BEIJING - Air quality slightly dropped in China's major cities in May compared with the previous month, the environmental watchdog announced on Wednesday.

Air quality in 74 monitored cities met the required standards set by the Ministry of Environmental Protection in 60.1 percent, or 18.63, of the 31 days in May. This is down 3.4 percentage points from the previous month, the ministry said in a report.

These cities had on average 0.68 heavily polluted day, or 2.2 percent of the days last month, up 0.2 percentage point from April, according to the ministry.

Pollution in the Chinese capital Beijing and its neighboring Tianjin municipality and Hebei Province was heavier than other parts of the country, with only 27.4 percent of days meeting the standard.

Compared with the previous month, the concentration of pollutants rose and air quality dropped in these areas in May because of meteorologic conditions, making pollutants hard to diffuse, according to the report.

The ministry quoted PM2.5 - airborne particles measuring less than 2.5 microns in diameter that can pose health risks - and O3 as the major pollutants in these areas.

South China's Pearl River Delta had more clear days than other parts of the country with nearly 90 percent meeting the standard, according to the report.

China has about 496 air quality observation points in 74 cities in 2012.