Being outside on a bad air day in Beijing is like smoking a pack of cigarettes, or more. What's the leading cause of death in Beijing now? Lung cancer.
Around the world, a new study says, at least 2.4 million people die every year from air pollution. The number's probably a lot higher now, since the data in the study stopped at 2000, and smog is getting a lot worse. (The number also doesn't include the people who die from indoor air pollution in the developing world, which is about as big).