BEIJING • Skies over Beijing have begun to clear, bringing a hint of blue for the first time in days even as toxic clouds of smog that had cast the metropolis in darkness continued to linger over cities north of the Chinese capital.

Concentrations of PM2.5 - the pollutants that pose the greatest risk to human health - reached as high as 269 micrograms per cubic m in Tianjin city and 202 in Hengshui in Hebei province as of 9am yesterday, showed data from the China National Environment Monitoring Centre. The World Health Organisation recommends that average exposure over a 24-hour period should be no higher than 25 mcg per cubic m.

PM2.5 levels in Beijing hit 666 mcg per cubic m on Tuesday, and weakened to 8 mcg per cubic m as of 9am near Tiananmen Square yesterday, the Beijing Municipal Environmental Monitoring Centre said.

Smog has enveloped several areas of China, including parts of Beijing, Tianjin and Hebei, since the night of Nov 26.

Beijing on Sunday raised its air pollution alert for the first time in over a year to orange, the second- highest level in its four-tier system. City leaders asked some factories to suspend or limit production and construction sites to stop transporting materials and waste.

The burning of raw coal and industrial emissions are major sources of pollution in northern parts of China, while vehicle emissions are the main contributor to Beijing's smog, the Ministry of Environmental Protection said in a statement.

The extensive smog has sparked renewed debate over air pollution. Smog was the fourth-most popular real-time keyword search on the microblogging site Sina Weibo at one point on Tuesday.

BLOOMBERG