To answer reader questions, I've put together a short Q&A; on the topic of cancer risk and whole-body scanning machines.

The following info is a summary of published reports from respected news sources about how these scanners ought to work in theory. I'm not discussing the specific equipment of any manufacturer.

What kind of machine does the TSA use?

The TSA has deployed two types of scanners. One type uses X-rays and emits some ionizing radiation. These machines are called "backscatter" scanners. The other type of machine uses high-frequency radio wave technology, does not emit ionizing radiation, and produces fuzzier images.

The TSA has 40 whole-body scanners in place. It has purchased 150 more. It intends to buy at least 300 additional ones soon, according to the Associated Press. A majority of these scanners will likely be "backscatter," or X-ray, machines.

How do the TSA scanners work?

A "backscatter" machine shoots a bean of X-rays at a traveler. Then the machine measures what bounces back from your body. It uses that information to create a two-dimensional image of what's underneath your clothes.

Is radiation involved?

As summarized by The New York Times, "The X-rays are a form of ionizing radiation, that is, radiation powerful enough to strip molecules in the body of their electrons, creating charged particles that cause cell damage and are thought to be the mechanism through which radiation causes cancer."

How many additional cancer deaths might be caused by the machines that would not have happened otherwise?

The Times' interviewed Arjun Makhijani, a nuclear physicist, who said that if a billion passengers were screened with the dose assumed by researchers, "that would mean 10 more cancer deaths a year."

No one knows for sure, primarily because it is unethical to perform potentially fatal experiments on people. It is also difficult to measure the effects of "a dose of radiation equivalent to 1 percent or less of the radiation in a dental X-ray."

On the other hand, the TSA says, according to the AP, that "the radiation is equivalent to what a person gets in two minutes of flying on an airplane." In other words, when you fly in a plane, you are exposed to more solar radiation than you are on the Earth's surface because the earth's protective atmosphere is thinner at higher altitudes. (Get details at the TSA website.)

The Times quoted a spokeswoman for the Transportation Security Administration, Kristin Lee, saying

that even for pregnant women, children and people whose genetic makeup made them more susceptible to X-ray damage, "It would take more than 1,000 screenings per individual per year" to exceed radiation standards.

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