Airport scanners designed to detect non-metallic weapons and explosives have sparked an uproar over radiation concerns, with two pilots unions warning their members to avoid the X-ray scans.

The Transportation Security Administration is installing two types of scanners in airports across the country. One type, called millimeter-wave technology, uses millimeter-wavelength radio waves to create a 2D image of the passenger. As this involves nonionizing radiation, similar to what's produced by a cell phone, most scientists say there are few known health risks.

But the other scanner uses backscatter X-ray technology, which involves tiny doses of ionizing radiation.

Last week, two unions representing more than 16,000 pilots advised their members to steer clear of the scans.

"Based on currently available medical information, [U.S. Airline Pilots Association] has determined that frequent exposure to TSA-operated scanner devices may subject pilots to significant health risks," Capt. Mike Cleary, president of the USAPA, said in a letter to members.

In a letter to his group's members, Allied Pilots Association president David Bates said "no pilot at American Airlines should subject themselves to the needless privacy invasion and potential health risks caused by the [Advanced Imaging Technology] body scanners."

But despite the concerns, many experts, including the American College of Radiology, have dismissed the risks from the scanners as negligible. TSA says one scan is equivalent to the cosmic radiation a passenger would receive for only two minutes of air travel. And the ACR says a passenger would need 1,000 scans using backscatter technology to equal the dose received by just one chest X-ray.

"The ACR is not aware of any evidence that either of the scanning technologies that the TSA is considering would present significant biological effects for passengers screened," the radiologist group said in a statement.

But some scientists note that though on an individual level the risk of cancer is slight, on a population-level or for vulnerable populations there could be cause for concern.

"If you think of the entire population of, shall we say a billion people per year going through these scanners, it's very likely that some number of those will develop cancer from the radiation from these scanners," David Brenner, director of the Center for Radiological Research at Columbia University and a professor of radiation biophysics, told CNN.

And in April, four scientists with the University of California, San Francisco petitioned the White House to undertake an "urgent second independent evaluation" of the technology. The scientists said the comparisons with cosmic radiation received during a flight and from a chest X-ray were misleading because the scanners involve low beam energies delivered mostly to the skin.

"Thus, while the dose would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high," the scientists wrote.

They also said the devices were mostly untested and that the radiation could have a greater impact on the elderly, children, those with compromised immune systems, women with a genetic susceptibility to breast cancer or anyone more sensitive to the effects of radiation.

The U.S. Food and Drug Administration dismissed the concerns, saying in an October response to the letter that the equipment has all been rigorously evaluated and that tests show the effective dose to the skin is 89,000 times lower than the recommended annual dose limit for the general public.

"This technology has been available for nearly two decades and we have based our evaluation on scientific evidence and on the recommendations of recognized experts," John L. McCrohan, deputy director for technical and radiological initiatives with the FDA's Center for Devices and Radiological Health, and Karen R. Shelton Waters, deputy assistant administrator with the TSA, wrote in a letter.

According to CNN, Peter Rez, a professor of physics at Arizona State University, conducted his own independent evaluation of the technology. He said reported figures underestimate the dose, which though tiny, is equal to about one-fiftieth or one-one-hundredth of a chest X-ray.

But he said the possibility of malfunction was the real worry.

"The thing that worries me the most, is not what happens if the machine works as advertised, but what happens if it doesn't," he told CNN.

As of last month, around 189 backscatter devices and 152 millimeter-wave technology ones were installed at around 65 airports. TSA said by the end of next year they hope to have 1,000 up and running.

Passengers who opt out of the scans are subject to increased scrutiny and "enhanced pat-downs." On Saturday, John Tyner, a California software engineer, made headlines and earned tens of thousands of clicks for his YouTube video recorded on his cell-phone of a spat with security officials after he refused to undergo scans or the "enhanced" frisking.

Read DOTmed News' earlier report on the scan's risks.