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.