Mr. Pruitt has begun to systematically change how the E.P.A. treats science. In April, he proposed a regulation that would limit the types of scientific research that E.P.A. officials could take into account when crafting new public health policies, a change that could weaken the agency’s ability to protect public health.

The new rules would require that the data from all scientific studies used by the E.P.A. to formulate air and water regulations be publicly available. Mr. Pruitt has touted that as a step toward increasing scientific transparency. “The era of secret science at E.P.A. is coming to an end,” he said in a statement. “The ability to test, authenticate and reproduce scientific findings is vital for the integrity of rule-making process.”

However, the change could sharply limit the research available to the E.P.A., because health studies routinely rely on confidential data from individuals.

Last year, Mr. Pruitt significantly altered two major scientific panels that advise the E.P.A. on writing public health rules, restricting academic researchers from joining the boards while appointing several scientists who work for industries regulated by the E.P.A.

These and other changes “will diminish the characterization of pollution as risky,” said William K. Reilly, who headed the E.P.A. under the first President George Bush. “This tolerance for more exposure to pollution is altogether different from anything we are used to.”

In a statement defending the changes to the committees, Jahan Wilcox, an E.P.A. spokesman, said that the agency “sought a wider range of voices” and “was thrilled with the response of over 700 applicants.” The boards, he said, are not only highly qualified but also “independent and geographically diverse.”

This year, Mr. Pruitt sent a memo to the E.P.A.’s Clean Air Scientific Advisory Committee ordering steps that could effectively diminish the role of scientific evidence in air pollution enforcement. The committee is required by law to prioritize the health effects of pollution, but Mr. Pruitt’s memo orders it to consider potential economic consequences of meeting tighter clean-air rules — for example, the possibility that tougher pollution standards could make air-conditioning more expensive, leading to more deaths from heat.