Air pollution in the Chinese capital has hit dangerous marks, reaching beyond the permissible level of pollution on the local environmental center's scale. Beijing residents are recommended to stay indoors by local authorities.

The Beijing Municipal Environmental Monitoring Center has reported the rising of air-quality indices since Friday in many parts of the city.

A warning scrolled across the monitoring center's website says that the density of PM 2.5 had reached 700 micrograms per cubic meter in many parts of Beijing and that the polluted air was expected to linger for the next three days.

The index indicates the level of airborne PM 2.5 particulates, at which particle matters are considered the most harmful to health. Air is considered good when the index is at 50 or below, but hazardous with a reading between 301 and 500, when people are warned to avoid outdoor physical activities.

The city's authorities have blamed a lack of wind and foggy conditions for the high concentration of air pollutants.

"It is expected that air pollution in Beijing will remain heavy during the daytime today... people are advised to stay indoors as much as possible," China's state TV quoted Beijing's environmental protection center as saying on Friday.

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© Reuters / Jason Lee
A man walks along trees on a heavy haze winter day in central Beijing, January 12, 2013.
According to rules issued by the city's government in December, all outdoor sports activities are to stop and factories have to reduce production if Beijing's official air-quality reading goes over 500.

Meanwhile, according to a Twitter account run by the US embassy in Beijing, air quality ratings for the city have ranged between "very unhealthy" and "hazardous" since Thursday, and reached the "beyond index" mark during Saturday afternoon. Monitors there recorded off-the-chart air-quality readings as high as 845 at 8pm on Saturday.

Readings are often different in different parts of Beijing. Chinese authorities and the United States also have different ways to calculate the air quality index, although their indices are "highly similar" at the two ends of the spectrum, according to the founder of the nongovernmental Institute of Public and Environmental Affairs in Beijing, Ma Jun.
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© Reuters / Jason Lee
People walk during a heavily hazy winter day in central Beijing, January 12, 2013.
For comparison, on Friday the 9pm readings for PM2.5 and ozone in Hong Kong's Central and Western districts, among the most polluted on the island, were around 60 and 20 respectively, according to the website of Hong Kong's Environmental Protection Department.

Air pollution is believed to be one of the major problems in China with its fast pace of industrialization, reliance on coal power and quick growth in car ownership. Due to polluted air Beijing is often covered with dense smog, while its many residents suffer from respiratory problems.
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© Reuters / Jason Lee
People walk on a pedestrian bridge on a very hazy winter day in Beijing January 12, 2013.
Several other cities, including Tianjin on the coast east of Beijing and southern China's Wuhan city also reported severe pollution over the last several days.