It gets pretty cold this time of year in Tibet. For centuries, the solution to this problem was a win-win: just burn that huge pile of yak dung that’s been accumulating all summer. For millions of nomadic Tibetans, it’s a system that works. But that system comes at a hefty cost. Tibetan homes have some of the worst indoor air pollution in the world, and the soot the dung fires release is a big contributor to climate change. Yak dung, when used as fuel, is arguably dirtier than coal but is definitely much cheaper. Particulate pollution from burning animal dung...