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.