Review 5: Population levels of household air pollution and exposures" (Geneva, 2014), clean fuels include electricity, liquid petroleum gas (LPG), natural gas and biogas, whereas dirty fuels burnt in open fires and leaky stoves include solids such as wood, crop wastes, charcoal, coal and dung. Available at: httpWwww.iiasa.ac.at/web/home/research/alg/ energy-inequality.html.