Airport security scanners arouse intense controversy

May 6, 2010

An entire industry has grown up around airport security, aimed at the estimated two billion passengers taking flights every year. This industry grows explosively.

An entire industry has grown up around airport security, aimed at the estimated two billion passengers taking flights every year. This industry grows explosively. Security already accounts for 35% of the operating costs at European airports, compared with just 8% in 2001. More than 40% of airport employees now are security- related staff. It is a multibillion-euro business, a self-propelled, surefire success.1

The latest addition is digital whole-body imaging of passengers, and this has been introduced at a number of European and American airports. Three different kinds of scanners are being used, all of which virtually strip-search the subject being imaged:
• active terahertz scanners
• passive terahertz scanners
• x-ray scanners
• x-ray scanners

Both terahertz-wave and x-ray scanners use backscatter techniques. Backscatter is the part of radiation or waves that has a scattering angle higher than 90º. It is scattered backward, as the name suggests, away from the irradiated object.

Terahertz (tHz) radiation is also known as submillimeter radiation or t-rays. The region of the electromagnetic spectrum from 0.1 to 10 tHz is a frontier area for research in physics, chemistry, biology, materials science, and medicine. Terahertz radiation lies in the frequency gap between infrared and microwave radiation and, unlike x-rays, it is nonionizing. What makes these waves so fascinating to scientists is their ability to penetrate materials that are usually opaque to both visible and infrared radiation.2

The biological effects of terahertz radiation are largely unknown. This form of radiation is not believed to harm DNA or tissues, though this claim has been questioned.3 In the field of radiation protection, no exposure is considered “justified” unless it produces a positive net benefit.4,5 As such, the responsible authorities should insist on proper research into the possible biological effects of terahertz radiation. Until that work is done, active terahertz scanners should not be used on humans.

The second class of scanners also uses terahertz radiation, but unlike the active scanners they work in a passive mode and do not emit any radiation. These machines receive natural radiation that emanates from the subject being examined. The concept is similar to Airport security scanners arouse intense controversy medical thermography (in other words, imagination might help). Don't expect to see much.

The third technique, x-ray backscatter scanning, seems to be the most efficient virtual search technique. A pencil-shaped x-ray beam is used to determine the spatial resolution of the final image and backscattered Compton photons are turned into images by large detectors. The machines on the market operate between 50 kVp and 125 kVp, with 1 to 1.5 mm of aluminum filtration.6,7 Around 25 kVp is sufficient for x-ray mammography.

Standards governing x-ray scanners for security applications are different from those for medical applications. Radiation protection requirements are not as strict as those for diagnostic or therapeutic medical devices.

X-RAYS THAT BOUNCE BACK

The new scanners had to be sold to the public as absolutely harmless. “The x-rays do not penetrate the body but bounce off the skin,” according to Johns Hopkins physicist Mahadevappa Mahesh, writing in the British Medical Journal.8 Similar statements could be found in numerous newspaper articles, and even on the website of the American College of Radiology.9

When the x-ray backscatter devices initially appeared, the message from proponents was that the x-rays would penetrate passengers' clothing, hit the skin, then fly back to the detector, a kind of biological reflection. Hidden guns, explosives, and liquids would be revealed on the x-rays' return journey. The equipment would expose individuals to a negligible x-ray dose and a “miniscule” increase in risk of cancer, according to manufacturers and government agencies.

The x-rays bounce back just as some raindrops bounce back from the surface of the ocean: some do, some don't. White lies remain lies. The question is: Who is accountable for these deliberately incorrect statements and why are they made?

Why these x-rays should be stopped or reflected by the surface of the skin remains an open question. We all know that x-rays penetrate the skin. Even at low doses they can travel through the entire body quite easily, for instance, in the case of infants and children. If you look at x-ray backscatter images, you will see some bones, air-filled spaces, and perhaps breast implants, all of which are behind the skin.

Leon Kaufman, one of the leading medical physicists in the development of clinical MRI equipment in the 1980s, published a short comment on these devices and the way that tens of millions of air passengers will be forced to undergo examinations. He asks whether airport body x-ray scanners are a great public health experiment.

