A mask fit test checks whether your mask properly seals against your face! You may not always be able to see or feel small air gaps in your mask, but a fit test helps tell you if you have any air gaps. This is good to know because if a mask does not seal properly, it lets contaminated air through without being filtered.
In a qualitative fit test, a sweet (Saccharin) or bitter (Bitrex) solution is put in a nebulizer. The nebulizer spreads the solution into the air, which is where most of it stays. We can't taste most particles, but we can still taste the extremely sweet or extremely bitter solution when it is aerosolized.
We run a series of tests: the person who is being fit-tested does a series of movements while wearing a mask. If they taste the sweet or bitter solution while masked, it tells them that the mask does not fit them well.
In a quantitative fit test, a particle counting device attaches to the mask and takes readings of the particle count both inside and outside the air. It then compares the particle counts to see if the air is being filtered.