Beijing: A new study has pinned down an overlooked source of outdoor pollution in Beijing -- residential cooking and heating -- which according to scientists contributes more to the city's choking smog than do the transportation sector and power plants combined. "Coal and other dirty solid fuels are frequently used in homes for cooking and heating," said Denise Mauzerall from Princeton University who was part of the research team. "Because these emissions are essentially uncontrolled they emit a disproportionately large amount of air pollutants which contribute substantially to smog in Beijing and surrounding regions," she added. According to the team from Princeton, the University of California-Berkeley, Peking University and Tsinghua University, households account for about 18 per cent of total energy use in the Beijing region but produce 50 per cent of black carbon emissions and 69 per cent of organic carbon emissions. In the Beijing area, households contribute more pollutants in the form of small soot particles (which are particularly hazardous to human health) than the transportation sector and power plants combined. In the winter heating season, households also contribute more small particles than do industrial sources. The high levels of air pollutant emissions are due to the use of coal and other dirty fuels in small stoves and heaters that lack the pollution controls in place in power plants, vehicles and at some factories, according to the study published in the journal Proceedings of the National Academy of Sciences. The region in the study included the cities of Beijing and the surrounding Tianjin and Hebei provinces.