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In the factories it's very dusty (and Noisy if you ever visit one). They likely wear one to keep down the issues with inhaling all the dust and lint?
I asked "she who cooks my dinner" about the mask wearing on the street, they do it where I am retired quite often, and she said something along the same line: keeps out street dust and smells I guess? She said some ladies do it when they have a cold or flu so they don't spread it to others around them, but I'm not sure about that last part.
You see it all the time flying on airplanes with East Asians. I expect that is to prevent picking up disease from other travelers on crowded airplanes which is probably a good idea?
My wife and I carry N95 masks with us. I wear one when sitting in a doctor's office, or other areas with lots of strangers. I place tape on the inside over the exhale valve to keep me from infecting others when I have a cough; remove the tape otherwise. Comes in handy in dusty or smoky areas. There are a lot of people who cough or sneeze on public transit, you really should wear a mask on public transit.