Cow dung cakes are used as fuel in traditional mud stoves. Hundreds of millions of housewives who, with the simple act of cooking, fill their homes with deadly pollutants. India, the world’s third largest climate polluting nation, has spent decades encouraging cleaner cooking technologies, with limited success. Such a shift would have little impact on India’s emissions of climate-warming greenhouse gases, and many of the alternatives pollute as well. Pure black carbon is a significant climate pollutant, with some studies suggesting it is responsible for up to 20 per cent of climate change. It is also a major component of the tiny particulate matter known as PM2.5 that is now one of the most common measures for air pollution. About a quarter of the world’s black carbon pollution comes from cookstoves, with the rest released by car exhaust, forest fires, agricultural burning and other sources.