Beijing warns residents after off-the-charts smog

"Because these dust particles are relatively fine, they can be directly absorbed by the lung's tiny air sacs. The airway's ability to block the fine dust is relatively weak and so bacteria and viruses carried by the dust can directly enter the airway," Huang said.

Huang said exposure to such high levels of pollution over the short term can cause bacterial and viral infections, and prolonged exposure could result in tumors.

Beijing's air started to worsen on Thursday, and Beijing's monitoring center said the pollution was expected to linger until Tuesday.

Weather conditions are a factor, as a lack of wind means pollutants can easily accumulate and fail to dissipate, said Pan Xiao Chuan, a professor at Peking University's public health department.

"Recent pollution doesn't mean there is an increase in the discharge of pollutants," he said.

The government started publishing PM2.5 readings last year after public demands for more detailed air quality data, prompted in part by a Twitter feed from the U.S. Embassy that reported readings from the building's roof. A growing Chinese middle class has become increasingly vocal about the quality of the environment. Hourly air quality updates are now available online for more than 70 cities.

Air pollution is a major problem in China due to the country's rapid pace of industrialization, reliance on coal power, explosive growth in car ownership and disregard for environmental laws. It typically gets worse in the winter because of weather conditions and an increase in coal burning for heating needs.

Several other cities, including Tianjin on the coast east of Beijing and southern China's Wuhan city, also reported severe pollution over the last several days.