BEIJING -- The Chinese Research Academy of Environmental Sciences (CRAES) Friday released a list of air pollutants in Beijing, Tianjin, Hebei and four nearby areas, a region worst hit by smog.

"The seven air pollutants in the region, mostly in north China, are PM10, PM2.5, sulfur dioxide, oxynitride, volatile organic compound, ammonia and nitric oxide," said Chai Fahe, chief scientist of CRAES. "The source of pollution comes from burning, industrial production, vehicles, dust, human activity, agriculture and natural plants."

"The discharge of major air pollutants needs to be cut by 40 to 80 percent compared with the 2015 level to meet the national standard, which put the PM2.5 density at 35 micrograms per cubic meter.

"The steel, concrete and petrochemical industries have the greatest potential for pollutant reduction.

"In the long term, the steel output in Beijing, Tianjin and Hebei should be controlled within 200 million tonnes per year to meet the target."

Last year, the average PM2.5 level in the region was 64 micrograms per cubic meter, down 9.9 percent on 2016.