Citizens of a northeastern Chinese city are concerned for their health in the wake of the "airpocalypse" of choking smog that is slated to worsen with winter rapidly approaching.

This week, Harbin has been cloaked in a gray haze that's being called "PM2.5" - the name for tiny, invisible particulate matter that scientists declared a cancer-causing hazard in a report published by the World Health Organization's International Agency for Research on Cancer days before reports of the haze surfaced.

"The air we breathe has become polluted with a mixture of cancer-causing substances," Head of the IARC Monographs Section Dr. Kurt Straif wrote in a press release obtained by the Guardian. "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."

PM2.5 reached levels of 1,000 micrograms per cubic meter in Harbin, surpassing WHO's suggested safety levels 40 times over. The city essentially closed down as a wall of grey haze settled, causing closure of public schools, while local airports and buses suspended service. Photos and video from Harbin depicted harrowed citizens moving through the thick fog, most of them donning face masks and scarves to shield themselves from the dirty air.

As winter creeps closer and the city prepares to turn on its largely coal-powered central heating system, citizens are nervous that the air quality will only worsen. According to CRI English, local farmers' tradition of burning crop stubble is also believed to add to the thick smog, which has already been linked to widespread respiratory illnesses in major Chinese cities. Medical experts who spoke with state-run news agency Xinhua warned there would surely be a spike in the number of people seeking out medical attention.

"The impact of air pollution on people will be gradual," doctor at the Second Affiliated Hospital of Harbin Medical University Deng Ying told Xinhua. "There won't be a sudden outbreak of symptoms, but normally, three to five days after the smoggy weather occurs, there is a peak in the number of people seeing doctors."

Some of Beijing, Tianjin and other areas of northern China have installed an "air pollution coordination control mechanism," which facilitates a reduction in coal use - a push organized by the State Council, which met in September to discuss scaling back on pollutant levels in the next few years, the New York Times reported.

Chinese social media users on Twitter-like site Weibo shared their grievances concerning the choking smog. User Wei Bang Zhu wrote on Tuesday that "someone should take responsibility for the smog...the price of pursuing high-speed development is that people end up being fed with smog. To venture out in an environment like this would be equivalent to teasing about one's own health and life."

"Too horrible, it's like the end of the world in American movies," another user, Justop88, posted.