Some motorcyclists in Zhengzhou, capital of Henan province, wear masks on Wednesday for protection from the smog. The air was heavily polluted in the city. Li Bo / Xinhua

Air quality in May around the country was slightly worse than it was the same period last year, with air quality in the Yangtze River Delta deteriorating the most, according to the Ministry of Environmental Protection.

The ministry released air quality statistics for May for 74 major cities on Wednesday. These showed that the number of days with good air quality for the 25 cities in the Yangtze River Delta region was only 18 out of the total of 31 - five days fewer than in May of last year.

Of the six major pollutants that are included in the calculation of air quality, only the readings for carbon monoxide were unchanged compared with the 2013 figures for the region.

Concentrations of the five others - PM2.5, which is particulate matter that is smaller than 2.5 microns and penetrates the lungs; PM10; sulfur dioxide; nitrogen dioxide; and ozone - all increased.

"Stronger sunshine, warmer temperatures and burning straw within the region are the three major reasons for the area's apparently deteriorated air quality," said an official from the ministry who requested anonymity.

Nanjing, Changzhou and Zhenjiang, all in Jiangsu province in the Yangtze River Delta region, were among the top 10 cities with the worst air quality in May.

This is the first time that three cities at once from the region have made the monthly list. The previous months' lists only saw two cities from the region at most at the same time.

The 13 cities from the Beijing, Tianjin and Hebei cluster had slightly better air quality in May compared with figures from May of last year, with three additional days with good air.

The situation for the nine cities from the Pearl River Delta region was virtually unchanged.

Nationwide, air quality in May grew slightly worse than in April and was worse than May of last year as well.

PM2.5, PM10 and ozone were the top three pollutants, with each accounting for about one-third of high- pollution days.

"The growing concentration of ozone is typical when the temperature gets higher, especially during the period from May to September," said Chai Fahe, deputy head of the Chinese Research Academy of Environmental Sciences.

Chai said pollution caused by PM10 played a significant role in May, partly because of sandstorm weather that is normal in April and May. But he said the concentration of both PM2.5 and PM10 have actually decreased compared with the previous month's figures because of vegetation growth at this time of year.

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