Many different masks are available, such as medical masks, activated carbon masks and industrial masks, in drugstores, supermarkets and online for "PM2.5 protection."
In some online stores, however, vendors boast that masks costing from 3 to 20 yuan stop PM2.5 particles. None, however, have the certificates that could help prove their claims.
"Right now there is no quality standard or guideline on PM2.5 protection masks and most so-called protection masks just use ordinary materials without proof of their effects," said Zhao Danqing from Shanghai Sinotextiles Co, which makes masks.
"With the public awareness of air pollution, making masks is such a profitable business. The government must issue a quality guideline to regulate the market and ensure public health," said Zhao.
Activated carbon masks and ordinary medical masks can't stop PM2.5 particles, while activated carbon itself produces particles, doctors warned.
Some industrial masks can block PM2.5, but they are not for daily use.
They may cause adverse reactions in people with heart disease or respiratory illness because they can limit air intake, experts said.