Polluted airborne particles  kill 7 million people a year, reports the World Health Organization. That news may not come as a surprise to anyone who has seen images of chimneys in Beijing, Delhi or Mexico. But those factories-or even the jammed roadways of modern cities-are not the biggest killer. Each year, some 4.3 million people die earlier than they should because of polluted air inside their homes, says the WHO. What's causing the air inside people's homes to be so poisonous that it kills around 11,000 people a day? Stoves. "Having an open fire in your kitchen is like burning 400 cigarettes an hour." says Kirk Smith, a professor at the University of California at Berkeley, whose research suggests that household air pollution from cooking killed between 3.5 million and 4 million people in 2013. Not all stoves cause this kind of harm. The ones Smith's talking about are those that the 3 billion people in the developing world use for heat and cooking, which burn solid fuels such as wood, coal, or crop waste instead of gas. The smoke from those fires produces harmful fine particles and carbon monoxide into homes. Poor _ then prevents that smoke from escaping, raising fine particle levels 100 times higher than the limits that the WHO considers acceptable. Breathing this air day in day out eventually causes a lot of diseases: more than a third of the 4.3 million die of a stroke, while a quarter die of heart disease. And around one-third of annual lung disease deaths worldwide are due to waste from coal stoves. Exposure tends to be extremely harmful for the people who spend the most time around the fire-usually women and young children. In fact, the WHO reports that household air pollution almost doubles the risk for childhood lung disease. What should be the deadly killer in a household kitchen?
Solid fuels. Coal stoves. Poor gas. Cooking smoke.