BEIJING — Beijing issued a red alert for smog, urging schools to close and residents to stay indoors for the second time in 10 days.
Restrictions begin Saturday and last until Tuesday, the city's emergency management headquarters announced on the Beijing government website.
Measures include pausing factory work and restricting road traffic, except electric vehicles.
The alert reflects a forecast of three days of PM2.5 – airborne particulate matter 2.5 microns and smaller – levels above 200 micrograms per cubic metre.
PM2.5 pollution is fine enough to penetrate deep into the lungs, and is associated with increased risk of heart attack, stroke, lung cancer and asthma.
The U.S. embassy's air quality index showed PM2.5 levels of 151 micrograms per cubic meter on Friday morning. The World Health Organization's recommended safe maximum is 25 micrograms.
Beijing made global headlines in early December during the Paris climate change talks when its air quality went off most scales with a PM2.5 reading of 678 near Tiananmen Square.
The city may struggle to clean up its air in time for the 2022 Winter Olympics, as the sources of its air pollution are not local, according to U.S. researchers.
Beijing "receives much of its pollution from distant industrial areas, particularly Shijiazhuang," 322 kilometres to the south-west, said Robert Rohde, co-author of a report on China's pollution.
Bad air contributes to 1.6 million deaths a year or roughly 17 percent of all deaths in China, according to a report by Rohde and co-author Richard Muller, in the science journal PLOS One.
Air pollution kills about 4,000 Chinese people a day, with coal burning a key contributor, it said.