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About half are what are known as millimeter-wave units, made by L-3 Communications, which are not the focus of safety concerns because they emit a less potent kind of radiation. The remaining machines are Rapiscan System’s “backscatter” scanners, which emit X-ray-like ionizing radiation. This kind of radiation in larger doses can cause cell changes leading to cancer.

“From a strictly radiation-safety standpoint, there would be no concern” with either type of scanner, said Richard Morin, a radiology specialist at the Mayo Clinic in Jacksonville, Fla. “For both of them, levels for radiation are pretty much insignificant.”

The Health Physics Society says the screening is justified if radiation doses do not exceed standard limits, and if the public is informed of the radiation exposure.

The National Council on Radiation Protection & Measurements, an independent group that advises the government on radiation issues, examined backscatter screening at the request of the Food and Drug Administration. It said non-ionizing scanners should be considered first, if possible. But it also recommended limits for backscatter radiation and said health risks would be minimal if doses were below those limits. The government insists that they are.

David Schauer, the council’s executive director, says he has no qualms about being scanned by backscatter devices, and would allow his three sons to be scanned, too.

TSA spokesman Nick Kimball said Thursday that both types of scanners are safe, cost about the same and are similarly effective. He said the TSA chose to use both types to keep the process competitive and to “drive innovation.”

He noted that unlike medical workers who deal with more potent radiation, TSA employees do not need to wear protective gear while operating the scanners.

Scanners are tested for safety before being set up at airports and tested again periodically once they are in place, said Daniel Kassiday, an FDA radiation expert.

The FDA has estimated that the risk of fatal cancer from the maximum allowable dose would be 1 in 80 million per backscatter screening. And doses from a single scan are considerably lower than the maximum, Kassiday said.

By comparison, the chance of dying in a car driven for 40 miles are 1 in 1 million.

Rez agrees the odds of getting cancer from the scanners may be low. But he calculates it’s about the same as the chance of being on a plane blown up by terrorists. And he says that makes mass scanning not worth the effort.

The government says independent testing proved the airport scanners are safe. Johns Hopkins University’s Applied Physics Laboratory did independent tests _ but only to determine how much radiation the devices emit, not to examine safety, said Helen Worth, a lab spokeswoman.

The amount of radiation the devices emit in a lab setting versus real-world use may be different, and a group of scientists from the University of California at San Francisco argues that tougher safety testing is needed.

The four scientists expressed their concerns in an April 6 letter to Holdren, the White House adviser. It took him six months to respond.

Though the scanner images do not reveal what’s beneath the skin’s surface, the radiation they emit could potentially affect breast tissue, sex organs and eyes, said David Agard, an imaging expert at the University of California at San Francisco.

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