世界卫生组织的国际癌症研究机构将大气污染归为致癌物。据称，有“充分证据”证实，暴露在大气污染下的户外会导致肺癌，而且还会有增加患膀胱癌的风险。

Cityscapes shrouded in thick smog have become a common scene in China. Last winter, Beijing's 'airpocalypse' garnered headlines worldwide and generated much anger and debate within China. But this week, air pollution levels in the northeastern city of Harbin surpassed the previous record levels in Beijing.

The city was essentially shut down after PM2.5, fine particulate pollution（粒状物污染） that is considered hazardous, reached levels of 1,000 micrograms per cubic metre – 40 times the safety level recommended by the World Health Organisation. Schools, motorways and an airport were closed on Tuesday as visibility in some areas of the city dropped to less than 10 metres.

Photos from Harbin showed residents covering their mouths with masks and scarves, moving like ghostly shadows through the fog. Cars and motorcycles moving slowly as traffic came to a standstill with traffic lights barely visible.

Just days previously, the World Health Organisation's International Agency for Research on Cancer (IARC) classified air pollution as carcinogenic. It stated that there is "sufficient evidence" that exposure to outdoor air pollution causes lung cancer and also linked it with an increased risk of bladder cancer. It said that exposure has increased significantly particularly in "rapidly industrial countries with large populations" such as China.

"The air we breath has become polluted with a mixture of cancer-causing substances", Dr Kurt Straif, Head of the IARC Monographs Section said in a press released. "We now know that outdoor air pollution is not only a major risk to health in general, but also a leading environmental cause of cancer deaths."

These findings, while they show the gravity of China's air pollution problem, do not come as a shock to Chinese residents. Increasingly the health implications of pollution have been discussed online in local media.