Chinese cities have reported fewer days with "good air" in the first half (H1) of 2017, with the Beijing-Tianjin-Hebei area suffering from higher density of key pollutants, official data showed.

The 338 cities monitored by the Ministry of Environmental Protection (MEP) enjoyed good or excellent air quality on 74.1 percent of the days in H1, down 2.6 percentage points year on year, according to Liu Zhiquan, head of the environmental monitoring department at the MEP.

During the same period, the density of hazardous fine particle matter PM2.5 remained unchanged from a year ago while the density of PM10 was down 2.2 percent.

Air quality in 13 cities in the Beijing-Tianjin-Hebei area was rated good or excellent on 50.7 percent of the days in H1, down 7.1 percentage points year on year. In June, the ratio was only 34.1 percent, 14.2 percentage points lower than a year ago.

The density of PM2.5 and PM10 in these regions also climbed 14.3 percent and 13.2 percent, respectively, indicating worsened air quality.

Of the country's 74 major cities, Haikou in southern China's Hainan Province had the best air quality while Handan in northern China's Hebei Province was the most polluted, MEP data showed.

The worsened air quality came despite the government's strengthened oversight of environmental violations.

In H1, China investigated and dealt with 17,169 violations of environmental protection laws and regulations, previous MEP data showed.

Over 610 million yuan (about 90.44 million US dollars) in fines were issued in 503 cases where violators must pay daily fines until they address their wrongdoings, up 131 percent and 64 percent year on year respectively.