Filtered air respirators must be used with a clean shaven face. Moustaches are usually okay, but anything which interferes with a tight seal, including stubble, will allow contaminants into your eyes and lungs. Bad times.
(infographic here)To be useful, a respirator/mask should be fitted and tested. I'm respirator certified at work. I have to take annual training and a fit check using a test respirator hooked to a computer which analyzes the air entering an exiting to insure an adequate seal. They also do a medical screening to make sure you don't have heart or lung issues which could make using a mask dangerous.
A filtered air respirator will add resistance to your breathing. This will make your body work harder to get the same amount of oxygen. This is important to remember if you're using it in a stressful environment.
It's very important to don the mask before you're exposed to the irritant. Otherwise, you have a rubber bucket of snot and tears strapped to your face.