I think maybe people around them, but I really don’t know the answer to that.
Masks really reduce aeration.
You are re-breathing your own carbon dioxide, which is taken into the red blood cells displacing oxygen. A local doctor was on a radio talk show recently. He put his oxygen meter on his finger, then put on a mask. In a couple of minutes his blood oxygen level went down several points. I then asked waitresses in a restaurant what they experienced after wearing their masks for a while. Many said headaches, drowsiness, and dizziness. I am an Industrial Hygienist, and I have monitored processes that used CO2. Headaches and sleepiness are common effects of over exposure.
This is most commonly seen when the A/C in a building doesn’t provide enough make-up air for the number of occupants, resulting in CO2 levels going up.
This experiment says otherwise.
I'm gonna do my own test with a variety of masks to see for myself. Not that any of the true believers in the hazards of masks will accept it or do their own tests. I'll let you know. :)