Buildings are shrouded by fog in Changsha, capital of central China's Hunan Province, Jan. 27, 2013. Local meteorological observatory issued a yellow alert for heavy fog on Sunday. [Photo: Xinhua]

China issued a blue-coded alert on Sunday as foggy weather forecast for the coming two days will cut visibility and worsen air pollution in some central and eastern Chinese cities.

The alert represents the least serious level, after red, orange and yellow in China's four-tier color-coded weather warning system.

The National Meteorological Center (NMC) said visibility in provinces including Hebei, Henan, Shandong, Jiangsu, Anhui and Zhejiang will be cut to less than 1,000 meters on Sunday. In some cities, where fog will be thicker, visibility will fall below 200 meters.

The NMC warned of a decline in air quality and advised residents in these regions to stay indoor or take precautions.

Meanwhile, a strong cold front will cause snowfall and temperature drops in Xinjiang Uygur Autonomous Region, with some parts experiencing blizzards and temperatures plunging more than 10 degrees Celsius, it forecast.

Beijing is blanketed by smog again on Sunday. At 10 a.m., air quality indices from most monitoring stations in the city proper exceeded 200, or Level V, a serious level, according to statistics from the Beijing Municipal Environmental Monitoring Center.

The Beijing Municipal Meteorological Center issued a yellow alert for haze at 11 a.m.. Visibility in some parts of the city was less than 3,000 meters around noon.

It forecast that a cold front will sweep through the capital, bringing strong winds to disperse the fog on Thursday.

The capital has seen a sharply increasing number of patients, especially the sick children, with respiratory symptoms, said Wei Tianni, vice president of the Sixth Hospital of Beijing.

Over sixty percent of new patients in the department of respiratory have the symptom of cough, which is related to haze and foggy weather, Wei added.

To reduce auto emissions, the environmental protection bureau said on Wednesday that the city will adopt a new standard for vehicle emissions starting Feb. 1.

New cars in Beijing will be subject to the new Beijing Standard V, which will be equivalent to the Euro V standard, said Fang Li, a spokesman with the bureau.

The auto emissions standards will contribute greatly to the reduction of the capital's PM 2.5, he said.

Earlier Jan., several consecutive days of smoggy weather choked Beijing and other cities in north and east China, as readings for PM 2.5, or airborne particles measuring less than 2.5 micrometers in diameter, far exceeded safe levels.