雾霾覆盖中国北部地区

Heavy smog continues to cover vast areas of China. Air quality index readings are at alarmingly unhealthy levels. The country's Central weather authorities have issued two orange alerts -- the second-highest warning -- for heavy smog and fog in northern China. Data show about 1-point-4 million square kilometers of land are covered by haze. Authorities say this is the most severe, and longest-lasting, smog this year. It also covers the widest area of the country.

Over 20 cities have issued red alerts for smog, while 50 cities have issued orange alerts. The PM2.5 reading in many cities in northern China, mainly in the Hebei and Henan provinces, exceeds 500 micrograms per cubic meter. The reading at one monitoring station in Shijiazhuang, the capital of Hebei province, exceeded a thousand micrograms per cubic meter. Visibility has dipped below 500 meters in Beijing.

Thick smog and extremely poor visibility obscures landmarks in the city. In north China's Tianjin, airport operations have been suspended, leading to 131 flights cancellations and 75 delays. All these cities have placed strict restrictions on industrial emissions and the use of vehicles.