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But “clean” is a nebulous term. Of those 28 million cookstoves, only 8.2 million — the ones that run on electricity or burn liquid fuels including liquefied petroleum gas (LPG), ethanol and biogas — meet the health guidelines for indoor emissions set by the WHO. The vast majority of the stoves burn wood, charcoal, animal dung or agricultural waste — and aren’t, therefore, nearly as healthy as promised. Although these cookstoves produce fewer emissions than open fires, burning biomass fuels in them still releases plenty of toxins. “As yet, no biomass stove in the world is clean enough to be truly health protective in household use,” says Kirk Smith, a professor of global environmental health at the University of California at Berkeley and the leading health researcher on cookstoves.