It would have been an excellent idea in Southern California not that long ago — a good mask massively reduces PM2.5, and PM2.5, especially on So Cal freeways, was quite high.
“Should have” is, at best, wishful thinking. In air filtration, a good rule of thumb is that, if an actual quantitative spec isn’t given, then the filter doesn’t filter. I’ve seen this with expensive commercial systems with stamped plans, I’ve seen it with very fancy European IAQ systems, and I’ve seen it with cars. Seriously, go look for a stated spec for a cabin air filter, which you generally won’t find, then drive on a freeway, bring a PM2.5 meter, and compare windows open to windows closed.
I live in Minneapolis - I see it all the time. Several times a day in fact. I still see people in outside settings wearing them as well. By now, if I see a person wearing one my only two thoughts are: a) this is an act of virtue signaling (Minneapolis is a VERY progressive city) or b) you are immunocompromised and or wearing it for medical or health reasons.
Since both of these are on opposite ends of the spectrum, I just hope its for medical reasons and being cautious as opposed to the former and get one with my day.
Don’t forget option C: in cold, dry weather, a disposable mask can be an impressive comfort improvement. And you can throw it away when it gets disgusting. They often outperform balaclavas.
Or option D: allergies. A well fitted disposable mask removes allergens, at least until it collects so many that it starts triggering allergies all by itself. Achoo!
I discovered option D during covid and it's like a superpower. I have allergies even through double dosing OTC medicine, but a good mask does wonders for me in spring.
Where do you live that you see this regularly?
Don't think it's happening in southern California.