Some of the millions of abandoned oil and natural gas wells in the United States are still spewing methane, and are a potentially large source of unrecorded greenhouse gas emissions, according to a new study.

Researchers at Princeton University measured emissions from dozens of abandoned wells in Pennsylvania in 2013 and 2014 and found they were emitting an average of 0.27 kg (0.6 lbs) of methane per day, according to the study published in the Proceedings of the National Academy of Sciences on Monday.

“These measurements show that methane emissions from abandoned oil and gas wells can be significant,” the study said. “The research required to quantify these emissions nationally should be undertaken so they can be accurately described and included in greenhouse gas emissions inventories.”

The U.S. Environmental Protection Agency is mulling whether to issue mandatory standards to reduce methane emissions from the oil and gas sector as part of President Barack Obama’s initiative to fight climate change.

Environmental groups have told the EPA that targeting methane instead of secondary volatile organic compounds, which the agency currently regulates, is more effective and can help the U.S. make more emissions cuts.

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