Living with smog

Unlike PM10, or coarse dust particles that can be filtered by the nose and expelled through the respiratory tract, fine particulate matters (PM2.5), often attached with viruses and microbes, can get into the lungs and enter the bloodstream, and then produce acute and long-term health effects. The individual particles are so tiny that they cannot be seen by the naked eye.

In 2013, the World Health Organization's International Agency for Research on Cancer classified air pollution as a leading environmental cause of cancer, placing it in the same category as tobacco, and UV and plutonium radiation.

A variety of other illnesses, including cardiovascular and respiratory diseases, such as stroke and asthma, are also linked to PM2.5 and PM10 exposure.

Xu, who has a 15-month-old son, says masks and air purifiers are essential to his life nowadays. Xu owns a 8,000-yuan ($1,304) imported air purifier.

Postings on how to choose a mask and air purifier are common on social media, and he decided on the particular brand of air purifier after long and thorough research.

He says his son is still so young, and he wants to give the toddler the best protection against air pollution.

A few days ago, when he got fined for violating traffic rules, he pitied the policeman instead of feeling sorry for himself, because it was a hazy day and the policeman was not wearing a mask.

It happened in February, when thick smog and haze persisted for more than a week in Beijing - the longest in duration since the capital started monitoring air quality in early 2012, and the air quality index stayed above 300 points for over a week.

An AQI reading above 200 points indicates the air is very unhealthy for people of all ages. When it goes above 300, it is hazardous, according to the United States Environment Protection Agency.

Purchases of masks in 2013 on Taobao.com, the country's largest online shopping platform, were 2.8 times higher than in 2012, and the purchases of air purifiers increased by 2.3 times.

But not everyone uses a mask correctly.