This combo shows the Xuanwu Lake under heavy fog in Nanjing, east China's Jiangsu province, and the same view taken under non-polluted weather on March 31, 2012. —Photo by AFP

The Pudong business district is shrouded by heavy air pollution in Shanghai. Several days of hazardous air quality across large swathes of northern China triggered an emergency response which saw schoolchildren kept indoors, factories closed and government cars parked. —Photo by AFP

Heavy smoke coming out from chimneys in the Wangjing community, Beijing. Shares of a Chinese face mask manufacturer soared as investors looked for opportunities to cash in on the severe air pollution. —Photo by AFP

Pigeons fly over the courtyard of a neighbourhood on a hazy day in central Beijing, China. One of Beijing's worst rounds of air pollution kept school children indoors and sent coughing residents to hospitals. —Photo by AP

A nurse puts in a drip for a baby diagnosed with respiratory diseases at a provincial children's hospital in Hefei, Anhui province. —Photo by Reuters

Picture shows a row of intravenous drips for flu treatments in Beijing. —Photo by AFP

A man covering his mouth with his hand as he rides amidst the heavy fog in Hefei, central China's Anhui province. —Photo by AFP

A woman wears a mask as she does her morning exercise outdoors in Fuyang, Anhui province. China's environmental watchdog ordered greater efforts to issue early warnings for air quality on Monday, as hazardous air pollution has hit many parts of the country in recent days, Xinhua News Agency reported. —Photo by Reuters

A cyclist wearing a mask prepares to cross a road in the severe pollution of Beijing. —Photo by AFP

Chinese artist dissident Ai Weiwei poses with a gas mask in protest of the bad air quality in Beijing, in this handout photo which was provided to Reuters. —Photo by Reuters