Central Beijing sits under a thick layer of smog Thursday, as a sandstorm swept north east China.

Story highlights

A large sandstorm has covered north east China in a thick smog

Levels of pollution reach beyond the index, residents warned to remain indoors

Beijing CNN —  

Beijing’s air quality reached hazardous levels Thursday, as a large sandstorm swept through the Chinese capital, turning the sky a murky yellow and forcing many residents to keep indoors.

Sandstorms are common across north east Asia during the spring, with prevailing winds carrying sand and dust particles from the Gobi Desert across the region.

But the speed with which the storm set in astounded locals – between 3 a.m. and 6 a.m. Thursday, the city’s air quality index (AQI) jumped from under 100 to over 500 – from “moderate” to a rating of “beyond index.”

By midday, the AQI level had hit a peak of 621, with PM2.5 levels of 684 micrograms per cubic meter.

The WHO’s recommended level of PM2.5 – the smallest and most harmful pollution particles – is 25 micrograms per cubic meter.

Authorities in Beijing warned residents to avoid outdoor activity.

Many residents took to social media to express their dismay:

A cold front is set to move across the region Friday, which will help to clear the air, according to CNN meteorologist Derek Van Dam.

Until then, dust and smog particulates will continue to affect air quality and reduce visibility across Beijing, Gansu, Hebei, Heilongjiang, Jilin, Shaanxi and Shanxi provinces, and the autonomous regions of Inner Mongolia, Ningxia and Xinjian, he said.

A toxic problem

While Thursday’s choking skies were attributed to sandstorms from Mongolia, China has a wider issue with air pollution.

Research by Nanjing University’s School of the Environment has linked smog with nearly one-third of all deaths in China, positioning it on a par with smoking as a threat to public health.

Published in November last year, the study analyzed over 3 million deaths across 74 cities throughout China in 2013.

The findings revealed that as many as 31.8% of all recorded deaths could be linked to pollution, with major cities in Hebei, the province that encircles Beijing, ranked among the worst.

China is the world’s largest emitter of greenhouse gasses, and it’s costly – the country’s economy lost roughly $535 billion due to pollution in 2012, according to the RAND corporation. The government publicly declared a “war on pollution” in 2014.

CNN’s Derek Van Dam, Joshua Berlinger and Steve George contributed to this report.