The head of the Transportation Security Administration testified today that the agency will perform a new, independent study on the safety of X-ray body scanners after lawmakers raised questions about a ProPublica investigation at a Senate hearing on aviation security.

The story, researched in conjunction with the PBS NewsHour, reported that while the radiation emitted from the machines is extremely low, scientific studies have concluded that they could still increase the risk of cancer. It also reported that the Food and Drug Administration went against the advice of a 1998 expert panel, which recommended the agency set a mandatory federal safety standard for the machines. Several members of that panel said they were concerned about widespread use of X-ray scanners, including in airports.

The TSA uses two types of body scanners to search for explosives — an X-ray machine that uses ionizing radiation, a form of energy that has been shown to damage DNA, and a millimeter-wave machine that uses radiofrequency technology, which has not been linked to cancer.

Sen. Susan Collins, the top Republican on the homeland security committee, recommended that the Department of Homeland Security independently evaluate the health effects of the X-ray scanners and “establish a goal of using radiation-free screening technology.”

Questioned about the story by Sen. Joe Lieberman, TSA Administrator John Pistole said that the agency has already conducted several independent studies showing that the radiation is equivalent to the dose received in about three minutes of flying at typical cruising altitude.

“But that being said, I am concerned that there is a perception that they are not as safe as they could be,” Pistole said. “And since we are using a different technology, that being the millimeter-wave scanner, that does not have that same perspective, I will take that back and we will conduct an independent study to address that.”

In recent years, the TSA has commissioned tests of the X-ray machines, also known as backscatters, by the Food and Drug Administration and the Johns Hopkins University Applied Physics Laboratory. In addition, survey teams from the Army Public Health Command visit airports to check the machines. The TSA says the results have all confirmed that the backscatters don’t pose a significant risk to public health.

But David Brenner, director of Columbia University’s Center for Radiological Research, said in a recent interview that while the dose is low, the chances of someone getting cancer increase as TSA puts millions of airline passengers through the machines.

“Why would we want to put ourselves in this uncertain situation where potentially we’re going to have some cancer cases?” he asked. “It makes me think, really, why don’t we use millimeter waves when we don’t have so much uncertainty?”

Robin Kane, the TSA’s assistant administrator for screening technology, told ProPublica and PBS that the health risk is small compared to the security benefit. Having both technologies is important, he added, to improve detection capabilities and find the most cost-effective solution.