Particles in the air was the main pollutant, said officials with the Beijing Municipal Bureau of Environment Protection. The city's weather "was bad for the spread of pollutants".

The pollution in June was mainly caused by car exhaust, while farmers burning stalks in neighboring Hebei, Henan, Shandong, Jiangsu and Anhui provinces were also to blame, the Beijing officials said.

There were only 6 days with substandard air quality for the corresponding month last year.

A satellite map showed smoke from these provinces was blown north causing particulate matter to reach 850 milligrams per cubic meter in Beijing on June 11.

In Hebei, environment watchdog identified farmer-set fires in 53 locations on June 11 and at 80 locations on June 12.

Beijing has aimed to have 245 days with "blue sky" this year, but it recorded only 110 days in the first six months.

Beijing issued a ban on burning straw in 2000. An inspection by the environment watchdog found no recent violations of the regulation in the city. However, burning straw is still common practice in rural areas of other provinces.

The Hebei government has also sent an inspection team to stop farmers from burning the chaff after Beijing was chocked.