“Contrary to every policy enacted since the effects of radiation were understood, we are engaging in a mass experiment of irradiation of not just adults, but also pregnant women, fetuses, children, women of child-bearing age, men who may conceive after exposure-in short, everyone unfortunate enough to have to be in an airport. The results will not be known for 20 years,” Kaufman wrote.10 “Imagine if, as an investigator at [University of California, San Francisco], I had asked my [institutional review board] to allow me to carry out an experiment involving indiscriminate exposure to x-rays of a randomly selected population with no informed consent.”

In addition to the x-ray luggage inspection machines that expose passengers and personnel to ionizing radiation,11 there is now an additional source of ionizing radiation at airports.

The U.S. Food and Drug Administration (FDA) does not oppose the detectors. Yet they do seem concerned about radiation doses from medical imaging, having announced their intention to develop and disseminate “patient medical imaging history cards” that will keep track of a patient's accumulated radiation dose.

“The amount of radiation Americans are exposed to from medical imaging has dramatically increased over the past 20 years,” said Jeffrey Shuren, M.D., J.D., director of the FDA's Center for Devices and Radiological Health. “The goal of the FDA's initiative is to support the benefits associated with medical imaging while minimizing the risks.” 12

The risk of cancer from a single backscatter check is admittedly low. The cumulative risk of repeated exposure to radiation may, however, be a public health threat, in particular for people working in the airline industry and frequent travelers. The risk of a terrorist attack seems far lower. Even those in favor of checking air passengers with ionizing radiation concede that you are more likely to die from their controls than from a terrorist act.

Manufacturers of this new scanning equipment appear to be aware of the potential problems and have been shifting responsibility upwards. As one vendor has observed: “It should be noted here that this is a risk imposed on members of the public by government and that those who are at risk may not be those who benefit.”13

The manufacturers also concede that the radiation penetrates the human body and is not bounced off: “While surely there are misunderstandings as to how x-ray backscatter personnel scanners function, it is true that these scanners utilize ionizing radiation, and that the x-rays emitted from them penetrate skin, as well as clothing.”14

I foresee class actions. We will see collective lawsuits from flight attendants, pilots, frequent flyers, and operators of airport x-ray machines who will claim that their leukemia or multiple myeloma, or their deformed child, is due to the radiation they were exposed to. They will sue the airport operators, the airlines, and the manufacturers. Let's see whether at this point the machines will be scrapped.

Even if the dose is considered trivial, remember the maxim: Better safe than sorry. I have no idea who is right and what will eventually happen. However, one should minimize risks.

I am also concerned that somebody could be tempted to play with the machines. “Let's try increasing the patient entrance dose a little bit to get a sharper picture.” Who vouches and checks? Will the scanners be totally safe in hands that are not completely trustworthy?

DISCLAIMER

I have nothing against security controls. I have been through them in many countries. They were all performed by concentration camp-type guards (who regard all passengers as potential enemies and killers) or by friendly professionals.

I am not aware of anybody in Europe or North America who has been killed in the air by a terrorist after airport security controls were tightened following the attack on the World Trade Center in New York. This shows that airport screening is quite sufficient as it is today. No such system will ever be foolproof. However, the principle of proportionality between control and genuine threat was left behind a long time ago. More than 3000 people in the U.S. are killed by fires at home every year. Working smoke alarms would reduce the likelihood of such fatalities considerably and would be a worthwhile investment. For the price of one airport scanner you could buy 5000 smoke detectors.

The practice of scanning passengers will satisfy politicians' have-to-do-something urge, despite the futility of this strategy. This just glosses over a problem without dealing with it properly. It is like treating cancer with aspirin.

As far as the usefulness of the checks, consider the views of U.S. lawyer Michael Dorf. “In the end, the best argument for implementing x-ray backscatter scans at airports may appeal more to the emotions than to hardheaded calculations of costs and benefits,” he concludes at the end of a lengthy article on the topic.15 “Despite statistics showing that air travel is substantially safer than automobile travel, many people are jittery about flying, even without the worry of potential terrorism. Full-body scans would be a way to give millions of travelers some added peace of mind. That is not quite the same thing as making air travel safer, but it is a real benefit nonetheless.”

My thoughts exactly. Peace of mind is most likely to be the only benefit that these machines will have for the general public. The powers that be think differently.

Editor's note: For a light-hearted take on airport scanners, click here to read A Dose of Dubbins.