The Environmental Protection Agency for years has issued costly clean air rules based, in part, on two '90s-era studies linking air pollution with death. 

But, critics say, the same agency has stymied efforts to access the data behind them. The transparency concerns have Republican lawmakers on a new campaign to end the use of what they dub "secret science." 

"Why would the EPA want to hide this information from the American people?" House science committee Chairman Lamar Smith, R-Texas, asked EPA Administrator Gina McCarthy at a hearing last week. 

Smith is among those pushing legislation to bar the use of "secret science" for EPA regulations -- namely, Clean Air Act rules that Republicans say are based on research hidden from public view. The bill has passed the House and now awaits action on the Senate floor. 

"The most expensive rules coming out of the EPA rely on secret science," Sen. Jim Inhofe, R-Okla., chairman of the Senate Committee on Environment and Public Works (EPW), said in a statement to "Americans deserve to have access to technical information and data being used to develop EPA rules that significantly impact their daily lives." 

For its part, the EPA has argued that releasing the data could compromise confidential personal information, and that it didn't have access to all the research anyway, among other issues. The agency made an effort to contact the original institutions behind the studies in 2013, but Republicans say they again would not hand over everything. 

During last week's hearing, McCarthy questioned why lawmakers have focused on this -- and why anyone would want to seek out this kind of granular information. 

"The EPA totally supports both transparency as well as a strong peer-reviewed independent science process, but the bill I'm afraid I don't think will get us there," she said. "I don't actually need the raw data in order to develop science, that's not how it's done. ... I do not know of what value raw data is to the general public." 

But Smith said the agency "has a responsibility to be open and transparent with the people it serves, and whose money it spends."

Further, Inhofe said the data pertains to everything from forthcoming emissions rules for power plants to mercury rules recently challenged by a major Supreme Court ruling. 

The Republican legislation -- called the Secret Science Reform Act of 2015 -- would bar the EPA from issuing certain rules unless all relevant research is named and publicly available for those who want it. In seeking the change, critics say the EPA's air quality rules for years have relied largely on two studies from the 1990s whose data is not entirely accessible -- including a 1993 Harvard study linking air pollution and mortality in certain U.S. cities, and another from the American Cancer Society.  

In the mercury case cited by Inhofe, the high court ruled last month that the EPA should have factored in the costs of recent rules targeting mercury and other pollution. McCarthy reportedly has said the "very narrow" ruling won't affect the separate and ongoing effort to draft new power plant emissions rules, which could be completed in a matter of weeks. The White House has taken a similar stance in downplaying the implications of the 5-4 decision. 

But the ruling nevertheless has emboldened critics. And the "secret science" legislation could add to that pressure. 

An EPW committee aide told the legislation, if approved, potentially could impact both the mercury and greenhouse gas emissions rules. 

"Really, this is just simple transparency," the aide said. 

The Secret Science Reform Act passed out of the EPW committee in April. It is sponsored by Sen. John Barrasso, R-Wyo., on the Senate side and Smith on the House side, where it passed 241-175 in March. A similar measure passed as an amendment last week to a House appropriations bill. 

The White House, though, already has threatened to veto the bill, and it's unclear whether Republicans could muster enough votes to override should it pass the Senate as well. 

In the veto threat, the White House warned the bill would "impose arbitrary, unnecessary, and expensive requirements that would seriously impede the Environmental Protection Agency's (EPA's) ability to use science to protect public health and the environment." According to the White House, some data is not made public to protect the privacy of test subjects; the White House called for more transparency but not an "overly broad bill." The EPA referred to this statement when asked for comment on the bill. 

Other opponents of the bill claim it is merely a tool for political gain rather than scientific improvement. 

"The legislation may sound reasonable, but it's actually a cynical attack on the EPA's ability to do its job," Dr. Andrew Rosenberg, director of the Center for Science and Democracy at the Union of Concerned Scientists, said in a statement. "The legislation wasn't designed to promote good science -- it was crafted to prevent public health and environmental laws from being enforced." 

Rosenberg said the law would prevent the EPA from using all sorts of data -- including health studies and business information -- because of privacy laws. This sentiment has been echoed by the American Association for the Advancement of Science, which sent a petition signed by 43 scientific institutions and universities opposing the proposal. 

But industry groups like the American Fuel and Petrochemical Manufacturers (AFPM) are fully behind the effort, saying Congress and third-party science groups should be able to review EPA health benefit claims. 

"This is supposed to be the experts here, and they don't even have all of it," the EPW committee aide said. "So this just really calls into question their judgment."'s Matt Fossen contributed to this report.