Amelioration is no cure

About 2.8 billion people - between a third and a half of the world's population - burn solid fuel such as wood, crop waste, charcoal, coal and dung to cook their food in open fires and leaky stoves. Inside those homes, cooking smoke is eye-stingingly visible, and it blackens the walls. But the invisible effect it has on the tiniest, branching airways of the lungs is what makes this a global crisis. Those smoke particles are taken up by cells that form part of the natural defences of the lungs. In doing that, they diminish the function of these cells, increasing the risk of infections and chronic illness. Crucially, smoke is a key driver of child pneumonia - a leading cause of death in children under five in many countries across the developing world. The World Health Organization (WHO) estimates that four million premature deaths every year - in children and adults - can be attributed to indoor air pollution. That is why in 2010, the United Nations Foundation and the US State Department launched the Global Alliance for Clean Cookstoves (GACC), with a mission to "foster the adoption of clean cookstoves and fuels in 100 million households by 2020".

Cleaner cookstoves, the alliance says "save lives and reduce illness". There is, however, a problem. A study run by Kevin Mortimer in Malawi over the last two years, and written up in the Lancet, shows that the cookstove alone does neither. A big clinical trial in Malawi was expected to show children are less likely to die of pneumonia if they live in a home where food is cooked on a smoke-free stove rather than an open fire. Instead it suggests the stove makes no difference. "We followed up and monitored the health of 10,000 children for this study, and our results basically show that there is no difference in cases of pneumonia between the intervention and the control group," he says. "The hard science - certainly from our study - is perhaps at odds with the claim that stoves along the lines of the ones we've used here save lives and reduce illness."

Where does this leave a huge UN-backed project to get 100 million clean cookstoves into homes in the developing world by 2020?

"Exposure to household air pollution is a problem of poverty," says Kevin Mortimer, a medical doctor and a respiratory health researcher at the Liverpool School of Tropical Medicine. "If you're not poor, you're not exposed."