世界卫生组织估计，在发展中国家城市的室外空气污染可能比发达国家城市高十倍，而通过燃烧木材和粪便造成的平均室内空气污染会高出一百倍。
The WHO estimates that while outdoor air pollution in developing-country cities may be ten times higher than in advanced-country cities, average indoor air pollution, caused by burning wood and dung, is a hundred times higher.