China's commercial hub Shanghai was blanketed by dense smog Friday, delaying flights and spurring sales of face masks.
Levels of PM 2.5 -- tiny particles in the air considered particularly hazardous to health -- rose to more than 600 micrograms per cubic meter in the afternoon, Shanghai's government said on its microblog.
That is 24 times the World Health Organization's (WHO) safety guideline of 25 micrograms, and reports said it was a record since such monitoring began.
Shanghai's air quality is normally far better than that of Beijing and other major cities in the north, but environmental authorities declared the highest warning of "severe pollution," issued when the ambient PM 2.5 concentration reaches 301 or more.
"Shanghai is no longer suitable for living, air pollution is too severe!" one user commented on a microblogging service.
The smog forced the city's two airports to cancel or delay hundreds of flights on Friday, official figures showed. It also boosted sales of face masks in drug stores, state media reported.
Environmental group Greenpeace blamed the smog on emissions from coal-fired power plants in the neighboring provinces of Jiangsu, Anhui and Shandong, it said on a verified microblogging account.
Cities across China have been hit by intense air pollution in recent years, much of it caused by emissions from coal-burning power stations, with PM 2.5 levels reaching as high as 40 times WHO limits.
China's pollution problems are blamed on rapid urbanization, dramatic economic development and climatic factors. Pollution tends to worsen as winter approaches.
Airborne particles have been linked to hundreds of thousands of premature deaths, and the problem has tarnished the image of Chinese cities including Beijing, which saw an almost 15 percent drop in tourist visits during the first half of this year.
China vowed in September to reduce levels of atmospheric pollutants in the capital and other major cities by as much as 25 percent to try to improve their dire air quality.