Air pollution went off the charts as a thick gray smog enveloped the city of Shenyang in northeast China's Liaoning Province on Sunday, according to local environmental authorities.

Fine particulate matter known as PM2.5, which is considered particularly hazardous, exceeded the maximum reading of 500 micrograms per cubic meter in some areas, while the air-quality index rose as high as 350 micrograms per cubic meter in some monitoring sites.

The provincial meteorological disaster warning center has issued a yellow alert for smog. The country has a four-tier warning system for extreme weather, with red being the most serious, followed by orange, yellow and blue.