Meera Subramanian in Nature:
After returning from her nine-and-a-half-hour shift as a security guard, Savita Satish Dadas begins plucking fenugreek leaves from their stems for dinner. She and her two children, along with three of their cousins, gather in a shed-like structure next to their house in the Satara District of Maharashtra, India. As goats and cows settle in for the night a few metres away, Dadas and the children sit down on a packed dirt floor around the family hearth. Whisps of smoke rise up from their chulha, the Indian name given to a traditional cooking-stove fuelled by wood and other organic matter often gathered from the countryside. Dadas's stove, like several of her neighbours', is sculpted out of clay. But many make a rudimentary three-stone fire — a triangle of elevated points to support a pot — that humans have used for millennia. Dadas feeds roughly chopped logs into the stove and her hands shape moistened flour into bhakri bread, the rhythmic movement illuminated by the flickering flames.
With this simple daily act, Dadas shares a connection with more than one-third of the world's population, the three billion people who depend on solid biomass fuels — such as wood, animal dung, agricultural waste and charcoal — or coal for their cooking needs. In India, a nation that is rapidly developing in many ways, 160 million households — some two-thirds of families — still rely on such fuel for their primary cooking energy source. Globally, the percentage of households that use biomass has slowly and steadily decreased over the past three decades1. But because the world's population has been rising so quickly, the number of people using solid fuels is not declining, says Kirk Smith, an environmental-health scientist at the University of California, Berkeley, who has studied the health implications of such cooking stoves for 30 years. “This is not going away.” And the urgency to transition billions of people around the world to cleaner forms of cooking has never been greater, in light of recent research revealing that emissions from traditional cooking-stoves pose a bigger threat than previously thought. Results from a global health study released earlier this year project that household air pollution from such fires causes more than four million premature deaths annually — more than one-quarter of them in India alone2. Earth's climate is also at risk from the smoke, which contains dark particles that absorb sunlight, alter atmospheric patterns and hasten glacial melting.