The japs supposedly do this to conceal their identity
China and SA places it is usually because of smog/air pollution.
Japanese do it from various reasons. They are pretty much germaphobes. Even the young kids in the arcades playing those "slap" games (think DDR for the hands) wear gloves and masks. You see people of all ages wearing them in pachinko parlors and on the trains. What started out as a caution for pollution (even though we sterilized their air in 1945), it is an ingrained custom. It more common in colder weather, people worrying about bad breath (their lunches can absolutely STINK at times), and young kids wear who are self conscious of orthodontics wear them a lot - or to hide pimples. During the spring, the pollen also gets pretty bad over there and people use it for that. For a while, it was actually a fashion trend among young people and that DBZ fad.
I just returned from Japan last week and to be honest, it was about the same number of people wearing masks then as it was when I was there last in 2018 and the first two times I was there in 2010 and 2012.
