Several years ago, the Transportation Security Administration (TSA) began installing and using advanced imaging technology (AIT) at airport passenger screening checkpoints as a secondary measure to detect security threats. By early 2010, AIT was widely implemented as a primary measure because AIT is more effective at detecting weapons, explosives, and other hazardous and/or concealed items hidden under clothing than older metal detector-based screening units. The two main types of AIT used are “backscatter” models, which use low levels of ionizing radiation, and “millimeter wave” models, which use radio waves.
Debate centers on exposure to ionizing radiation from backscatter screening. Although, backscatter units use extremely low levels of ionizing radiation, concern exists that any increase exposure to radiation is biologically dangerous. Although few data exist about the safety of millimeter wave scanners, they are not believed to have carcinogenic potential, so the focus of this discussion is on backscatter screening.
Ionizing radiation refers to radiation that has sufficient energy to ionize atoms or molecules (cause separation of electrons from an atom) in biological systems. The electrons and positively-charged ions released as a result of ionization can cause cellular damage. X-rays, gamma rays, beta particles (high-speed electrons), neutrons (heavy uncharged particles), and alpha particles (heavy charged particles) are the principal types of ionizing radiation encountered. Of these types, x-ray and gamma rays have the lowest rate of energy transfer.
Backscatter scanning units direct an x-ray beam over the surface of the body; the x-rays are low intensity, and therefore do not travel deep into tissues or through the body as those of a medical x- ray would. Instead, the majority of the rays are reflected back from the skin. Detectors translate the reflection pattern into an image that is examined by security personnel. The backscatter pattern is dependent on material property, and thus distinguishes between organic and inorganic materials.
The imperatives raised in these resolutions are diminished somewhat based on the U.S Transportation Security Administration’s (TSA) decision in early 2013, to remove the backscatter models from U.S. airports by June 2013 and replace them with millimeter wave models. This followed an October 2012 announcement that the TSA had removed backscatter scanners from the majority of large airports, placing them in smaller airports. According to news reports, the backscatter models removed from airports in 2013 will likely be placed in federal buildings and other locations in which security measures are needed instead. However, depending on the frequency of exposure for employees and visitors of locations in which the backscatter units may eventually be placed, the concerns raised in the Council resolutions continue to warrant examination.
In conclusion, no studies have demonstrated negative health effects in passengers scanned by backscatter units, and the cancer risk from exposure appears to be miniscule. The Council on Science and Public Health believes that no data currently exist to suggest
that passengers should avoid being screened by backscatter scanners. However, it supports continued research on the safe use of the scanners, as well as maintenance, calibration, survey, and officer training procedures that are meant to ensure that the units operate as intended. The Council notes that passengers who do not wish to undergo backscatter screening may opt for alternative screening. The Council also notes that no adverse health consequences are known to occur from millimeter wave models that have replaced the backscatter models.