Gas stoves leak significant amounts of methane when they are being ignited and even while they are turned off, according to a new report, adding to the growing debate over the effects of gas-powered appliances on human health and climate change.

The small study — based on measurements from cooktops, ovens and broilers in 53 homes in California — estimated that stoves emit between 0.8% to 1.3% of the natural gas they consume as unburned methane, a potent greenhouse gas. During the course of a typical year, three-quarters of these emissions occur when the devices are shut off, the study showed, which could suggest leaky fittings and connections with gas service lines.

Over a 20-year period, emissions from stoves across the United States could be having the same effect in heating the planet as half a million gas-powered cars, the study estimated.