Most residential and commercial buildings use appliances powered with fossil fuels for space and/or water heating. Collectively, these appliances emit twice the level of smog-forming nitrogen oxides as the nation’s gas-fired power plants and almost as much planet-warming carbon dioxide. Yet while the U.S. Environmental Protection Agency (EPA) has limited gas power plants’ NOx emissions since the 1970s and their greenhouse gas emissions since 2015, the agency has never regulated emissions from residential appliances and restricts emissions from only a tiny fraction of heating systems in commercial buildings.