Natural gas stoves in >40 million U.S. residences release methane (CH )─a potent greenhouse gas─through post-meter leaks and incomplete combustion. We quantified methane released in 53 homes during all phases of stove use: steady-state-off (appliance not in use), steady-state-on (during combustion), and transitory periods of ignition and extinguishment. We estimated that natural gas stoves emit 0.8-1.3% of the gas they use as unburned methane and that total U.S. stove emissions are 28.1 95% confidence interval: 18.5, 41.2 Gg CH year . More than three-quarters of methane emissions we measured originated during steady-state-off. Using a 20-year timeframe for methane, annual methane emissions from all gas stoves in U.S. homes have a climate impact comparable to the annual carbon dioxide emissions of 500 000 cars. In addition to methane emissions, co-emitted health-damaging air pollutants such as nitrogen oxides (NO ) are released into home air and can trigger respiratory diseases. In 32 homes, we measured NO (NO and NO ) emissions and found them to be linearly related to the amount of natural gas burned ( = 0.76; ≪ 0.01). Emissions averaged 21.7 20.5, 22.9 ng NO J , comprised of 7.8 7.1, 8.4 ng NO J and 14.0 12.8, 15.1 ng NO J . Our data suggest that families who don't use their range hoods or who have poor ventilation can surpass the 1-h national standard of NO (100 ppb) within a few minutes of stove usage, particularly in smaller kitchens.