[redcatcherb412]

blood and muscle oxygen levels are not affected for healthy individuals wearing a face ... The study evaluated use of a three-layer cloth face mask

Now let's see a detailed definition of a 'healthy individual' for this study.

[daniel1212]

Now let's see a detailed definition of a 'healthy individual' for this study.

Considering that only about 30% of Americans are healthy (over 70% overweight, besides other issues) then that is a valid question. As is the viability of the study if 'healthy individual' means one of the 30%.