California filed a lawsuit against the Environmental Protection Agency Tuesday for reversing a 1995 memo establishing the “once in, always in” policy for “major” sources of emissions.

EPA Administrator Scott Pruitt issued his own memo undoing the more-than-two-decades-old policy on January 25. California Attorney General Xavier Becerra and the California Air Resources Board filed a lawsuit against the “unconscionable” and “illegal” action, according to a statement Tuesday.

“Instead of prioritizing the health of hardworking Americans, EPA Administrator Scott Pruitt wants to let major polluters off the hook,” Becerra said in the statement. “The EPA must be held accountable.”

The EPA classifies sources of emissions such as manufacturing and energy plants as either “major” or “area” sources of pollution. A source is considered major if it emits more than 10 tons of a Hazardous Air Pollutant (HAP) annually, or if it emits 25 tons of any combination of HAPs in a year.

If classified as a major emitter, the plant or factory is subject to Maximum Available Control Technology (MACT) standards, which grow more stringent over time because MACT standards are based off the most technically advanced and lowest-emitting sources in the U.S., according to Harvard University’s Environmental Policy Initiative.

The 1995 “once in, always in” policy of the EPA meant once a source was classified from an area source to a major source and subject to MACT standards, the MACT standards could never be lifted off the plant or factory. Major sources that reigned in HAP emissions below the 10 and 25-ton thresholds — a common outcome under the strict MACT standards — could not be re-classified as area sources.

Since Pruitt rescinded the policy, many currently classified major sources of emissions will seek to reclassify themselves as area sources “in order to take advantage of more streamlined record keeping and reporting requirements applicable to such facilities,” according to Manufacturing Industry Advisor’s blog.

“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,” EPA’s Office of Air and Radiation Assistant Administrator Bill Wehrum said in a statement announcing the updated department policy. “It will reduce regulatory burden for industries and the states, while continuing to ensure stringent and effective controls on hazardous air pollutants.”

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