The Environmental Protection Agency (EPA) loosened regulatory compliance standards Thursday for certain sources of air pollution previously considered "major."

William Wehrum, head of the EPA’s air office, put out regulatory guidance repealing the “once in, always in” policy, in which facilities like power plants or factories considered “major” sources of hazardous air pollutants were always regulated as such, even if the facilities’ owners took measures to reduce pollution.

“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 said in a statement. “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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The previous standard had been enforced since 1995. “Major” air pollution sources are subject to much stricter rules for what they must do to reduce emissions such as mercury compounds and benzene.

The EPA argued that the “once in, always in” standard disincentivized companies from reducing pollution and targeted it as part of the Trump administration’s overarching goal of cutting regulatory burdens.

“Nothing in the structure of the [Clean Air Act] counsels against the plain language reading of the statute to allow major sources to become area sources after an applicable compliance date,” Wehrum wrote in his guidance.

The Natural Resources Defense Council slammed the move, saying it will cause the biggest increase in air pollution in United States history.

“This is among the most dangerous actions that the Trump EPA has taken yet against public health,” John Walke, the group’s clean air director, said in a statement. “This move drastically weakens protective limits on air pollutants like arsenic, lead, mercury and other toxins that cause cancer, brain damage, infertility, developmental problems and even death. And those harmed most would be nearby communities already suffering a legacy of pollution.”