A mask fit test is a way to know how well a mask works for the person wearing it. You don't need to do a fit test to benefit from wearing an N95-style mask and most people wearing an N95 can achieve around 90% effective filtration without a fit test.
A fit test gives you confidence that you are probably getting less than 1% inward leakage when wearing your mask. This means that rather than getting around 90% protection, you're probably getting around 99% - a 10x improvement in protection!
Step 1
In order to get a good mask fit your face needs to be clean, razor-shaven (if you have facial hair). If you cannot razor shave your facial hair, a large elastic elastic rubber band may be used to achieve a good fit.
Step 2
Make sure your mask intuitively fits you well and you don't feel jets of air leaking out of it when wearing it. This kind of test is usually called a "seal check." The video above demonstrates how to do this kind of test. If you pass this "seal check" step you are probably getting at least 90% protection.
Technical details
If interested, please see the technical details on this method of fit testing.
