Air quality nationwide in July was better than in June, according to air quality readings of 74 cities released by the Environmental Protection Ministry on Friday.

Compared with June, more than half of the cities enjoyed good air quality for at least 21 days in July. The concentration of major pollutants was also lower.

However, six cities in Hebei province remained on China's top 10 worst polluted cities list from June.

Air quality in Beijing, Tianjin and Hebei province was better in July, with the ratio of good air quality days rising from 24.2 percent in June to 36.5 percent in July. The main pollutant for these regions was PM2.5, fine particles smaller than 2.5 micrometers in diameter that can invade even the smallest airways.

Nationwide, more than 70 percent of bad pollution days were mainly caused by ozone. Ozone pollution began to worsen as major cities moved from North China to the South.

In the Yangtze Delta region, 98 percent of days with high pollution in July were caused by ozone, which dragged down the region’s total quality rate. The number of days with good air quality was reduced by almost 40 percent in July compared with June.

Readings from the Pearl River Delta region further highlighted ozone pollution in the region. Ozone was the key pollutant for all the bad-air days. But the region’s total quality rate increased slightly, from 88.5 percent in June to 90.3 percent in July.