The Environmental Protection Agency (EPA) has ended a decades-old policy for the classifying major sources of hazardous air pollutants under section 112 of the Clear Air Act (CAA).

Under a guidance memorandum issued Thursday, sources of hazardous air pollutants previously considered as major sources may be reclassified as area sources when the facility limits their potential to emit below major source thresholds, EPA said.

"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," said EPA’s Bill Wehrum, assistant administrator of the Office of Air and Radiation. "It will reduce regulatory burden for industries and the states, while continuing to ensure stringent and effective controls on hazardous air pollutants."

The "once in always in" policy "has been a longstanding disincentive for sources to implement voluntary pollution abatement and prevention efforts, or to pursue technological innovations that would reduce hazardous air pollution emissions," EPA said.

"States, state organizations and industries have frequently requested rescission of this policy, which was one of the most commonly cited requests in response to President Trump's Executive Order 13777," which requires federal government agencies to identify regulations that should be repealed or modified.

The CAA defines a major source as a one that emits or has the potential to emit 10 tons per year of any hazardous air pollutant, or at least 25 tons per year of any combination of hazardous air pollutants.  Sources with emissions below that threshold are classified as area sources.

EPA established the "once in always in" policy in 1995, determining that facilities subject to major source standards would always remain subject to those standards, even if production processes changed or controls were implemented that eliminated or permanently reduced that facility's potential to emit hazardous air pollutants, Wehrum said.

The EPA's decision was cheered by Sen. Shelley Moore Capito (R-WV), who, along with Sen. John Barrasso (R-WY), had called on EPA Administrator Scott Pruitt to withdraw the Clinton-era policy.

"Maintaining the outdated and misguided 'once-in-always-in' policy just doesn't make sense. Rather than reward facilities for doing the right thing and working to decrease emissions, it makes it harder for them to innovate and improve operations," Capito said.

But Sen. Ed Markey (D-MA) said rolling back the policy "could be the worst environmental sin yet from the Trump administration."