Chen, 35, is a sporty Beijing-based designer and writer who has been cycling intensively since 2008. He regularly wears masks when he trains, picking several brands based on their function.

"My masks have to be professional,” he said.

“Respro is best for cycling, because it can filter both PM2.5 [particles] and vehicle exhaust. Totobobo is breathable and light so it’s good for running; 3M9010 is good for outdoor family activities,” he said.

Like most people in China, Chen used to wear masks only when outdoors, but after learning about the effects of PM2.5 he did research and invested in masks that could offer more protection.

PM, short for particulate matter, is the term for particles found in the air including dust, dirt, soot, smoke and liquid droplets. According to the US Environmental Protection Agency (EPA), particles smaller than 2.5 micrometres in diameter, PM2.5, can be inhaled and absorbed into the gas exchange regions of the lung, endangering the respiratory system.

According to EPA guidelines, air quality is considered unhealthy if the average concentration of the PM2.5 particles is more than 100 micrograms per cubic metre.

The poor air quality has led to huge sales of face masks. Taobao.com, China’s largest online retailer, has sold hundreds of thousands of masks since last year, most of which are designed to protect against PM2.5.

“3M8210 are our best sellers. Around this [year's] Spring Festival, we had over 10,000 masks sold within weeks,” said Xiao Lu, a saleswoman at Panfeng Househould Products.

Panfeng is a leading online retailer on Tmall.com, a spin-off from Taobao.com. She said she had noticed that fewer masks were sold during the summer months.

“Only a few thousands masks are sold each month,” she said. “Obviously, the air becomes better compared to this winter.”

She has also found that the fashion aspect matters to customers, too.

“Young people tend to like bright colours. Men prefer blue or black masks. Right now, UV proof masks are popular.”

The most important factor for customers, she said, is whether the mask is practical and comfortable. Some invest in masks with the most cutting-edge technology, such as ones with activated carbon.

Price matters too, with the majority of customers opting to buy the cheapest masks. For example, on Taobao.com one 3M8210 mask costs on average about 2 yuan (HK$2.50).

“I think in the end, only practical masks will last a long time,” said Wu Wenxi, 28, a private seller of a popular brand of activated carbon masks, which she sells for 0.5 yuan each.

She said her shop had sold N95 masks since 2003, when Sars hit China. “When Sars went away, sales of masks fell,” she said. “Now more types of masks have come out because of PM2.5.”

Every day she checks the PM2.5 report to decide whether she should wear a mask that day. “You’ve got to wear one if the PM2.5 number is above 200,” she said.

Sometimes PM2.5 reports from different sources can be confusing, she said. “Foreign sources tend to report higher PM2.5 numbers compared to domestic sources. I feel safer referring to a higher number and wear a mask.”

Some people refuse to wear cheap masks. Rena, 29, spends 200 yuan on her Totobobo mask, 100 times more than a popular activated carbon mask.

An Uygur girl from Urumqi, Xinjiang province, Rena has enjoyed living and working in Beijing for the last nine years in a white-collar job.

She admitted she was concerned about the air quality. “Going back to Urumqi means less job opportunities and the air is not necessarily better,” she said. “Staying in Beijing means wearing a mask most days. It’s not very comfortable.”

Worse still, many people can be taken aback by the shape of the Totobobo mask.

“It’s like having fish gills on my face. I can see why people give me strange looks,” she said. Her solution is to put a normal medical mask on top of her other mask.

“But I can’t cover my face forever,” she said. “I’d prefer to live in a cleaner environment.”

Like Rena, mask enthusiast Chen is making plans for the future, too. Although he has several professional masks that are appropriate for most types of weather, he dreams of mask-free days, perhaps in another place.

“Europe could be an ideal place to live,” he said.