The study, which concluded that pollution from glider trucks was no worse than from trucks with modern emissions controls, was cited by Mr. Pruitt when he announced in November he would make the regulatory change requested by Ms. Black. Last June, Mr. Oldham had signed a letter endorsing the study.

The Obama administration had moved to eliminate the exemption for the glider trucks, given the health threat associated with their emissions. The E.P.A., in its own analysis, estimated last year that gliders emitted nitrogen oxide levels during highway operations that were 43 times as high as those from trucks with modern emissions control systems.

The E.P.A., in a written statement Wednesday, said that Mr. Pruitt’s move to exempt the glider trucks was based on a legal determination that the agency did not have the authority to regulate them, not the findings of the Tennessee Tech study.

“E.P.A. did not rely upon the study or even quote directly from it,” the statement said, adding that the agency “only noted the existence of the study,” and its findings, when Mr. Pruitt moved to exempt the glider trucks.

A lawyer for Fitzgerald did not respond on Wednesday for a request for comment.

The letter from Mr. Oldham followed a separate letter on Friday from Darrell Hoy, interim dean at Tennessee Tech’s College of Engineering, directed to faculty leaders at the university. It urged Mr. Oldham to suspend the university’s support for the study, which Mr. Hoy said was largely handled by a graduate student, not an engineering expert.