A woman walks across a bridge in front of a chimney billowing smoke from a coal-burning power station in central Beijing February 25, 2011. A scientific study published in The Lancet journal this week, suggests air pollution triggers more heart attacks than using cocaine and poses as high a risk of sparking a heart attack as alcohol, coffee and physical exertion. China is the world's top emitter of greenhouse gases along with the United States. To match Reuters Life! HEART-AIR/POLLUTION REUTERS/David Gray (CHINA - Tags: ENVIRONMENT SOCIETY HEALTH)