Fear Porn, that's why. ChYnA
On the 7 train that connects Manhattan to the bustling ethnic enclave of Flushing, Queens, it’s becoming more and more common to see riders wearing surgical masks in public. It’s a phenomenon that’s long been common in East Asian countries. And ever since the 2002 SARS outbreak and the 2006 bird flu panic, the practice has crossed over into immigrant Asian populations in the US. Now, with Ebola fears still on high, many immigrant Asians aren’t taking chances—despite the fact that the number of known US Ebola infections has now dropped to zero, and assertions by public health authorities that Ebola is almost certainly not airborne-transmissible.
The reality is that the woven-cloth surgical masks provide minimal protection from environmental viruses anyway. (Surgeons use them to protect patients from their mouth-borne germs, not the other way around.) But the masks’ actual prophylactic utility is, in a way, secondary to other reasons they’re being worn, which is why they’re likely to become more common in the future—even among non-Asians.
The custom of facemask-wearing began in Japan during the early years of the 20th century, when a massive pandemic of influenza killed between 20 and 40 million people around the world—more than died in World War I. There were outbreaks of the disease on every inhabited continent, including Asia (where it devastated India, leading to the deaths of a full 5% of the population). Covering the face with scarves, veils and masks became a prevalent (if ineffective) means of warding off the disease in many parts of the world, until the epidemic finally faded at the end of 1919.
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https://qz.com/299003/a-quick-history-of-why-asians-wear-surgical-masks-in-public/