I've had a few people back in the states ask me about the face mask thing and why people wear them. There are a couple reasons for the mask, maybe a little pseudo-sciency, but reasons nonetheless.
I started wearing one if the bluish green surgical masks a couple of weeks ago on my roommates suggestion. I get really bad pollen allergies, and was sneezing for like 10 minutes solid, and the next day she brought some home for me to wear. She said that it's common for people wear them to help with allergies. I feel like this makes sense, because it's a physical barrier between your face and pollen. I haven't had a ton of allergy flare-ups since I've been here, so I can't say definitively weather or not it really works, but I buy it.
The second reason that people wear a mask is to prevent the spread of illness. You would wear it if you were sick, so that you wouldn't get anyone else sick. Sick days in china are not really a thing, and you probably have to show up to work or class once or twice while your sick, so you could wear one at that time, on the subway, etc. The drawback is that they make your face really hot and are really annoying to wear inside.
The third reason for a mask is to prevent breathing in dirty air when you are out in the road. I see so many people riding bicycles, on their motorbike, or just walking around wearing a mask out in busy streets. I feel like the pseudo-science comes in a little bit here, but I invite you to suspend your suspicion for just a moment to imagine being in really dirty air. Like your standing behind a tractor for a few minutes, a California afternoon when the southern half the state is on fire, your sister left the bottle of nail polish open in the smallest bathroom in the house, a campfire that hasn't quite taken yet and is mostly just smoke... These are the kind if smells that you will get out on the street. The kind if smells which you can't tell if they simply smell bad or if they are actually dangerous. A classmate has even told me she gets headaches when she goes out for too long, so you really can't help but wonder. When faced with these strange smells, you cannot help but want to cover your nose and mouth. Having a mask on can really help with this.
There are a few different kinds masks, which are the surgical type masks, and then novelty cloth masks. As far as I can tell, neither if these masks have any type if extra protection, other than just being cloth over your face, hence my suspicion of pseudo-science. The novelty ones are funny, baecause they come in children's sizes, men and women's styles, big and small, Lacey and pink, checkered... Quite funny.
I myself bought this really great shiny pink lepoard one that covers most if my face. Will says it makes me look like a ninja hehe. Not only is it totally adorable, it gets warmed up from your breath and keeps you really warm! It keeps the dangerous and gross smells down and potentially helps my allergies stay under control. It helps keep me healthy and keeps out roadside dirt particles. And it covers my face and makes me look less like a foreigner! So for me and many others, the mask is definitely working.