Paul Billings, a senior vice president of the American Lung Association, noted that the Harvard study came on top of other reports that linked the pollutant to premature deaths. “This pollution already kills tens of thousands of Americans every year. This is an affirmation of a standard that already does not provide adequate safeguards to public health,” he said.

Because the Harvard study was only published last week, after White House lawyers had already largely completed a lengthy review of the proposed rule, Mr. Wheeler said that its findings were not included in the E.P.A.’s legal rationale.

“The Harvard study has just been released. It has not been yet been peer reviewed or fully vetted,” said Mr. Wheeler. He added, “We think the information is interesting, and we want to know more about it.”

But he also took aim at its authors, some of whom have publicly criticized decisions made by the Trump administration.

“The scientists seem to have a bias,” he said.

Francesca Dominici, a professor of biostatistics at Harvard who led the study, said she was “disappointed but not surprised” by the administration’s announcement.

She added, “it is an unwise decision in light of the pandemic. There has been a constant tactic over the last few years by the administration to dismiss science in general.”

Mr. Lazarus, the Harvard lawyer, said that he expected that E.P.A. would ultimately be legally forced to incorporate the findings of the Harvard study into the rationale for the rule before it is made final, likely later this year. “It will eventually be part of the legal record,” he said. “Historically, Harvard’s public health studies have been central to E.P.A. public health rules.”