Dense clouds enveloped the Chinese capital where the air quality index of PM10 – particles of pollution that are less than 10 micrometers in diameter and can enter through the nose and travel to the lungs – have exceeded monitoring charts, according to the Beijing Ecological Environment Monitoring Center.

Weather authorities in Beijing warned people not to go outdoors for exercise and other activities, and have asked drivers to stay vigilant and reduce speed because of low visibility.

Concentrations of PM10 particles hit 1,667 micrograms per cubic meter by 6 a.m. local time, according to Beijing’s monitoring center on Wednesday, with the agency calling it “the most severe sandstorm to date this year.”

That figure is more than 37 times the daily average guideline of 45 micrograms per cubic meter set by the World Health Organization.

Beijing is regularly hit with sandstorms in the spring with the smog made worse by rising industrial activities and rapid deforestation throughout northern China.

Nearly a dozen provinces issued yellow warning signals from Wednesday to early morning Thursday, including Shaanxi, Shanxi, Hebei, Heilongjiang, Jilin, Liaoning, Shandong, Henan, Jiangsu, Anhui and Hubei, according to China’s Meteorological Administration.

China uses a four-tier weather warning system, with red representing the most severe warning, followed by orange, yellow and blue.

The current sandstorm originated from Mongolia on Tuesday and gradually moved towards central and eastern China, according to Chinese weather forecasters. A lack of rainfall and low pressure winds also meant sandy particles have also drifted into the country.