Steubenville, Ohio, was included in the influential 1993 Six Cities Study. This mural on a grocery store shows native son Dean Martin. (Michael S. Williamson/The Washington Post)

In the annals of science there aren’t many reports that had as much impact as Harvard’s Six Cities Study of 1993. It showed a dramatic association between long-term exposure to air pollution and higher risk of an early death. It influenced government pollution standards that research shows have saved thousands of lives.

But the Six Cities Study, as well as many other scientific research papers, could be deemed unreliable and discarded by the Trump administration under a proposal announced Tuesday by Environmental Protection Agency Administrator Scott Pruitt.

Pruitt, who as attorney general in Oklahoma battled the EPA before becoming its leader last year, has pushed forward a new rule that would limit the kind of scientific research used by the agency when crafting regulations. Research would be banned if its underlying data had not been made public or been independently reproduced.

“If you don’t publish the data, you only publish the conclusions. … That’s simply wrongheaded,” Pruitt said.

Such a rule could potentially affect any study relying on confidential health records and clinical data. The Six Cities Study and a follow-up 1995 report using confidential data from the American Cancer Society are prime examples.

Supporters of Pruitt say that in crafting regulations in the past, the EPA has relied on “secret science.” Moments before Pruitt announced the rule Tuesday, Rep. Lamar Smith (R-Tex.) took the rostrum to praise the effort as a way of “putting a stop to hidden agendas.”

Leaders of the scientific community expressed outrage, saying that such a restriction, which must go through a 30-day comment period and will probably face legal challenges, would suppress solid science.

This showdown has been building for a quarter-century.

Douglas Dockery, a Harvard University research scientist, was the lead author of the Six Cities Study and saw it through an exacting peer-review process. The results were so stark and surprising that Dockery and his collaborators embarked on the second, larger study, using the American Cancer Society data, to make sure the first one was right.

In the first study, Dockery and his colleagues looked at six communities — two with clean air (Portage, Wis., and Topeka, Kan.), two with serious air pollution (Steubenville, Ohio, and St. Louis) and two with air quality between those extremes (Watertown, Mass., and Kingston, Tenn.). They found that greater air pollution was strongly associated with a higher risk of an early death and that the association persisted even when factoring in cigarette-smoking habits, occupational hazards and other health risks.

Fine particulates, so tiny that they can penetrate deep into the lungs, contribute to deaths from cardiovascular and respiratory diseases, Dockery said in an interview this week. People living in Steubenville, with the dirtiest air, lost roughly two years of life because of their exposure to air pollution, compared with people in Portage, Dockery said.

The follow-up study examined 154 cities and produced a similar result. This research influenced regulators at the EPA. In 1997, when they crafted the National Ambient Air Quality Standards, they included a standard for fine particulate matter, 2.5 micrometers in diameter or smaller. (A human hair is about 70 micrometers wide.)

The researchers then turned over their data to the Health Effects Institute — a neutral organization funded jointly by the EPA and industry — to conduct a full review. “That was a very anxious time,” Dockery said this week.

“I knew that we had analyzed the data as honestly and accurately as we knew how,” said Six Cities co-author C. Arden Pope, who was the lead author of the American Cancer Society study and is now a professor of economics at Brigham Young University.

The institute’s review broadly validated the quality of the research of Dockery and his colleagues. The data was of “generally high quality,” and the reanalyses “replicated the original results” and “did extend and challenge our understanding of the original results in several important ways.” For example, mortality linked to air pollution was higher among people with less than a high school education, the institute found. The reanalyses also noted an association between sulfur dioxide pollution and mortality.

Since then, scores of studies in many countries have affirmed a connection between air pollution and health problems, according to Dockery and Pope.

“You always wonder when you have a new finding like that whether it’s true or not, and whether it will stand up to scrutiny,” Dockery said. “And it has over these — where are we now? — 25 years. It’s been tested and retested.”

But the research has remained controversial politically, if not scientifically, and has been a target of attacks by EPA critics. These are anxious times for many scientists who are concerned about a federal government headed by a president and political appointees who have expressed doubt about the consensus on climate change and other issues.

Pruitt’s actions follow a failed legislative effort by Smith and other conservative members of Congress known as the Honest Act. The Pruitt proposal is titled Strengthening Transparency in Regulatory Science.

Some proponents of “transparency” borrow language that has been used by scientists who have been pushing for better reproducibility in research and open access to data. Brian Nosek, a University of Virginia professor who is a leader of that movement, said the goal is not to suppress scientific research. In an email to The Washington Post, Nosek said that policymakers should know which findings have been reproduced and how transparent the research data is, “but that doesn’t justify a blanket prohibition of evidence that doesn’t meet the aspirational ideal.”

Pope said: “Almost all of us believe that open, transparent, peer-reviewed, reproducible science is what we want. On the other hand, all of us want to meet the [legal] requirements and privacy and confidentiality ethics that are required to do some of this research.”

Gretchen Goldman, an expert on air pollution and the research director for the Center for Science and Democracy at the Union of Concerned Scientists, said of the move by Pruitt and his allies: “The goal was always to stave off science-based policies, not to promote transparency. What they’re doing now is couching that language in ways to confuse the public and make this sound innocuous.”

Dockery noted that air quality has improved greatly in the United States since he began his research in 1974. Health outcomes have also improved. Those are not unrelated facts, he said.

“People are living longer in the United States and other countries in proportion to how much the air quality has improved,” he said. “I continue to be very proud of the impact we’ve had.”

Brady Dennis and Juliet Eilperin contributed to this report.