A view of the smokestack of the 47-year old Cheswick coal-fired power plant October 26, 2017, in Springdale, Pennsylvania.

(Robert Nickelsberg/Getty Images)

At a Glance

The "once in, always in" policy, part ofthe Clean Air Act, is being revoked. The policy, in place since 1995,mandated that a source of pollution deemed "major" would always remain so and be regulated as such.Opponents of the move say it will allow hundreds of facilities to increase dangerous emissions.

The U.S. Environmental Agency hasreversed adecades-old policymeant to reduce toxic airpollutant from "majorsources" of air pollution.

,William Wehrum, head of the EPA's air office, announced Thursday.The policyin place since 1995mandated that a source of pollution deemed "major", such as coal-fired power plants,would always remain so and be regulated as such.

Wehrum said the policy was a misinterpretation of the Clean Air Act and didn't take into account when such facilities no longer had the potential to emit pollutants that fell within prescribed criteria.

The as a one that has the potential to emit 10 tons per year or more of a listed hazardous pollutant or 25 tons per year or more of any combination of hazardous air pollutants.

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The new interpretation allows facilities classified as "major sources"to be reclassified as "area sources" when emissions fall below major source thresholds. Once facilities are reclassified, they are subject to different regulatory standards.

“This guidance is based on a plain language reading of the statute that is in line with EPA’s guidance for other provisions of the Clean Air Act,” Wehrum in the EPA press release.“It will reduce regulatory burden for industries and the states, while continuing to ensure stringent and effective controls on hazardous air pollutants.”

Republicans applauded the measure, particularly those from states that rely on gas, oil and coal, the Associated Press reports.

"The EPA's decision today is consistent with President Trump's agenda to keep America's air clean and our economy growing," said Senate Environment Committee Chairman John Barrasso of Wyoming. "Withdrawal of this policy means manufacturers, oil and gas operations, and other types of industrial facilities will have greater incentive to reduce emissions."

Opponents of the measure said the EPA's decision will allow hundreds of U.S. facilities to increase dangerousemissions of toxic pollutants.

“This is among the most dangerous actions that the Trump EPA has taken yet against public health,"John Walke, clean air director at the Natural Resources Defence Council, . "Rolling back longstanding protections to allow the greatest increase in hazardous air pollutants in our nation’s history is unconscionable."