More questions on the radiation dose of TSA full body scanners

How safe are they? Keeping things secret for 'National Security' can cover up a host of sins. [Link]

some scientists with expertise in imaging and cancer say the evidence made public to support those claims is unreliable. And in anew letter sent to White House science adviser John Holdren, they question why the TSA won't make the scanners available for independent testing by outside scientists.

The machines, which are designed to reveal objects hidden under clothing, have the potential to close a significant security gap for the TSA because metal detectors can't find explosives or ceramic knives, which can be just as sharp as the box cutters that hijackers used on 9/11.

They are also important for TSA's public relations battle over the alternative, the "enhanced pat-down," which has bred an epidemic of viral videos: A 6-year-old girl is touched from head to toe. A former Miss USA says she was violated. A software programmer warns a screener, "If you touch my junk, I'm going to have you arrested."

After the underwear bomber tried to blow up a Northwest Airlines plane on Christmas Day 2009, the TSA ramped up deployment of full-body scanners and plans to have them at nearly every security line by 2014.

There are two types of body scanners. Millimeter wave machines emit a radio frequency similar to cellphones. Backscatters work like a fast-moving X-ray. In the latter, the rays bounce off the skin and create a fuzzy white image of the passenger's body. Because the beam doesn't go through the body, most of its radiation is received by the skin.

The TSA says the backscatter technology has been evaluated by the Food and Drug Administration, the National Institute for Standards and Technology and the Johns Hopkins University Applied Physics Laboratory. Survey teams are using radiation-detecting dosimeters to check the machines at airports. The TSA says the results have all confirmed that the scanners don't pose a significant risk to public health.

According to the agency and many radiation experts, the dose is so low, even for children or cancer patients, that someone would have to pass through the machines more than a thousand times before approaching the annual limit set by radiation safety organizations.

But the letter to the White House science adviser, signed by five professors at University of California, San Francisco, and one at Arizona State University, points out several flaws in the tests. Studies published in scientific journals in the last few months have also cast doubt on the radiation dose and the machines' ability to find explosives.

A number of scientists, including some who believe the radiation is trivial, say more testing should be done given the government's plans to put millions of passengers through the machines. And they have been disturbed by the TSA's reluctance to do so.

"There's no real data on these machines, and in fact, the best guess of the dose is much, much higher than certainly what the public thinks," said John Sedat, a professor emeritus in biochemistry and biophysics at UCSF and the primary author of the letter.