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 indoor cooking using solid fuels like wood, coal, and cow dung. He told the WHO, "Having an open fire in your kitchen is like burning 400 cigarettes an hour."

Carlos Dora, coordinator for the WHO Interventions for Healthy Environments unit, prefers to put it more scientifically. "The home with a dirty cookstove using coal can reach 2,000 or 3,000 micrograms per cubic meter of particles," he says. That's 200 to 300 times the WHO's average daily standard for maximum concentration of the fine particles of air pollution—about 25 to 100 times smaller than a human hair—that can settle deeply in the lungs.

Breathing those microscopic dirty particles causes more than lung cancer, which is responsible for only 6 percent of the seven million premature deaths caused by air pollution. The vast majority of deaths—69 percent—are due to heart disease or stroke.

Evidence of the cardiovascular harm from air pollution has increased since the WHO's last report. That's one reason the estimated number of deaths has risen so dramatically. The other reason is that the organization has developed better ways to measure pollution outside of urban areas.

"There are more epidemiological studies showing a clear connection between air pollution and heart disease and stroke. The evidence now is robust," says Dora. "And we have better models of how pollution travels and more monitoring stations in rural areas."

Almost three million people around the world use dirty solid fuels to do their cooking. Women and children, whose days often center around gathering fuel and cooking, are more exposed to pollutants than men. Yet death rates attributed to air pollution are higher, at 49 percent, among men than among women, at 42 percent. Men in general have more risk factors—like smoking or a high-fat diet—than women do, says Dora. "Any one risk factor has a higher impact on people who already have a number of other risk factors."