Residents of China's capital urged the government to curb pollution after it issued a health warning for severe smog that blanketed the city on Tuesday.
Tens of thousands of people supported an online pledge by a local legislator and property developer to push for a clean-air law in Beijing. Environmental group Greenpeace urged action to reduce coal smoke and vehicle emissions.
Health officials warned the city's 20 million residents to minimize outdoor activity as pollution levels rose amid severe smog that has affected many northern Chinese cities this month.
The Beijing Centre for Disease Control and Prevention also urged people to avoid crowded areas because the cold, polluted air had raised the risk of spreading seasonal influenza and other airborne viruses.
The smog that has affected Beijing since mid-January was caused by a combination of air pollution and weather conditions, it said.
The US embassy in Beijing said it measured Tuesday's peak density of PM2.5 at 526 micrograms per cubic metre, more than 20 times the safe limit recommended by the World Health Organization.
PM2.5 refers to particles smaller than 2.5 micrometres, which pose a serious health hazard because they can be embedded deep into the lungs.
Beijing recorded PM2.5 of 900 micrograms per cubic metre earlier this month.
The pollution and fog caused the cancellation of several dozen flights from Beijing and other northern cities on Tuesday.
"I try to avoid going out, and I wear a mask when I do go out," said student Zhou Ziqing.
Zhou said the government's current anti-pollution measures were ineffective.
"They should restrict cars with large emissions and enhance the current traffic restrictions," she said.
The government should also improve treatment of emissions from Bejing's winter heating plants and curb pollution from nearby cities, Zhou said.
Property mogul Pan Shiyi told 14 million followers of his Sina Weibo microblog that he would press for clean-air legislation at the Beijing Municipal People's Congress, of which he is a member.
More than 25,000 users supported his proposal within three hours, state media said.
The government has identified vehicle emissions as the major contributor to the capital's air pollution.
But Greenpeace said coal smoke was an equally important factor, especially from outlying industrial cities.
"In Beijing, traffic pollution and regional pollution take 50 per cent respectively," Greenpeace China spokeswoman Zhou Rong said.
Seasonal atmospheric conditions have trapped pollutants at a lower altitude than usual this month, mixing particles from coal smoke with vehicle emissions closer to the ground, Zhou said.
"I think the most immediate measure that could achieve results would be to improve the quality of petroleum, especially in Beijing," she said.
"As for the longer term, treatment of pollution from industry and power plants must follow," Zhou said.
State media quoted new Beijing mayor Wang Anshun, who took office Monday, as saying the city government would study new "concrete measures" to cut air pollution.
"The current environmental problems are worrisome," Wang said.
"The total number of vehicles should increase slowly," he said, referring to Beijing's policy of restricting the number of vehicles on the city's roads, which has risen to 5.18 million from 3.13 million in 2008.