Since the attempted explosion of an airliner as it was landing in Detroit on Christmas Day by an alleged terrorist from Nigeria, global air safety experts have been scrambling to enact new safety measures. A quick answer has come in the form of whole-body scanners that use low-level radiation to allow screeners to see through clothing to identify hidden weapons or explosives.
These things have been around for a while, but, outside of a few pilot locations, haven’t really gained much attention until now. Given the circumstances of the Detroit incident, we shouldn’t be surprised that airports all over the world are rushing these systems into use.
Most of the press has been concerned with privacy issues—the systems essentially strip passengers naked—and focused far less on health safety matters. Still, the safety issue is starting to engage the public. Recent news reports have suggested the new scanners are basically safe. But a more nuanced look at the question suggests the answers are not yet all that clear.
There are two technologies in use in the U.S.: Backscatter technology uses x-rays delivering less than 10 microrem of radiation per scan, equivalent to the radiation one receives inside an aircraft flying for two minutes at 30,000 feet, according to the American College of Radiology. Another approach relies on millimeter-wave technology, which uses radio waves in the millimeter-wave spectrum. Two rotating antennae cover the passenger from head to toe with low-level radiofrequency energy.
The ACR said it was not aware that either of the scanning technologies that the Transportation Security Agency is considering would present a significant biological threat for passengers screened. Indeed, ACR chair Dr. James Thrall was quoted on ABC news as saying, “the individual x-rays themselves are very low energy. And unlike the x-ray spectrum that we use in medicine, the backscatter x-rays don't really penetrate to the organs in the body.” Click here for the article.
The ABC article also had some useful comparisons: If, after a body scan, a passenger had four hours of flying and a two-and-a-half hour layover in Denver, given the increased proximity to the sun from the high altitudes, the scan would be equal to about one seventieth of the overall radiation exposure.
OK, so far so good for backscatter radiation. It’s an ionizing radiation risk, but certainly smaller than the risk from the plane flight that follows. What about the millimeter-wave technology that operates in a different part of the spectrum?
The National Council on Radiation Protection & Measurements, which vouched for the ACR’s position on the backscatter radiation scanners, hasn’t reached a conclusion on the millimeter-wave technology, its president, Thomas Tenforde, Ph.D., told Diagnostic Imaging. The NCRP, which operates under a Congressional charter, would like to take a look at the new technology, but hasn’t had the opportunity so far.
Millimeter-wave scanners are probably within bounds, Tenforde said, but there should be an effort to verify that they are safe for frequent use. According to Tenforde, standards have been established for RF exposures up to 300 gigahertz, but millimeter-wave technology may operate outside those established standards, and potential bioeffects need to be evaluated.
Also, one study by a group at Los Alamos National Labs argues that terahertz radiation, which, although in the RF spectrum, is much higher than the 300 GHz level, does have bioeffects. The study also cites other papers finding bioeffects from exposure to the THz spectrum. The Los Alamos researchers say that while the forces generated are tiny, resonant effects could allow THz waves to unzip double-stranded DNA, creating bubbles in the double strand that could significantly interfere with processes such as gene expression and DNA replication. The study describes a model for assessing the bioeffects of THz radiation and is drawing attention from critics of the whole-body scanners.
A Wikipedia site devoted to the millimeter-wave scanners says they operate just below the THz radiation range, and does include, under the heading “possible health effects,” a short description of the Los Alamos research.