BEIJING, Jan. 25 -- Air quality in major Chinese cities in north and east China dropped significantly last month due to heating in winter and adverse weather, the Ministry of Environmental Protection (MEP) said Monday.

The ministry issued air quality figures on Monday for 74 major Chinese cities in December, saying that in the three key areas of the Yangtze River Delta, Pearl River Delta and the combined Beijing, Tianjin and Hebei region, only the Pearl River Delta saw better air quality.

Air quality in the other two regions worsened and PM2.5, a major pollutant, increased significantly compared to the same period of 2014, the ministry said.

Luo Yi, head of the department of environmental monitoring of the MEP, said the drop was caused by heating in winter and adverse weather, which led to five rounds of poor air quality in north China.

The PM2.5 indexes of Beijing, Tianjin and Hebei Province climbed in December compared to the previous year. For instance, Beijing's PM2.5 index increased by 166.7 percent.

The air quality of seven cities in south China, including Xiamen, Fuzhou, Kunming, Huizhou, Jiangmen, Haikou and Shenzhen, were the best in China and reached air quality standards on every day of December.

Baoding, Xingtai, Hengshui, Handan, Shijiazhuang, Langfang and Tangshan, all in Hebei Province, plus Jinan, Zhengzhou and Harbin, were the top 10 cities for worst air quality in December.