Ozone and particulate matters worsened air quality in the Pearl River Delta region in October, making it the most polluted area in the country, the Environmental Protection Ministry reported on its website on Tuesday.

In the Pearl River Delta region, only 22.2 percent of days in October had good air quality. The number in September was 77.6 percent.

Though there was no severe pollution in October, all nine cities in the region had good air quality for fewer than 15 days, according to the report.

Ozone remained the top pollutant, accounting for 20 days of the 24 polluted days. The average level of ozone in the region rose by almost 40 percent compared with the level in September.

Particulate matter also significantly contributed to the region’s heavy pollution, with the level of PM2.5 — particulate matter smaller than 2.5 microns in diameter that can penetrate deep into the lungs — rising by almost 90 percent and the PM10 level rising by more than 70 percent compared with the previous month.

The ministry attributed the cause of the severe pollution in the region to the shortage of precipitation and the increase of solar radiation intensity in October.

Nationwide, almost half of the 74 cities mentioned in the report had good air quality in fewer than 15 days in October. The number of such cities was only 19 in September.

Levels of all types of pollutants, except for ozone, rose in October due to the change of season from autumn to winter, the report said.

Aside from the Pearl River Delta region, air quality in the Yangtze River Delta region and the Beijing, Tianjin and Hebei province cluster also worsened.

The major pollutant in the Yangtze River Delta region shifted from ozone in September to PM2.5 in October.

Six cities in Hebei province ranked in the top 10 most polluted cities. But Zhangjiakou in the province ranked ninth in the list of cities with the best air quality in October, together with Lhasa and eight other cities along the coast.

This is not the first time Zhangjiakou, northwest of Beijing, made the list of the top 10 least polluted cities. The city also made the list in March.

"The location and relatively high altitude of Zhangjiakou are not geographical conditions that trap smog for long periods," said Chang Chunping, associate professor of the School of Resources and Environmental Science at Hebei Normal University.

"The city also usually has strong wind, which is good for diffusing poor air conditions," said Chang, who used to conduct fieldwork in Zhangjiakou.