Indeed. Nor were they wearing the same one again and again for months on end, and stuffing it in their jeans pocket in between.

True, I used a fresh one every day during the worst of it, but I have to say I've gone back to doing exactly what you describe. The only place I still need one is public transport and now that it's getting hot (Spain) I often go outside without a coat, so my pocket is the only place to leave it. And it seems like a waste to replace it after one 15 minute ride on the subway.

And I'm not the only one, I've seen a lot of people wearing ones that are all fluffy from long use and sometimes washing (some people really wash disposable masks). I'm sure that will increase shedding a lot.

On the contrary, there's been plenty of doctors and lab technicians wearing masks full time and for decades!

Not to mention semiconductor workers.

And industrial worker dealing with hazardous materials.

Like hanoz says above, what professionals wear is not really comparable to what people wear on the subway these days. It's the cheapest of the cheap Chinese crap, and usually used so long and mishandled that it is becoming really fluffy. Especially now that nobody really cares anymore and the use of them is incidental they end up being lugged around more than worn and not frequently replaced.

A fresh mask doesn't shed a lot but I've seen ones that are all hairy and stringy from being rubbed around in pockets with other stuff. I'm sure they shed a lot more material than a fresh mask.

And a lot of the types you mention would not wear masks like this but powered respirators (e.g. people working in clean rooms)

Personally I have major issues the masks for past medical trauma reasons but I really don't think they are healthy to keep around forever.