Yes air pollution is definitely a factor(e.g China) but in some cases it predates that, in Japan the habit is even older..

But the single most important event that elevated masks from being a luxury item to an everyday product for the masses was the Spanish flu, which killed tens of millions around the world between 1918 and 1920.

In Japan alone, 450,000 perished according to some estimates, with an additional 280,000 believed to have died on the Korean Peninsula and in Taiwan, which were under colonial Japanese rule at the time.

Saburo Shochi, a famously long-lived academic, was often interviewed about his experience during the pandemic. In a story that ran on Nikkei Medical in 2008, the 90th anniversary of the start of the Spanish flu outbreak, Shochi recalled losing his classmates to “the bad cold.” Shochi said most of his family, including himself, then around 10 years old, caught the disease and were unable to get out of the futon for days. The infectious nature of the virus eventually became known, and people started wearing masks, which seemed to offer protection from the influenza, he said.

Educational posters from the period feature slogans such as “reckless are those who don’t wear masks.” And for those who couldn’t afford to buy masks, newspapers began giving instructions on how to make them at home, much like the online mask-making tutorials that flourished during Japan’s latest mask shortage.