A woman wears a mask as she visits Tiananmen Square in Beijing. China will look into establishing a nation-wide trading system for pollution permits as part of efforts to use market mechanisms to help clean up its environment.

Beijing - Beijing schools cancelled outdoor flag-raisings and took sports classes indoors while hospitals saw a spike in respiratory cases on Monday because of the extremely hazardous air in China's notoriously smoggy capital following a weekend of off-the charts pollution.

City authorities ordered many factories to scale back emissions, and were spraying water at building sites to try to tamp down the dust and dirt that worsens the noxious haze that has hung over the city since late last week.

Demand spiked for face masks and air purifiers, and hospitals saw surges of up to 30 percent in residents seeking help for breathing problems, state-run media outlets reported. Schools in several districts were ordered to cancel outdoor activities, and in an unusual public announcement, Beijing authorities advised all residents to “take measures to protect their health.”

“It's really terrible. I'm extremely upset, but there's really nothing much I can do,” said a Beijing resident out for a morning stroll. Like many Chinese, the man would give only his surname, Kang.

Another man, a 60-year-old retiree surnamed Chen, said his elderly relatives had moved to stay with family members outside the city to avoid the pollution. “I'm in pretty good shape, but the older folks have a lot of problems with their hearts - breathing, and high blood pressure,” Chen said.

Levels of PM2.5 particle pollution over the weekend reached the highest levels since the Beijing government began publicly releasing figures early last year, and in separate monitoring by the US Embassy, they were at 886 micrograms per cubic meter in a reading that was labelled “beyond index.”

By Monday levels had declined to 245 micrograms on the Beijing government scale - down from a high above 700 - but still way above the level of 25 considered safe by the World Health Organisation.

PM2.5 are tiny particulate matter less than 2.5 micrometers in size, or about 1/30th the average width of a human hair. They can penetrate deep into the lungs, and measuring them is considered a more accurate reflection of air quality than other methods.

Beijing's air started to worsen on Thursday, and Beijing's monitoring centre said the pollution was expected to linger until Tuesday.

Weather conditions are a factor, as a lack of wind means pollutants can easily accumulate and fail to dissipate, said Pan Xiao Chuan, a professor at Peking University's public health department.

“Recent pollution doesn't mean there is an increase in the discharge of pollutants,” he said.

The government started publishing PM2.5 readings last year only after a public outcry for more detailed air quality data. A growing Chinese middle class has become increasingly vocal about the quality of the environment. Hourly air quality updates are now available online for more than 70 cities.

Air pollution is a major problem in China due to the country's rapid pace of industrialisation, reliance on coal power, explosive growth in car ownership and disregard to environmental laws. It typically gets worse in the winter because of an increase in coal burning for heating needs, and due to weather conditions.

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. - Sapa-AP