Nearly half a century after the Clean Air Act instituted the world’s most stringent emissions controls, the problem of air pollution is far from being solved in the US or anywhere else. Pollution has proved much more persistent, and exposure to it much more damaging, than anyone expected. Today, 91 percent of people worldwide live in areas where air pollution levels exceed the World Health Organization’s recommended limits. The result is a global health emergency, as three new books reveal in sobering detail.