Air quality in 74 major cities was deemed unsafe for nearly half of the days in the first six months of this year, with the area comprising Beijing Tianjin-Hebei area found to be the most polluted region, the Ministry of Environmental Protection said on its website yesterday. In an interview with People's Daily yesterday, Environmental Minister Zhou Shengxian said that air pollution had become so severe that the country could "no longer afford further delays in its cleanup efforts". During the first half of the year, air quality was rated "severely polluted" in Beijing-Tianjin-Hebei for 26 per cent of days, and the primary cause was PM2.5 - tiny airborne particles smaller than 2.5 microns in diameter. Only 31 per cent of days in the region saw air quality on par with national standards, according to the ministry. And seven cities in the region ranked among the nation's 10 most polluted cities in that period. Beijing, notorious for its thick smog, had "good" air quality on about 40 per cent of days in the first half of the year, according to officials, while the air quality in Shijiazhuang , the provincial capital of Hebei , was within national safety standards only 10 per cent of the time. The average concentration of PM2.5 in Shijiazhuang was 172 microns per cubic metres - nearly five times the level considered safe. Among the mainland's major city clusters, the Pearl River Delta region had the best air quality, according to official figures, with 80 per cent of days registering "good" air quality. The ministry statement also for the first time talked about ground-level ozone pollution, which it called a major problem that had yet to catch the public's attention to same degree as PM2.5 levels. Ozone pollution is a powerful respiratory irritant that can be harmful to people's lungs, especially children and the elderly. It can also make smog worse. It is formed by a reaction between volatile organic compounds - mainly from vehicles, chemical plants and refineries - and nitrogen oxides. Zhou told People's Daily that the State Council had approved 35 measures to improve air quality, following a 10-point plan released in June, but details of the plan remain unclear. He also vowed that there would be harsher penalties for polluters, and that investment would be increased in clean-up programmes, while local governments will be held responsible for failing to meet air quality targets. The war on air pollution has been deemed a top priority by the new leadership amid mounting public discontent, but progress has been slow as the central government's desire to cut coal consumption and production capacity in energy-intensive sectors have been met with strong opposition from local authorities who fearing an economic slowdown.