BEIJING — A thick layer of deep gray smog swept into Beijing on Friday afternoon, bringing what the authorities said would be five days of the worst air pollution in a year.
The city issued its first red alert for air pollution of 2016, the most severe notice in a four-tier system, requiring schools to close and half of all privately owned cars to stay off the roads.
As lead-colored air blanketed the capital, moving in from the industrial heartland of surrounding Hebei Province, residents posted photos on social media showing how the skies had changed to gloomy gray at sunset from bright blue in the morning. Some people posted online maps showing Japan and South Korea with clean air readings.
Red emergency signs on Beijing’s main roads alerted drivers to the smoggy conditions, and workers wore protective masks for their journeys home.
In a show of frustration, some people on social media circulated a message from the popular movie actress Zhang Ziyi, who wrote last month that she was flying out of the city because she did not want her baby to wear a protective mask.
Beijing was not the only city affected. On Thursday, the Ministry of Environmental Protection urged 22 other cities in the northeast to issue red alerts.
By 8 p.m. on Friday, many neighborhoods in Beijing registered an air quality index of more than 300. The United States government rates readings above 200 “very unhealthy,” and 301 to 500 as “hazardous.” In such conditions, people should avoid unnecessary exercise, the official American guidelines say.
Under an emergency air-pollution response system introduced several years ago, the Chinese authorities issue red alerts whenever the air quality index was forecast to break 200 for more than four days in a row, or to go above 300 for more than two days.
Regular episodes of smog have occurred in northern China this year. The poor conditions stem from a combination of local emissions, unfavorable weather and pollutants from elsewhere, Bai Qiuyong, head of the National Environmental Monitoring Center, told the state-run newspaper China Daily.
The geography of Beijing makes it especially vulnerable to bad air. The city is bordered to the south and east by the coal-consuming industries that emit pollutants, and to the north and west by mountains that trap the emissions.
The environmental activist group Greenpeace urged the Chinese government to accelerate the transition to cleaner energy.
“As Beijing braces for five days of serious air pollution, the culprits are in clear sight — coal-burning heavy industry,” said Dong Liansai, a Greenpeace campaigner.
The red alert requires heavy-polluting industries in northeast China to curb production until Wednesday.
But during earlier episodes of heavy pollution, factories in Hebei Province flouted regulations aimed at reducing pollution, and they needed to be “strictly punished,” Greenpeace said.
“We have to wait till the smog kills hundreds everyday like SARS once did, so the government would deal with it seriously,” one Beijing resident wrote on Weibo, the Chinese equivalent of Twitter, referring to the outbreak of severe acute respiratory syndrome in 2002-3. “That’s the Communist Party. So bear with it.”