What is a p100, n95 and pm2.5?
PM 2.5 means particulate matter 2.5 micron. Its the size of dust/dirt/contaminant particle most difficult for filters to catch. Bigger is caught by the "mesh" of the fibers and smaller is caught by the electrostatics which make the tiny particles veer and slam into the fibers and stick. The 2.5 micron size is hardest to catch by the "mesh", but massive enough to not veer toward the individual fibers as fast. (You may not realize it, but the fibers of these masks have a small charge captured inside them. They are really amazing technology, they are not just a synthetic cloth. They actually get zapped into having a charge stuck inside the fibers that attracts particles because small particles in the air statisticly have a slight charge imbalance.)
Unfortunately, it turns out that particles at the 2.5 micron size are small enough to get trapped in the lung tissue, but still large enough for your body to react to them. They more exposure you have, the higher your chances of having lung disease, even though you don't smoke, don't work in a coal mine, play with asbestos, etc. So doing what you can to cut down on your exposure should benefit you later in life.
P95 is a filter that stops 95% of the 2.5 micron particles.
P100 is a filter that stops 99.98 % (I think) of the 2.5 micron particles.
N95 are the dust and medical masks (typically white or green) which are made of 3 layers of material similar to P95 filter media. N95 is the "real thing" tested more stringently, but were diverted to medical use only during the pandemic. KN95 are the chinese-made equivalents, but which aren't tested to the same degree. And now are in many colors, not just white.
Disclaimer: You will get more precise / correct definitions from Wikipedia. This is just as best as I recollect in the moment. Its roughly correct but probably has errors.
