It's so obvious. Even OSHA published studies from decades ago warning of the danger of wearing 'respirators'. These are mostly the N-95 face masks that were first produced by 3M Corporation for coal industry and coal dust.

OSHA safety standards, specifically 29 CFR §1910.134. OSHA Director of Safety Programs stated that the scientific research conducted by OSHA establishes that having an oxygen level below 19.5% may be deadly, or result in organ failure and severe bacterial infections (e.g. pleurisy). Wearing a muzzle immediately reduces oxygen flow well below 19.5%, in clear violation of OSHA safety standards. These guidelines were hidden and overwritten by tyrannical abuse of power. People were lied too. I never wore a face mask and refused to do so. Being a scuba diver, you learn how precious air really is.