The Beijing-Tianjin-Hebei area had a monthly average of 59.1% of good air days from January to April, a year-on-year rise of 2.2%, the Ministry of Ecology and Environment said on May 23.

A decline of densities of PM2.5 and PM10 was apparent from the same period last year.

The ministry said that in the first four months of this year, 76.7% of the air days in the 338 cities monitored nationally were ranked as good, a year-on-year increase of 0.9 percent, while densities of PM2.5 and PM10 decreased compared with last year.

The 10 cities with the worst air quality in April were Xingtai and Shijiazhuang, Hebei province; Xian, Shaanxi province; Handan, Tangshan and Baoding, Hebei province; Taiyuan, Shanxi province; Zhengzhou, Henan province; Xuzhou, Jiangsu province; and Changzhou, Hebei province .

The Beijing-Tianjin-Hebei area had a percentage of 58 of good air days in April, down 13.7 percent year-on-year. The average PM2.5 density saw a 5.5 percent decrease, but PM10 increased 1.7 percent from the same period last year.

The nation's air quality generally improved in the first four months but some regions witnessed decreases in air quality for the following reasons: first, poor meteorological conditions. The higher temperature of April and less rainfall led to a steady-state environment which slows dispersion of pollution.

Second, an increase of industrial production and construction activities resulted in a significant emissions increase. Lastly, frequent storms in April caused PM10 density to increase dramatically in some regions and cities due to dust and sand.