Officials on Sunday issued an alert due to the dense and harmful pollution that for two days has covered 17 Chinese provinces and which has become the worse case of its kind in this capital's history.

Authorities have asked citizens across more than half this populous Asian country to remain in their homes to avoid the effects of the pollution, which in Beijing was called "severely harmful or dangerous."

The capital's air quality is the worst ever registered and the situation is similar in three other cities in the northern province of Hebei and the eastern province of Tianjin, the head of the Greenpeace's Office of Climate and Energy for East Asia, Zhou Rong, told Efe.

"It's so terrible that it's difficult to breathe," Zhou said.

Many Beijing residents went about their daily chores or to work wearing masks after environmental authorities advised their use, along with the suggestion to avoid exhausting exercise.

The heavy pollution, however, did not seem to cut the regular amount of traffic out on Beijing streets on Sunday and some people decided to continue with their daily habits, including engaging in Tai Chi in the morning or skating on the frozen surfaces of lakes in some of the city parks.

In the Internet forums, those most aware of the environmental situation in the Chinese capital asked the local government to declare the coming days to be holidays until the fog of noxious chemical vapor and particulate matter dissipates.

Beijing's children's hospital blamed the pollution for the large number of children suffering from respiratory illnesses over the past few days, with the number of patients due to those causes exceeding 7,000 per day last week, the South China Morning Post reported.

In the capital, the poor visibility - under 100 meters (yards) - led airport authorities to cancel at least 25 national and international flights and to delay five others for more than an hour.

The latest air quality figures in major Chinese cities published by the state-run news agency Xinhua show that, after Beijing, the cities with the worst conditions are Shijiazhuang, Changchun, Shenyang and Xian in the north, as well as Zhengzhou and Chengdu in the central part of the country.

High levels of air pollution in 2012 caused an estimated 8,500 premature deaths in Beijing, Shanghai, Canton and Xian, Greenpeace said. EFE