With the TSA putting more and more full-body scanners in U.S. airports, experts are battling over the potential risks for human health.
But a new study published online yesterday by the Archives of Internal Medicine suggests that backscatter x-ray scanners -- one of two types of scanners in use and the only kind to emit actual radiation -- are extremely low-risk.
Dr. Rebecca Smith-Bindman, an expert in Radiology and Biomedical Imaging at the University of California, San Francisco, coauthored the study.
"The radiation exposure is pretty darn small," she said. "You'd have to go through the scanner 200,000 times to get the dose of one CT scan."
The TSA reported earlier this month that the full-body scanners are safe, however the agency's statements were accused of being biased and received with skepticism.
Smith-Bindman, an independent researcher, said she undertook the study in part because she is a regular flyer and had been avoiding the scanners, opting for pat-downs instead. She and her coauthor wanted quantifying both the dose of radiation emitted by the backscatter scanners and the potential harm, considering the link between radiation exposure and cancer risk.
According to a recent report from the National Council on Radiation Protection and Measurements, Americans are exposed to an average of 0.01 microsieverts (a unit used to measure radiation) per minute. The most common sources are the sun and radon released from the earth.
According to the study's authors, backscatter x-ray scanners expose people to 0.03 to 0.1 microsieverts per scan, which they say is equal to three to nine minutes of radiation you'd get in the course of daily living.
They add that travelers are exposed to far more naturally occurring radiation in-flight because of the proximity to the sun. (The TSA estimates that the scanners result in the equivalent exposure each person receives in about two minutes of flight.)
Dr. David J Brenner, director of Columbia University's Center for Radiological Research, agreed that the radiation risks from scanners are small. But he said that when you consider the possibility of 700 million scans per year (the estimated frequency of flights in the U.S.) there's a justifiable concern about the overall population risk.
"It's a very small risk, yes, but there's a very large number of people exposed to that risk," he said. "It's like the lottery. If you buy a ticket, your individual 'risk' of winning is very small. But someone is going to win."
He also mentioned that an alternative method exists -- the so-called millimeter wave machine scanner. The machine takes a comparable picture and costs around the same, but releases no radiation; instead, it bounces harmless electromagnetic waves off a person's body.
"Somebody is going to end up with cancer," Dr. Brenner said, "And if it were the case that there was only one technology around, you could argue that these risks are pretty uncertain and certainly, it's better to have safe skies. But there is another technology, so why wouldn't you use it?"
Smith-Bindma's research focused on estimating the cancer risks associated with very low doses of exposure. (She admitted the models they used were imperfect, but the only ones available.) They zeroed in on several hypothetical high-risk groups -- among them, 1 million frequent fliers who take 10 roundtrip, cross-country flights per week.
They found that of those 1 million frequent fliers, 600 cancers could be associated with radiation received from flying so close to the sun, and four cancers could stem from the scanners. Smith-Bindman cautioned that those numbers must be considered in the context of the estimated 400,000 cancers that could occur to those 1 million frequent flyers over the course of their lifetimes.
The researchers also modeled the cancer risks associated with a hypothetical group of 2 million, 10-year-old girls who travel every weekend to see their parents, flying 52 times per year.
"Of those 2 million," Smith-Bindman said, "One additional breast cancer could occur over their lifetimes. But that's within the context of the 250,000 of them that will get breast cancer just because they're girls."
She concluded that as best as they could estimate, the cancer risks associated with backscatter scanners are truly trivial.
Lauren Gaches, a TSA spokeswoman, said the agency "competitively bids technologies" and makes its scanner selections after extensive research and testing.
"By investing in multiple supplies of imaging technology solutions," she said, "TSA is able to leverage competition to procure systems, which provide the best value to the government."
She added that both backscatter and millimeter wave technologies are "safe" -- saying the FDA, the National Institute for Standards and Technology, and the Johns Hopkins Applied Physics laboratory have all tested them.
But while Smith-Bindman agreed the risks are minimal, she concluded her report by advocating for more testing.
"I'm convinced they're safe," said Smith-Bindman. "But I think it would be prudent for the TSA to permit further independent testing of them, for due-diligence."