The air pollution levels that prompted China to issue its first "red alert" for smog in Beijing pose serious health dangers to people who live there.
The heavy smog carries risks for everyone, but especially people with heart disease or lung conditions such as asthma or emphysema, said David Lang chairman of the department of allergy and immunology at Cleveland Clinic, who visited China in October.
Air pollution can cause cancer, according to the World Health Organization. It also can trigger heart attacks in people with underlying heart disease, said John Groopman, a professor of environmental health sciences at the Johns Hopkins Bloomberg School of Public Health in Baltimore who has worked in China for 35 years.
Children are also at greater risk, because they spend more time playing outside and have smaller lungs, Lang said. Heavy pollution can permanently damage children's lungs, putting them at greater risk of lung disease as adults, said Janice Nolen of the American Lung Association. Officials in China urged Beijing schools to close to protect children from the unhealthy air.
In all, about 7 million people worldwide die every year due to air pollution — about 1 in 8 deaths, according to WHO.
Air quality levels are measured on a scale that begins at 0, with levels under 50 considered healthy, Lang said. Pollution levels in the United States rarely exceed 100, the level considered unhealthy.
As of Tuesday morning, the air quality rating in Beijing was 200, on the border between "unhealthy" and "very unhealthy," Lang said. On days when air pollution is this bad, people with asthma and similar conditions should avoid physical activity, and healthy people should reschedule outdoor plans, he said.
"There's no question that this is injurious to health," Lang said, noting that pollution irritates the lungs. "You start coughing after a while."
When Lang visited the Chinese city of Xian in October, the sky was dark gray, even in the middle of the afternoon. He and colleague, who has asthma, bought protective masks.
While these masks can block large particles of soot, they don't block the smallest particles of pollution, which can get deep into the lung, Groopman said.
This isn't the first time that a city's air has become dangerously dirty.
In 1952, London experienced a "killer fog," a mix of soot and and dense fog that killed thousands of London residents in four days. In 1966, an intense smog over Thanksgiving weekend was blamed for the deaths of 166 people in New York.
The United States hasn't seen air pollution levels like China's in decades, thanks largely to the Clean Air Act of 1970, Groopman said.
Before the Clean Air Act, there were days in Los Angeles when the smog was so thick that "you couldn't even see that there were mountains and hills around," Groopman said.
Efforts to clean the air in the USA have produced dramatic changes, Groopman said.
"There are blue skies over Baltimore today," Groopman said. "Not that long ago, when the steel plant was working and there was a lot of heavy industry, the skies were orange."