Most cities out of more than 330 monitored saw their annual air quality readings fall below national safety standards last year, the national environmental authority said on Thursday.

Just 21 percent of the 338 cities had air quality that surpassed the national standards — mainly in southern and western regions.

The vast majority of northern cities still faced severe pollution, according to a report by the Ministry of Environmental Protection.

The 73 cities with good air quality were located mainly in Fujian, Guangdong and Yunnan provinces and in the Tibet autonomous region, the report said.

In contrast, seven of the bottom 10 were in the northern province of Hebei, with the city of Baoding ranking last.

In the Beijing-Tianjin-Hebei region, the 13 cities monitored reported that air quality met national standards for 52.4 percent of the year, meaning that for the rest of the time residents had to contend with light or severe smog.

Soaring coal consumption to fuel heating systems during the winter was the main factor in the regional deterioration of air quality, said Luo Yi, head of the ministry’s Environmental Monitoring Bureau.

Smog affected the trilateral region five times in winter, with the concentration levels of PM2.5 increasing by 9.6 percent year-on-year, the ministry said, adding that Baoding saw severe smog for eight consecutive days.

In Beijing, the PM2.5 concentration level soared by 75.9 percent year-on-year from Nov 15, when the heating service started, until the end of the year.

But for the nation as a whole, air quality improved last year and the concentration levels of three major airborne pollutants — sulfur dioxide, nitrogen dioxide and PM10 — fell by at least 6.3 percent, Luo said.

The average concentration levels of PM2.5 and PM10 in the 338 cities monitored fell by more than 11 percent year-on-year, the report said.

For the 74 major cities, 11 reported good air quality, three more than in 2014. Of these cities, air quality better than the national standard was reported on 71.2 percent of the days last year, demonstrating that efforts to control air pollution had worked, the ministry said.

“But in the north, we have more problems when it comes to improving air quality,” Luo said, adding that curbing pollution this year is still a priority for the governments concerned.

The ministry said the Beijing-Tianjin-Hebei region has launched a pilot project involving an emergency response to smog. The project took effect on Thursday and will end on March 31.

Cities in the trilateral region will take the same emergency actions under unified alert levels, the ministry said in a statement.

When smog affected the region in December, different cities adopted different emergency responses to the severe pollution.

The ministry has told cities in the region to make joint efforts to control air pollution, which will be a more efficient system, the statement said. It added that the core cities of Beijing, Tianjin, Tangshan, Baoding, Langfang and Cangzhou will take the lead on this.

After the pilot project, the joint emergency response system will be expanded to more cities, the ministry said.