BEIJING, May 16 (Xinhua) -- Cities in the Beijing-Tianjin-Hebei region saw more days with "good" air quality in April, official data showed Tuesday.

An average daily Air Quality Index (AQI) of under 100 is defined as good under Chinese air quality standards.

The 13 cities in the region reported good air quality 23 days on average, up 16.5 percent year on year, according to the data released by the Ministry of Environmental Protection (MEP).

Beijing saw 22 days with good air quality in April, up from 18 days in the same month last year, a year-on-year increase of 20 percent. The average density of PM 2.5, one major air pollutant, fell by 22.1 percent year on year.

Air quality improved in China's 338 prefecture-level-and-above cities in April, with the average density of PM 2.5 dropping by 4.8 percent year on year.

Hebei's Handan, Shanxi's Taiyuan and Hebei's Tangshan recorded the worst air quality in April among the 74 cities nationwide monitored by the MEP.

Cities in the Yangtze River and the Pearl River deltas reported worsening air quality in April.