Also Hong Kong. I always thought it may be related to population density. Another thing I've noticed in Hong Kong is sanitizing gel dispensers placed pretty much everywhere - corridors in metro stations, hotels, etc. Seems reasonable with the amount of people living there per square meter of space.

I'm not sure it's population density or at least not that alone. There are parts of the West with comparably high densities, especially in large cities, and covering your face is generally frowned upon.

I suspect that the cultural focus on cleanliness has a lot to do with it, but given that it's Hacker News there may be someone with a more exact idea of the origin.

You wear one if you're sick. So that you don't get other people sick basically.

Contrast with my experience in Sydney, where catching the bus during flu season is bloody hazardous. Being coughed and sneezed on gets old quickly, so I totally understand the surgical mask thing; it's a basic courtesy.

Yes. Though I am not sure if it's actually effective, or only a superficial courtesy.

It may even be counter-productive, if it means you no longer feel the need to cover your mouth when coughing.

Isn't surgical mask more effective than covering your mouth when coughing?

Allergies is one major motivator.

Allergies should be a good reason not do that, at least from a public health perspective.

It's most likely due to the country's brush with SARS and avian flu. http://www.bbc.com/news/world-asia-china-21680682

I wonder if this is a consequence of the SARS outbreak ~10 years ago.

No, it predates it, to the 80's at least, to my personal knowledge, and I have no reason think it's not older than that.

You will find it elsewhere in SEA, but the prevalence in Japan is extremely high. I've been in train carriages where 40% of its passengers are wearing them.