My cousin and her family lived in Dehli for about 14 years, they moved last year to Kathmandu. They've worn face masks when outside (when they went outside) pretty much that entire time, her kids have grown up using them, ect, though it's very rare for people to wear them there until the last several years. I know she felt extremely weird about wearing them for awhile.
A few years ago, WA state had the worst air quality that I've ever experienced here thanks to massive wildfires and puger sound's weird and frequent inversion zones. Regular ashfalls, tasting the smog and wildfire ash as soon as you stepped outside, haze, ect. Even walking from my house to my car was triggering chest pains and wheezing. I tried a box of those disposable filtration masks which helped, my my cousin ordered and sent me the type of mask her family wears (not flimsy, comfortable, cute pattern) and while I got plenty of weird looks I could breathe and resume my delivery work without feeling like I was going to suffocate.
Pollution is no joke. I wish more people got air pollution masks, at least for their kids, in areas where it's frequent. And that they were subsidized too (but yeah right) especially in cities that are extremely dangerous (plenty of those in the US too!)