A thick smog engulfed the Chinese city of Shenyang on Sunday and Monday, sending air pollution levels 50 times above what's deemed safe by the World Health Organization (WHO) and marking the highest pollution on record since the country began monitoring air quality in 2013, according to the Associated Press.

The smog grounded flights, closed highways, and prompted officials to tell residents to stay indoors. Visibility in the northeastern industrial city of 5 million was just a few dozen feet, according to the New York Times.

The smog is particularly dangerous because of its high concentration of particulates measuring less than 2.5 microns (PM2.5) in diameter, which can be inhaled deeply into the lungs and absorbed into the bloodstream. PM2.5 pollution can contribute to a host of health problems, such as heart disease, stroke, emphysema, and lung cancer.

Fairyland or doomsday? Heaviest #smog this year shrouds NE Chinese city of Shenyang, PM 2.5 hit over 1400 pic.twitter.com/GLo82Abk4R — China Xinhua News (@XHNews) November 9, 2015

A report by Berkeley Earth, a research organization focusing on climate change and emissions, estimated that air pollution in China contributes to the deaths of an estimated 1.6 million people every year.

While the WHO recommends a safe level of PM2.5 to be no more than 25 micrograms per cubic meter over 24 hours, levels in the city of Shenyang had reached up to 1,400 micrograms on Sunday, according to China's official news agency Xinhua.

"As far as we are aware from the data we have been observing over the past few years, this is the highest ever PM2.5 level recording" in the country, Greenpeace campaigner Dong Liansai told AFP.

Last month, the organization said 80 percent of Chinese cities exceeded the national standard on PM2.5 — 35 micrograms per cubic meter — while levels in 367 Chinese cities were more than four times more stringent WHO guidelines.

As popular discontent continues to grow, the problem of air pollution is becoming a thorn in the side of the ruling Communist party, which is slowly moving to make amends. In August, China's upper legislative body passed a law that will restrict several sources of smog and make information about environmental conditions more readily available to the public.

Xinhua, which has downplayed air pollution problems in the past, also appears to be changing its tune.

Two years ago, when the country was suffering from severe smog, the state-run China Central Television (CCT) released a list outlining five "surprising benefits" of smog, including its ability to "unify" the Chinese people and make the country "more equal."

And as recently as March of this year, the popular government censored the release of a Chinese documentary "Under the Dome" about air pollution, made by a former CCT reporter. That same month, Premier Li Keqiang said the government had failed to respond adequately to public concerns over pollution, citing tension over the documentary as an example.

On Monday, state news outlets were following the story closely, with CCT reporting that experts "suggest children, the elderly, and people with respiratory issues to stay indoor." Xinhua labeled the event as the "heaviest smog this year," and as the day grew to a close, their reporting reflected the severity of the situation: "Patients with respiratory ailments jammed local hospitals. Face masks were sold out."

While China suffers from ongoing air-quality concerns, pollution tends to be worse in the winter, due to the country's reliance on coal as a primary source of power.

Coal accounts for an estimated 66 percent of China's energy consumption, with the latest official statistics estimating that the country consumed more than four billion tons of coal in 2012.