assuming: 1) 100% capture of air flow 2) no fatigue (ie capture is same at t=0, and t=later) anyone with glasses will tell you 1 isn't a very good assumption.

assuming: 1) 100% capture of air flow 2) no fatigue (ie capture is same at t=0, and t=later) anyone with glasses will tell you 1 isn't a very good assumption. As masks saturate in moisture, back pressure causes (1) to be more false.