The top Republican on the Senate homeland security committee is seeking answers after the head of the Transportation Security Administration backed off a promise to study the health effects of the X-ray body scanners used at airports.

In a letter sent Wednesday to TSA administrator John Pistole, Senator Susan Collins of Maine said she was “disappointed” to hear the news, especially after the European Commission prohibited such scanners because of concerns that the radiation emitted by the machines could lead to cancer.

In a second letter, Collins asked the TSA to place larger signs at checkpoints with X-ray scanners advising travelers, particularly pregnant women, about the radiation and the option of undergoing a pat-down instead. “I am disappointed that this simple precaution has not yet occurred,” the senator wrote.

Currently, checkpoints have an 11-by-14-inch sign in front of the machine that states the screening is optional but emphasizes the images the machines produce rather than any possible health risks. Passengers often have a few seconds to read the print before being flagged to walk through the scanner.

Asked to comment, a TSA spokesman wrote in an email that the agency “will respond directly to Senator Collins to address her questions.”

The letters come three weeks after a Senate hearing, in which lawmakers asked questions about a ProPublica investigation, conducted in conjunction with the PBS NewsHour, of the potential health risks of the X-ray scanners. At the hearing, Pistole agreed to a request from Collins to conduct a new independent study of the scanners.

At another Senate hearing a week later, Pistole said he had just received a draft report by the Department of Homeland Security’s inspector general that might render a new study unnecessary.

The full report has not been released. A summary obtained by ProPublica said that the inspector general found the radiation was within industry standards. But it contained several recommendations for the TSA to ensure that inspections and training are completed.

“My understanding is that the IG report will examine whether or not TSA is doing an adequate job of inspecting, maintaining and operating [the body-scanner] machines,” Senator Collins wrote in her letter. “This is not the same as conducting an independent study on the health effects of those [scanning] machines emitting ionizing radiation.”

The TSA uses two types of body scanners. An X-ray machine, also known as a backscatter, looks like two large blue boxes and emits extremely low levels of ionizing radiation, a form of energy which strips electrons from atoms and has been shown to cause cancer. The other machine, known as the millimeter-wave scanner, looks like a round phone booth and uses radio waves, which have not been linked to cancer.

In maintaining that the machines are safe, Pistole has pointed to studies the TSA commissioned with the Food and Drug Administration, the Johns Hopkins University Applied Physics Laboratory and the Army Public Health Command.

Those tests show that the radiation emitted from the backscatter is equivalent to the naturally-occurring radiation received in about two or three minutes of flying at altitude.

But two peer-reviewed research papers concluded that because the TSA is planning widespread use of the scanners, such trivial amounts could lead to additional cancer cases. One paper, written by Rebecca Smith-Bindman, a radiologist at the University of California, San Francisco, estimated that the backscatters would lead to six cancers over the course of a lifetime among the approximately 100 million people who fly every year. Another paper, by David Brenner, director of Columbia University’s Center for Radiological Research, reached a higher number – potentially 100 additional cancers every year.

Smith-Bindman concluded that “there is no significant threat of radiation from the scans,” given that the same 100 million people would develop 40 million cancers over the course of their lifetimes. In this large pool, it would be impossible to link specific cancer cases from cumulative exposure to the backscatter machines.