHA test would pass our test.

Our test is based on materials originally put out by the US Army for fit testing in field settings.

How can I get more information about home and community fit testing?

We’re sharing resources on our twitter account: