According to WCHS, Jackson told a US Senate subcommittee that she signed the standards prior to meeting with the group to testify about the EPA’s latest budget proposal.

While limits have been under discussion by the agency since Clean Air Act amendments were made in 1990, about 50% of the impacted plants have since made the changes the new rules require.

“Nearly half of the country's coal-fired plants continue to do nothing to limit the amounts of these poisons that they spew into the air,” she said in her address.

“EPA’s new action will ensure that companies all across the country follow the same rules.”

Despite presenting figures to the Senate group about the health benefits to come from the standards – she estimates 17,000 premature deaths will be prevented annually – she received criticism for the EPA’s enforcement and regulation actions.

Alaska Senator Lisa Murkowski told Jackson she just wanted the agency to do its job.

"And implicit in the agency doing its job is fair treatment to those you regulate," she reportedly told the EPA head, noting that there have been Clean Air permits in her state delayed for years.

Murkowski said many asked her to do something about containing the EPA, and many wanted to know what was happening with it and whether they should be concerned.

“I think the answer to that is yes,” she said, according to the news outlet.

Many power generation companies, including large utility FirstEnergy, had already announced plans to close some older coal-fired plants in light of the new standards.

The rules are expected to be in place by 2016, and power companies will have three years to comply, four if upgrade installation work is underway.