That could be true due to the natural occurence of the fog.

Fog blamed for Beijing's 'poor' air quality

Heavy smog and fog on Sunday made Beijing the most polluted of 47 cities monitored nationwide, according to the official environmental watchdog.

According to the Beijing municipal environmental protection bureau's website, air quality in Huangcun in the capital's Daxing district and Liangxiang in Fangshan district had already been classified "hazardous" by midday on Sunday.

The US embassy in Beijing also qualified the air near its compound in Chaoyang district as "hazardous" in its own measurements made available on the popular micro-blogging site, Twitter.


http://www.chinadaily.com.cn/china/2...t_11391663.htm