Spoiler alert: the most effective mask is the one you wear.
Mask tips: never use one with an exhaust valve (risk to others), avoid single-ply fabric. (quite poor filtration).
Wearing one under your nose is stupid.
They don’t have any real effect on oxygen levels or carbon dioxide levels…even in COPD patients. Surgeons, and now even ED doctors, wear them all day. That desperate asphyxiation feeling is mental. It’s just humidity and one’s baseline anxiety and fears.
There is little we can do that matters more in Covid prevention than masks, vax, and ventilation.