Seven million people die each year because of exposure to air pollution. That's one in eight deaths across the globe, making air pollution the single greatest environmental health risk on Earth. The estimate was reported this week by the World Health Organization; based on 2012 numbers, it is double the WHO's previous 2008 estimate.

What's more shocking is that the air pollution is often in the victim's own home. More than half of those deaths are caused by indoor pollutants, which in the developing world largely come from indoor cooking stove fuels like wood, coal, and cow dung.

Since the 1970s, Kirk Smith, professor of global environmental health at the University of California at Berkeley, has studied the effects of