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.
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.