It's a silent, invisible killer, but it takes a major toll. According to the World Health Organization, more than 1.3 million people die from outdoor air pollution every year. And while the skies have become much cleaner in nations like the U.S. and Japan, the air just seems to be getting worse in rapidly-growing developing nations like China. Fixing the problem won't be easy much of the pollution is due to the burning of coal, which is growing rapidly around the world. The good news is that technology exists today to remediate air pollution, and the Rio Summit can help by getting developed nations to share that know-how with developing ones.