Almost three billion people still burn dung, twigs, and other traditional fuels indoors to cook and keep warm, generating noxious fumes that kill an estimated two million people each year, mostly women and children. Likewise, just a hundred years ago, the average American family spent six hours each week during cold months shoveling six tons of coal into the furnace (not to mention cleaning the coal dust from carpets, furniture, curtains, and bedclothes).