BEIJING, May 27 (Xinhua) -- Air quality in the Beijing-Tianjin-Hebei region improved in the first four months of 2018 compared with the same period of last year, according to the Ministry of Ecology and Environment.

Good air days accounted for an average of 59.1 percent of total days in the 13 cities in the region, up 2.2 percentage points year on year, the ministry data showed.

The density of PM 2.5, a key indicator of air pollution, dropped 18.8 percent year on year during the period, while the density of PM 10, another major pollutant, fell 12.9 percent.

Beijing saw the ratio of good air quality days increase by 3.9 percentage points to 64.7 percent in the period, with PM 2.5 and PM 10 density down 22.4 percent and 9.9 percent, respectively.

Nationwide, 338 cities had an average of 76.7 percent good air quality days, up 0.9 percentage points year on year.

However, air quality in April alone worsened in the Beijing-Tianjin-Hebei region. The average proportion of days with good air quality was 58 percent, down 13.7 percentage points from April last year, with lower PM 2.5 density but higher PM 10 density.

A ministry statement attributed the decrease from the previous month to frequent sandy and dusty days, increased industrial and construction activities, and static weather patterns, which make it difficult for airborne pollutants to disperse.

Tackling pollution has been listed by the government as one of "the three tough battles" that China aims to win in the coming three years.