One thing to note is just because the mask is advertised to work and you feel it "fits" good or has fancy filters etc. As a nurse we get fitted yearly with 3m masks or duckbilled n95 masks. They will place our masks on and put a clear headsuite thing in, spray a fine mist of bitter tasting mist, make us move our heads up down side to side, talk. And at no point should you have any bitter taste sensation. Just like everything else in this world. A mask is a tool and only as effective as the user using it. Like others have stated, the flu and other viruses are majority droplet, meaning about any face mask will work. Airborne vectors such as TB need a n95 mask. But even then it takes a prolonged exposure time.
The Air Force taught us to use an effective method to check the seal. Place both hands on the filters and inhale. If the filter inlets are covered, there should be no air coming in at all. The rubber gaskets (diaphragms) covering the exhalation holes should automatically prevent air from coming in.
If you don't "suck" the mask tight against your face, you don't have an effective seal or mask.
Locate the failure point/s:
1. Seal between mask and skin is not good.
2. Diaphragms that prevent back-flow of exhaled air through exhaust holes need replacing/straightening out.
3. Filters are not sealing at attachment points.
4. There's a leak somewhere else.
If the mask is working, but it's still not sealing completely to your face, you may need a different size or style of mask.
If it seals when you inhale, and then becomes loose when you exhale, you should still not trust it. All it takes is a few seconds of not breathing in (creating a seal) or out (creating positive pressure) to any real degree for the broken seal to be a problem -- especially if your next breath is inhaling.
We were treated to some tear gas once we had our masks adjusted and tested. We had to remain in the little training shed for 5 minutes with masks on to ensure the seals were good. Then we removed our masks and stayed for 10-15 seconds just to get a whiff of the gas before exiting.
I didn't feel any effects until the bus ride back. The guy in front of me had his window open, and the tear gas particles on his field jacket blew right into my face!
