Stanford scientists discover that the health and climate impacts of natural gas stoves are bigger than originally believed.
Natural gas stoves and appliances have been noticed to warm the planet in two ways: by generating carbon dioxide by burning natural gas as a fuel, and by leaking unburned methane into the air. A new Stanford-led study reveals that the methane leaking from natural gas-burning stoves inside US homes has a climate impact comparable to the carbon dioxide emissions from about 500,000 gasoline-powered cars.
The adverse climate effects that originate from home methane leaks contributes to about a third as much warming as the carbon dioxide generated by combustion of the stove’s natural gas, and sometimes exposes users to respiratory disease-triggering pollutants. The findings, published January 27 in Environmental Science & Technology, come as legislators in numerous US municipalities and at least one state – New York – weigh banning natural gas hook-ups from new construction.
“Surprisingly, there are very few measurements of how much natural gas escapes into the air from inside homes and buildings through leaks and incomplete combustion from appliances,” explained Eric Lebel, the study lead author and graduate student from Stanford’s School of Earth, Energy and Environmental Sciences. “It’s probably the part of natural gas emissions we understand the least about, and it can have a big impact on both climate and indoor air quality.”
Natural gas stoves are contributing to a growing problem
Although carbon dioxide is more abundant in the atmosphere, methane’s global warming potential is about 86 times higher over a 20-year period. Methane also threatens air quality by increasing the concentration of the tropospheric ozone, exposure to which causes an estimated 1 million premature deaths annually worldwide due to respiratory illnesses. Methane’s relative concentration has grown more than twice as fast as that of carbon dioxide since the beginning of the Industrial Revolution, due to human-driven emissions.
While pipeline leaks of natural gas, which is more than 90% methane, have been studied extensively, natural gas-burning cooking appliances have received comparatively little attention.
Over one-third of US households – more than 40 million homes – cook with gas. Unlike other gas appliances, such as space and water heaters that are usually placed away from living quarters, cooking appliances directly expose people to their emissions.
These emissions can include formaldehyde, carbon monoxide and nitric oxides that can trigger asthma, coughing, and difficulty breathing, occasionally resulting in hospitalisation. Hood use and ventilation help reduce concentrations of nitrogen oxides and other co-produced pollutants in kitchen air, yet surveys show that home cooks on average use hoods for kitchen ventilation only 25%-40% of the time.
Findings and implications
To better understand cooking appliances’ potential climate and health impacts, the researchers measured methane and nitrogen oxides released in 53 California homes, not only during combustion, ignition, and extinguishment, but also while the appliance was off, something most previous studies had not completed. Their study included 18 brands of gas cooktops and stoves ranging in age from three to 30 years.
The highest emitters were cooktops that ignited using a pilot light instead of a built-in electronic spark. Methane emissions from the puffs of gas emitted while igniting and extinguishing a burner were on average equivalent to the amount of unburned methane emitted during 10 minutes of cooking with the burner.
Interestingly, the researchers found no evidence of a relationship between the age or cost of a stove and its emissions. Most surprising of all, more than three-quarters of methane emissions occurred while stoves were off, suggesting that gas fittings and connections to the stove and in-home gas lines are responsible for most emissions, regardless of how much the stove is used.
Overall, the researchers estimated that natural gas stoves emit up to 1.3% of the gas they use as unburned methane. While the US Environmental Protection Agency (EPA) does not report emissions from specific residential natural gas appliances, it does report methane emissions for residential appliances collectively. From stoves alone, the researchers estimated that the total methane emissions discovered were substantially more than the emissions currently reported by the EPA for all residential sources.
Larger stoves tended to emit higher rates of nitric oxides. Using their estimate of emissions of nitrogen oxides, the researchers found that people who do not use their range hoods or who have poor ventilation, can surpass the EPA’s guidelines for one hour exposure to nitrogen dioxide outdoors (there are no indoor standards) within a few minutes of stove usage, particularly in smaller kitchens.
“I don’t want to breathe any extra nitrogen oxides, carbon monoxide or formaldehyde,” concluded study senior author, Rob Jackson. “Why not reduce the risk entirely as switching to electric stoves will cut greenhouse gas emissions and indoor air pollution.”