If you live in one of the 40 million American households with a gas stove, it could be leaking even when it’s turned off.

According to a new study from Stanford scientists, many stoves are constantly giving out gases that can warm the planet and pose serious health risks when breathed in. The research found methane missions from gas stoves across the United States are roughly equal to the carbon dioxide released by half a million gas-powered cars in a year.

To estimate the impact of these emissions, researchers measured three key gases from stoves in 53 homes across seven California counties. The team chose two gases — methane and carbon dioxide — because of their contribution to climate change, and selected nitrogen oxides because of their known risk to human health. The scientists set up plastic dividing walls between the kitchens and other rooms and used instruments that measure wavelengths of light to determine the concentration of certain gases.

The team estimated that stoves release between 0.8 and 1.3 percent of the natural gas they consume as unburned methane. To their surprise, they found that more than three-quarters of the methane emissions happened when both old and new gas stoves were turned off. “Over a 20-year time scale, the global warming potential of methane leaks is 86 times greater than carbon dioxide,” Stanford reports.

The most significant health risks happen when the stove is lit, the authors note, because the process creates nitrogen dioxide as a byproduct. Increasing airflow by using a range hood can help reduce the personal health risk of natural gas-burning appliances, but most individuals report rarely using their ventilation system.

In a small kitchen, it only took a few minutes of unventilated stove use to generate emissions levels above national health standards. According to a meta-analysis from 2013, children living in homes with gas stoves were 42 percent more likely to experience symptoms associated with asthma, and 24 percent more likely to be diagnosed with lifetime asthma.

In addition to health risks, natural gas burning stoves also endanger the planet by releasing methane. The results of the study have furled efforts by scientists and activists to encourage Americans to switch to all-electric stoves and appliances.

【小题1】Which gas leaking from stoves warms the earth most?

A．Methane.

B．Carbon dioxide.

C．Nitrogen oxide.

D．Nitrogen dioxide.

【小题2】What does the underlined word “they” in paragraph 4 refer to?

A．The measuring instruments.	B．The American families.
C．The team members.	D．The gas stoves.

【小题3】When does a gas stove present the greatest health risks?

A．When it is old.	B．When it is new.
C．When it is turned on.	D．When it is turned off.

【小题4】What would the researchers urge people to do?

A．Equip kitchens.	B．Switch to clean fuels.
C．Use sustainable energy.	D．Purchase electric stoves.