BEIJING (BLOOMBERG) - Beijing has warned its 20 million residents to limit outdoor activities to avoid the ill effects of heavy pollution blanketing the city.

The warning comes as a thin layer of gray haze settled over the city in China's north and serves as a reminder of conditions almost a year ago that forced children home from school and saw municipal officials limit factory output.

Beijing's municipal government issued its first-ever red pollution alert on Dec 7 last year as acrid-smelling haze cloaked the city.

On Thursday, (Nov 10) concentrations of PM2.5 - the particles that pose the greatest health risks - stood at 201 micrograms per cubic m near Beijing's Tiananmen Square as of 3pm, according to the city's municipal air-monitoring website.

The World Health Organization recommends PM2.5 exposure of no more than 25 over a 24-hour period.

Beijing's frequent bouts of smog have forced the government to take tougher action to avoid any social unrest sparked by frustration over the city's fouled and polluted air.

Beijing phased out coal-fired heating facilities in 75,000 households during the first 10 months of the year, cutting consumption of the fuel by 225,000 metric tons, Xinhua News Agency reported on Nov 9, citing the city's Environmental Protection Bureau.

Beijing is not the only city suffering. Air pollution levels in the neighbouring cities of Baoding and Shijiazhuang in Hebei province are at medium, data from the China National Environment Monitoring Centre showed.

Vehicle emissions are the major source of pollution in the capital, the Ministry of Environmental Protection said in a Nov 5 statement. The ministry said it has dispatched 12 inspection groups to carry out checks on what major areas, including Beijing, Tianjin, Hebei, Shandong, Heilongjiang and Jilin, are doing to tackle heavy pollution.