A visitor walks past the Bird's Nest, the main stadium for the 2008 Olympics, in thick fog on Sunday. [Photo by Wang Jing/China Daily]

BEIJING - The capital's meteorological bureau issued its first heavy fog alert of the winter on Sunday, as low visibility forced the closure of six highways and caused delays or cancellations to more than 200 flights to or from the city.

But although it is called a "fog alert", Ma Jun, director of the Institute of Public and Environment Affairs, said the hazes that have been smothering Beijing are really "smog".

"Fog is the liquefaction of vapor in the air, which should be transparent in color, while the dim haze we are suffering is smog, the combination of pollutants and fog," Ma explained.

Lu Bin, a CITIC Trust employee who was at the airport to board a flight to Kunming, told China Daily he bought face masks in the supermarket "to protect himself from the heavy pollution".

"The foggy days were not so frequent in the past and the sky was not as dim," said the Beijing native.

According to the Air Pollution Index released by the Ministry of Environmental Protection, which monitors the air quality of 120 key cities and municipalities, the capital topped the list of the worst polluted cities across China on Sunday.

S. Ming Sung, chief representative for Asia-Pacific at the Clean Air Task Force, a United States-based nonprofit organization dedicated to reducing atmospheric pollution, told China Daily that the firing up of the city's heating and the large number of vehicles on the roads have contributed to the heavy smog.

Many residents in Beijing are concerned about the haze.

Xu Ning, a mother of a 3-year-old boy in Beijing, said she canceled her son's swimming class "because of the smog".

People with respiratory diseases, children and the elderly should try to stay indoors during smoggy days, said Pan Xiaochuan, a professor with the School of Public Health at Peking University.

Not only Beijing is suffering foggy days, the National Meteorological Center of China issued a heavy fog alert on Sunday for Tianjin municipality, Hebei province, Henan province, Shandong province, Anhui province and Jiangsu province, warning that some areas may have visibility of only 200 meters and suggesting people in fog-affected areas cancel outdoor activities due to the poor air quality.

However, according to the center snow and rain will bring an end to the foggy days in the coming week.