Frequent severe smog caused more than half of China's 74 major cities to fail national air quality standards for about two weeks in December, the national environment authority reported on Monday.

Intense smog blanketed the Beijing-Tianjin-Hebei region on five occasions, making it the most polluted cluster in December, though Baoding and Hengshui cities had eight smoggy days in a row, said Luo Yi, head of the environmental monitoring bureau at the Ministry of Environmental Protection.

A statement from the ministry also noted that all 13 cities in the Beijing-Tianjin-Hebei region experienced soaring concentrations of PM2.5, harmful particulate matter with diameter less than 2.5 microns, in December.

In Beijing, the?air quality reached hazardous levels on 13 days, and PM2.5 readings increased by 166.7 percent year-on-year, Luo said.

The severe smog covering the vast northern and northeastern regions dragged more cities to the bottom of air quality lists in December, including seven cities in Hebei province, Zhengzhou in Henan province, Harbin in Heilongjiang province and Jinan in Shandong province.

Baoding, in Hebei province, occupied the bottom position.

Soaring coal consumption for heating, especially in rural regions, and the windless weather contributed significantly to the frequent smog, Luo said.

Elsewhere in China, the cities in the Yangtze River Delta region, including Shanghai, also experienced poor air quality. Just over half the days in December had air quality higher than the national healthy standards.