The haze in China’s northern and central regions remained on Sunday. Beijing authorities warned against outdoor activity and businesses reduced emissions as air quality hit one of the worst levels on record in more than a decade.

The Beijing Observatory issued the first orange alert for smog in the city’s history on Sunday.

The density of PM2.5 particulates surpassed 700 micrograms per cubic meter in many parts of the city, according to the Beijing Municipal Environmental Monitoring Center.

The World Health Organization considers a safe daily level to be 25 micrograms per cubic meter.

PM2.5 are tiny particulates that can penetrate deep into the lungs. Measuring them is considered a more accurate reflection of air quality than other methods.

Beijing authorities recommended that children and old people should stay indoors, and others should avoid outdoor activities.

As Beijing's air started to worsen on Thursday, 54 Beijing enterprises reduced their emissions by 30 percent, and 28 construction sites have stopped earth-work so far, the official Xinhua News Agency reported on Sunday.

A common source of PM2.5 is from the burning of fuels in vehicles and power plants.

Hyundai Motor Group (China) Ltd., a Chinese subsidiary of South Korea's Hyundai Motor Group, stopped production for a day on Sunday, Xinhua said.

Pan Xiao Chuan, a professor at Peking University's public health department, told the Associated Press that pollutants can easily accumulate and fail to dissipate with lacking wind in recent weather.

"Recent pollution doesn't mean there is an increase in the discharge of pollutants," he was quoted as saying.

Air pollution is a major problem in China due to the country’s rapid pace of industrialization.
The Beijing monitoring center has said the pollution is expected to linger until Tuesday.

Other Chinese cities, including Tianjin, Wuhan and Chengdu, have also reported severe air pollution over the last few days.