Radiation in Passenger Screening: busting the myths

Thousands of passengers go through airport security checkpoints daily. We are all very familiar with the traditional search for metallic threats and weapons through the common use of Walk-Through Metal Detectors (WTMD) or Hand-Held Metal Detectors (HHMD) combined with physical pat-down or body search. But since the 2009 ‘Underwear Bomber’ incident, and the subsequent disruption of other plots and the interruption of chatter which have indicated the potential use of non-metallic threats by terrorists, the application of Full-Body Scanners, Whole Body Imagers (WBIs) or Advanced Imaging Technology (AIT) has become increasingly prevalent. Amir Neeman examines these technologies and evaluates the risks related to their usage in an airport screening environment compared to their security and operational benefits.

ASI Radiation1While some Advanced Imaging Technology (AIT) systems are based on X-ray transmission, others rely on X-ray Backscatter and most of those in regular use are based on millimetre wave. The widest deployment of such AITs are by the Transportation Security Administration (TSA) in U.S. airports.
 
AITs allow airport security personnel to effectively detect threats and contraband (with the potential to be concealed within or beneath clothing) without the physical contact necessitated by a physical pat-down search.

While AITs are undoubtedly more effective in the detection of a broader range of threats than the more traditional screening technologies, such as magnetometers, they have also generated much more controversy. One key concern is the public safety implication of exposing passengers, aircrew and airport employees to frequent exposure to potentially harmful electromagnetic and ionising radiation. But are such concerns based on fact or are they just the result of scaremongering by the media and civil libertarians?

X-ray Backscatter AIT
OSI Systems (Rapiscan), American Science and Engineering (AS&E), and Tek84 Engineering Group are the leading manufacturers of the backscatter systems for passenger screening. This technology generates small amounts of X-rays which reflect off passengers’ skin placed in front of the scanner. The scattered ionising energy of the X-rays is then picked up by backscatter detectors and processed to produce a two-sided image. The resulting image is then converted into a cartoon-like body outline – an avatar – with potential threats highlighted on it. Traditional backscatter scanners require one operator to direct the passenger through the scanner – a process taking around 15 seconds – while another operator analyses the image.

Unlike mobile phone signals, or millimetre-wave scanners, the energy emitted by a backscatter X-ray is a type of ionising radiation that breaks chemical bonds. Ionising radiation is considered carcinogenic even in small doses, but at the doses used by backscatter scanners this effect is believed to be negligible for an individual.

While X-rays used for medical imaging penetrate through the body, X-rays used in airport full-body scanners mostly interact with or near the surface of the skin. Several key radiation safety studies were conducted between 2009 and 2011 by the U.S. Food and Drug Administration’s Center for Devices and Radiological Health (CDRH), Rapiscan’s Third-Party Radiation Testing group, the National Institute of Standards and Technology’s (NIST) Office of Law Enforcement Standards, and Johns Hopkins University Independent Assessment group. Each assessment proved the effective dose rate to be below the American National Standards Institute/Health Physics Society’s standard annual dose limit of 250 μSv over a 12-month period.
 
The effective dose estimates from a single scan range from 0.015 μSv to 0.1 μSv. To put these numbers into perspective, air travel (depending on route and altitude flown) can expose a passenger to 0.04 μSv per minute from cosmic radiation. Another perspective is that a passenger would have to pass through a backscatter scanner 1000–2000 times to equal the dose from a medical chest X-ray; alternatively, consider one backscatter scan being equivalent to the dose received from three to nine minutes of normal daily living from background radiation!

The TSA operators themselves typically receive less than 100 μSv per year, which is well below the Occupational Safety and Health Administration’s occupational safety health limit of 50,000 μSv per year.
In 2012 the European Commission had a similar study conducted by the Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR). The Committee found that the effective dose, which takes into consideration the type of radiation and the sensitivity of the body parts exposed, is the best parameter to assess the health risk from ionising radiation.

The effective doses per scanned passenger are 0.02 to 0.1 µSv for backscatter systems and 0.1 to 5.0 µSv for transmission. The organ doses have generally the same order of magnitude. For persons scanned three times every working day, security scanning would result in an incremental effective dose of approximately 300 µSv (0.3 mSv) per year for the backscatter technique and close to 3 mSv per year for the transmission technique (assuming doses of 0.4 and 4 µSv per scan, respectively). The latter would exceed the dose limit for the general public and hence would not comply with the current radiation protection standards for the very extreme of the most frequently screened and therefore highest exposed group. The former remains within the range characterised as negligible by the US National Council on Radiation Protection and Measurements (NCRP). The potential magnitude of cancer risk from doses received from AITs cannot be estimated, but is likely to remain so low that it cannot be distinguished from the effects of other exposures including both ionising radiation from other sources (including natural) and background risk due to other factors.

Another reason why scepticism still surrounds backscatter units is because of studies which, by their very nature, make the general public question their safety. Some of the studies have been less than rigorous to put it mildly and should not be regarded as providing definitive proof of the conclusions reached; Marquette University’s College of Engineering (Milwaukee, Wisconsin) conducted a study concluding that ionising radiation emitted from backscatter scanning devices extends to organs deeper than the skin, but even that study found that the dose of ionising radiation was still lower than the established health standards.