Since the volume of gas trapped by a mask is miniscule compared to the lungs, I wonder if it even matters. Might be better to just try to measure blood concentrations.
Yeah I was surprised they didn't measure blood concentrations--feels like the actual metric we should care about. I remember like, a lot of mic-drop social media posts about people being like "I wore a mask for 16 straight hours and my spo2 is 100% (or whatever), quit whining"