Get yourself a cheap Pulse Oxymeter from your local pharmacy. ($15-$30+)
Put it on your finger and check your blood oxygen level.
Now put on your favorite mask, and just breath for a while.
Note your Oxy level after 5 minutes, 10 minutes, 15 minutes, 30 minutes.
15 minutes walking in a store with a common procedure mask brought my Dad to tunnel vision level with sweating and tremors.
Now when he's forced to wear one, we have the kind with exhalation valves, to reduce CO2 re-breathing.
It's better, but his blood oxy level still drops a few percent while masked.
(I'm pulling the valve flapper and the N99 filter out of another, to make a Stealth Breathable version.) 
~Easy
