The TSA says the radiation from its security scans amounts to about a thousandth of the amount a patient receives from a standard chest X-ray, or an amount "equivalent to two minutes of flying on an airplane."

But a physics professor at Arizona State University in Tempe not only conducted his own study, finding the radiation exposure 10 times what the TSA estimates, but also argues that the health risks aren't mathematically worth taking.

Prof. Peter Rez explained to MSNBC that while the risk of getting a fatal cancer from the screening is minuscule, it's about equal to the probability an airplane will get blown up by a terrorist. Either way, the professor argues, dead is dead.

"There is not a case to be made for deploying [the scanners] to prevent such a low probability event," Rez says.

Join tens of thousands of Americans in a petition demanding action against the intrusive airport screening procedures implemented by Janet Napolitano and send a letter to Congress, President Obama and others telling them exactly what you think about the issue.

Furthermore, a team of scientists from the University of California San Francisco have written a letter to the White House warning that the scanners present – above and beyond the risks to the general population – "potential serious health risks" to certain segments of society, such as the elderly and the pregnant.

"There is good reason to believe that these scanners will increase the risk of cancer to children and other vulnerable populations," say the cosigners of the letter, which include experts in biochemistry, imaging, X-rays and cancer research. "We are unanimous in believing that the potential health consequences need to be rigorously studied before these scanners are adopted."

The backscatter X-ray technology used in airport security scanners penetrates the skin only about 1/4 inch before the rays are scattered, whereas medical X-rays transmit completely through the body. The TSA has determined, therefore, that the amount of radiation emitted from the airport scanners is significantly less than at the doctor's office.

The University of California scientists, however, disagree.

"The X-ray dose from these devices has often been compared in the media to the cosmic ray exposure inherent to airplane travel or that of a chest X-ray," the professors' letter states. "However, this comparison is very misleading: Both the air travel cosmic ray exposure and chest X-rays have much higher X-ray energies, and the health consequences are appropriately understood in terms of the whole body volume dose. In contrast, these new airport scanners are largely depositing their energy into the skin and immediately adjacent tissue, and since this is such a small fraction of body weight/volume, possibly by one to two orders of magnitude, the real dose to the skin is now high."

 The professors are calling on the administration to specifically reexamine potential risks to the following groups:

• Older travelers, those greater than 65 years of age, who may be at particular risk from the mutagenic effects of the X-rays;
• A fraction of the female population especially sensitive to mutagenesis-provoking radiation leading to breast cancer, women typically exempted from X-ray mammograms, for example;
• The population of immuno-compromised individuals, such as HIV and cancer Patients;
• Children and adolescents;
• Pregnant women and their unborn children;
• And men in general, because of the proximity of the testicles to skin, which is most highly effected by the backscatter rays.

The TSA claims that the machines' safety has been evaluated by the Food and Drug Administration's Center for Devices and Radiological Health, the Commerce Department's National Institute for Standards and Technology and the Johns Hopkins University Applied Physics Laboratory.

"In summary, the potential health risks from a full-body screening with a general-use X-ray security system are minuscule. Several groups of recognized experts have been assembled and have analyzed the radiation safety issues associated with this technology," the FDA states. "As a result of these evidence-based, responsible actions, we are confident that full-body X-ray security products and practices do not pose a significant risk to the public health."

When New York Times reporter Susan Stellini called these research organizations to ask about their evaluations, however, she discovered the machines were primarily tested for whether the amount of radiation emitted meets guidelines established by the American National Standards Institute, an organization she suspects may be operating with a conflict of interest.

"Guess who was on the committee that developed the guidelines for the X-ray scanners? Representatives from the companies that make the machines and the Department of Homeland Security, among others," Stellini writes. "In other words, the machines passed a test developed, in part, by the companies that manufacture them and the government agency that wants to use them."

Both Rez and the team from University of California have also brought up yet another "red flag" with the airport scanners.

"The scary thing to me is not what happens in normal operations, but what happens if the machine fails," Rez told the TImes. "Mechanical things break down, frequently."

"Because this device can scan a human in a few seconds, the X-ray beam is very intense," the California professors' letter states. "Any glitch in power at any point in the hardware (or more importantly in software) that stops the device could cause an intense radiation dose to a single spot on the skin. Who will oversee problems with overall dose after repair or software problems?

"The TSA is already complaining about resolution limitations; who will keep the manufacturers and/or TSA from just raising the dose, an easy way to improve signal-to-noise and get higher resolution?" the professors continue. "Lastly, given the recent [underwear bomber incident], how do we know whether the manufacturer or TSA, seeking higher resolution, will scan the groin area more slowly leading to a much higher total dose?"

The scientists' letter, addressed to Dr. John P. Holdren, assistant to the president for science and technology, concludes, "We urge you to empower an impartial panel of experts to reevaluate the potential health issues we have raised before there are irrevocable long-term consequences to the health of our country. These negative effects may on balance far outweigh the potential benefit of increased detection of terrorists."

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Nov. 22, 2010