Airport body scanners pose no significant radiation threat, even to frequent flyers, who are exposed to far more radiation during travel at high altitudes, authors of a review concluded.
The scanners expose people to less than 1% of the radiation associated with cosmic rays at typical flight altitudes. A single exposure to a backscatter x-ray scanner is equivalent to three to nine minutes of radiation encountered in normal daily living.
Nonetheless, deployment of whole-body scanners at airports should not proceed in the absence of definitive studies to determine more precisely the risks and benefits, according to an article published online in Archives of Internal Medicine.
"In medicine, we try to balance risks and benefits of everything we do, and thus while the risks are indeed exceedingly small, the scanners should not be deployed unless they provide benefit-improved national security and safety -- and consideration of these issues is outside the scope of our expertise," Rebecca Smith-Bindman, MD, and Pratik Mehta, of the University of California San Francisco, wrote in conclusion.
"Issues have been raised regarding the efficacy of scanners, and if the scanners are not deemed efficacious, they should not be used."
On Dec. 25, 2009, a Detroit-bound passenger smuggled plastic explosives aboard an airliner, revealing a limitation of current airport screening devices. Since then the Transportation Security Administration has installed almost 500 whole-body scanners at 78 U.S. airports, and twice that many devices are expected to be in operation by the end of 2011, the authors wrote in their introduction.
Two types of scanners are in use. The millimeter-wave scanner emits low-energy waves estimated as a fraction of the energy emitted by a cell phone. The more commonly used backscatter x-ray scanner emits low-dose x-rays, which are absorbed entirely by the most superficial layers of the skin, the authors continued.
Although the detailed images generated by both types of scanners have raised privacy issues, the potential health risks center on the x-ray scanners, which employ ionizing radiation.
But the radiation doses emitted by the scanners are so low that the potential risks are unknown and difficult to quantify, the authors wrote.
Individuals in the U.S. are exposed to an estimated 6.2 millisieverts of ionizing radiation each year, an amount equivalent to 0.1 microsievert (µSv) per minute, according to the National Council on Radiation Protection and Measurements. The two most common sources of radiation are medical procedures and environmental background radiation.
Backscatter whole-body scanners expose individuals to 0.03 to 0.1 µSv per scan, the equivalent of three to nine minutes of radiation from natural sources.
Levels of naturally occurring radiation are increased at higher altitudes, such as those used by airliners. Although the levels change with altitude, radiation exposure during a flight averages about 0.04 µSv per minute of flight time. Thus, backscatter x-ray scanners deliver an amount of radiation equivalent to one to three minutes of flight time.
"Put into context of the entire flight, if a woman embarks on a six-hour flight, she will be exposed to approximately 14.3 µSv of radiation from the flight and 0.03 to 0.1 µSv from passing through the scanner at the airport," the authors wrote. "Thus, the scan will increase her exposure by less than 1%."
Given those calculations, concerns that vulnerable individuals should avoid the scanners are unwarranted, they added.
Offering other common examples for context, Smith-Bindman and Mehta noted that a person would have to pass through an airport scanner 50 times to get the same radiation exposure associated with a single dental x-ray, 1,000 times to equal the exposure of a chest x-ray, 4,000 times to equal the exposure of a mammogram, and 200,000 times to equal the exposure of a single combination abdominal-pelvic CT scan.
Estimating the cancer risk associated with airport scanners is even more difficult than quantifying the exposure, the authors continued.
Risk estimates normally rely on extrapolation from published studies of higher-dose exposures. Such extrapolation from the scanners' exceedingly small radiation doses is questionable and perhaps inappropriate.
Radiation exposure from the scanners is concentrated in the skin. No accepted mathematical models exist for determining the relationship between skin exposure and the risk of skin cancer. Moreover, the distribution of exposure differs from that of the whole-body exposure assumed by available mathematical models.
Noting that 100 million people have a total of 750 million flights per year, Smith-Bindman and Mehta estimated that radiation exposure from airport scanners would cause six excess cancers.
In contrast, 40 million cancers would be expected over the same individuals' lifetimes.
Frequent flyers represent one population potentially vulnerable to radiation exposure from airport scanners. Assuming one million of these passengers take 10 six-hour trips per week for a year, airport scanners would cause four cancers.
That compared with an estimated 600 excess cancers from radiation exposure during the flights and 400,000 cancers over the passengers' lifetimes.
Young children who fly frequently are another potentially vulnerable population. Using a five-year-old girl as an example, the authors estimated that two million girls flying once a week would have one excess breast cancer. In contrast, 250,000 of the girls will develop breast cancer over their lifetimes owing to the 12% lifetime risk of the disease.
"Based on what is known about the scanners, passengers should not fear going through the scans for health reasons, as the risks are truly trivial," the authors wrote in conclusion.
"If individuals feel vulnerable and are worried about the radiation emitted by the scans, they might reconsider flying altogether since most of the small, but real, radiation risk they will receive will come from the flight and not from the exceedingly small exposures from the scans."
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