As millions of U.S. travelers get ready for the busiest flying day of the year, scientists still can’t agree over whether the dose of radiation delivered by so-called backscatter machines is significantly higher than the government says. This is despite months of public debate between the White House, the U.S. Food and Drug Administration, and independent scientists.
Full-body scanners have been installed at many U.S. airports. The machines use either low-energy, millimeter wavelength radiation, which is harmless, or X-rays, which can potentially be hazardous. X-rays can ionize atoms or molecules, which can lead to cancerous changes in cells. Even if the government has significantly underestimated the dose of radiation delivered by an X-ray scanner, it is likely to be relatively small.
The low-energy X-rays emitted by the second type of scanner—also known as a backscatter machine—can pass through clothing but not skin or metal. This makes it possible to spot concealed weapons or explosives, although it also reveals a person, essentially, in the nude. To address this, the U.S. Transportation Security Agency is working on software that converts an image of person into a stick figure, or a blob, without obscuring objects that might pose a security threat. Passengers can also opt to be frisked instead of scanned.
In April, four scientists at the University of California, San Francisco, wrote a public letter to the White House warning that the government may have underestimated the dosage of ionizing radiation delivered to a person’s skin from a backscatter machine by one or two orders of magnitude. The scientists, who have expertise in biochemistry, biophysics, oncology, and X-ray crystallography, pointed out that the government’s estimate was based on radiation exposure for the entire body. During scanning, the majority of radiation will be focused on the surface of the body, meaning a more concentrated dose of radiation is delivered to the skin.
The Health Physics Society has worked with the FDA to determine the safety of backscatter machines. Spokeswoman Kelly Classic says a dummy made of acrylic is used to measure exposure to ionizing radiation. Sensors attached to the surface of the dummy determine the dose of radiation a person would get from the machine.
The FDA asserts that its method is correct. “This is how we measure the output of X-ray machines and how we’ve done it for the past 50 years,” says Classic.
But backscatter machines do function differently than most medical imaging systems, and some scientists say the information presented by the government may have underestimated the safety risks. Glenn Sjoden, a professor of nuclear and radiological engineering at the Georgia Institute of Technology, says comparing 1,000 backscatter X-rays to one chest X-ray (as the government’s estimates do) doesn’t accurately describe of the risk.
When a person is exposed to higher-energy radiation, it passes through the body delivering a similar dose of radiation throughout. The energy of the backscatter X-rays is lower than in a chest X-ray, which means it stops at the skin. “It is not a good analogy,” says Sjoden. “Because of the physics being applied in the backscatter devices, your whole body is not getting the dose.” Instead, he says, it’s mostly the skin getting the dose.
Sjoden says it should be possible to measure a full body dose of radiation from a backscatter machine and extrapolate the dose given to the skin. Both he and the USCF scientists are also concerned that backscatter machines could accidentally give someone a higher dose of radiation, or that a TSA employee could increase the dose to get a better view of a person.
However, Sjoden isn’t as worried that the estimated exposure is dangerously wrong. “You would have to be a heavy traveler to accumulate a large dose.”
But Sjoden still questions whether enough studies have been done to determine the risk across the population, especially since children, the elderly, and some people with genetic predispositions to cancers are more sensitive to the effects of radiation.
“My initial gut reaction is, that’s a lot of work to do,” said Sjoden. “Whether or not it’s been thoroughly investigated, I was not able to tell from what’s out there.”
Ed Nickoloff, professor of radiology at Columbia University and chief hospital physicist at Columbia University Medical Center, says the data isn’t yet clear either way. “At this point, until I knew more information, I’d tell people to take the pat-down,” he says.
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