The Environmental Protection Agency (EPA) issued a proposed rule Wednesday to implement language of the Clean Air Act (CAA) that allows "major sources" of hazardous air pollutants (HAP) to reclassify as "area sources" after acting to limit emissions to below levels that define major sources.

The rule, if finalized, would end EPA's "once in, always in" policy, relieving reclassified facilities "from regulatory requirements intended for much larger emitters and encourage other sources to pursue innovations in pollution reduction technologies, engineering, and work practices," EPA said.

The proposed rule would implement EPA's reading of the CAA described in a January 2018 guidance memo withdrawing the "once in, always in" policy.

"'Once in, always in' policies discourage facilities from deploying the latest pollution control technologies or modernizing in ways that increase efficiency and reduce emissions," said EPA Administrator Andrew Wheeler. "Today's proposal would remove a major regulatory burden and incentivize investments in technologies that improve air quality and public health."

The CAA defines a major source as one that emits or has the potential to emit 10 tons/year of any hazardous air pollutant, or at least 25 tons/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.

Of the estimated 7,920 sources subject to national emissions standards as a major source, EPA estimated that nearly half could become area sources.

Also on Wednesday, EPA said Bill Wehrum, assistant administrator of the agency's Office of Air and Radiation, was resigning. Wehrum, who was a partner and head of the administrative law group at Hunton & Williams before joining EPA in November 2017, is tied to a House Energy and Commerce Committee investigation, according to published reports.