SHANGHAI (BLOOMBERG) - Dense smog continued to shroud China's largest cities on Saturday (Dec 26) though an approaching cold front began to clear the skies, a day after a Beijing air-pollution alert forced the cancellation of more than 200 flights from the nation's capital.

Beijing's air was "severely" polluted while Shanghai was "heavily" polluted, environmental authorities had said early on Saturday (Dec 26). By the afternoon, the concentration of PM2.5 - particles that pose the greatest health risks - near Tiananmen Square had dwindled to about 134 micrograms per cubic metre from 340 micrograms earlier, according to the Beijing Municipal Environmental Monitoring Center.

That's lower than a peak of 647 on Christmas morning, when officials in the capital raised pollution alert levels to orange, the second-highest on a four-grade scale.

The World Health Organization recommends PM2.5 exposure of no more than 25 over 24 hours. The reading was 89 for Shanghai as of 2pm Saturday (Dec 26) versus 157 in the morning, that city's government said.

"'The air quality improved significantly in the afternoon" helped by cold weather, the Beijing environmental center said on its website.

Chronic air pollution has spurred renewed calls for the government to make better forecasts and act faster to clear the skies. Beijing this year has imposed two red alerts, the highest on the scale, prompting measures including school closures, traffic restrictions and factory operation limits.

The government has shut down 17,000 companies for pollution offenses and another 28,600 were ordered to halt operations as of October, according to the environmental protection ministry. In all, about 50 cities in northern and eastern China have issued air pollution alerts, the China Daily reported on Friday (Dec 25).