In a rare step, the science committee of the U.S. House of Representatives voted yesterday to subpoena the Environmental Protection Agency (EPA) for data from key studies used to justify air pollution regulations.

Following a heated exchange of letters between Committee Chair Lamar Smith (R-TX) and ranking member Eddie Bernice Johnson (D-TX), the committee approved its first subpoena in 21 years. The vote was strictly along party lines, with Democrats opposing the measure.

The information in question includes the underlying data in a 1993 paper considered to be groundbreaking work on the impact of air pollution. In the so-called Six Cities Study, Harvard researchers followed more than 8000 participants for 14 to 16 years and found an association between death rates and particulate matter, or soot, in the air. The study informed EPA’s 1997 decision to tighten its air quality standards and continues to underpin Clean Air Act regulations. Smith has repeatedly requested that EPA hand over raw data from the Six Cities Study and from a related (and much larger) American Cancer Society study known as “Cancer Prevention Study II,” plus all subsequent reanalysis of the studies. He accused the agency of using “secret science” to justify environmental regulations.

Smith gave EPA Administrator Gina McCarthy an ultimatum in a 22 July letter, threatening a subpoena if the information—which would include personal health information from study participants promised confidentiality—wasn’t turned over by the end of the month.

On Tuesday, Johnson responded to Smith’s subpoena threat with outrage. In her letter, she claimed that his evidence for questioning the validity of the studies was shoddy and that a subpoena would violate the trust of hundreds of thousands of Americans who had participated in the Six Cities Study and other research included in the subpoena. She demanded that Smith clarify who would receive the data and for what purpose.

In his response, the chairman acknowledged that the data would need to be “de-identified” to protect the privacy and health information of the participants.

Economist C. Arden Pope of Brigham Young University in Provo, Utah, one of the authors on the Six Cities Study, says that turning over what Smith requests would undoubtedly violate the confidentiality agreement made with participants. “It’s extremely hard to give a data set that will allow you to replicate the results in these studies that doesn’t include information that then allows you—with an Internet search of obituaries—to quickly figure out who the people were,” he says.

Smith also explained in his letter that he intended to share the data with “independent scientists for review.” But Johnson, in yesterday’s opening statement, accused Smith of intending to pass the data to “industry hacks” who would try to discredit the research for financial gain.

According to the subpoena that Smith signed following the vote, EPA now has until 19 August to turn over all relevant data. The agency has previously said it has provided Smith with all the relevant material that it has.