Use of full body scanners at airportsBMJ 2010; 340 doi: http://dx.doi.org/10.1136/bmj.c993 (Published 24 February 2010) Cite this as: BMJ 2010;340:c993
- Mahadevappa Mahesh, chief physicist and associate professor of radiology
- 1Russell H Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, 601 N Caroline Street, Baltimore, MD 21287-0856, USA
Since the attempted bombing of an aeroplane bound for the United States on Christmas Day 2009, several countries have made or are in the process of making a decision about mandatory use of full body scanners at airports. “Full body scanners” or “whole body scanners” can be classified as either “millimetre radio wave” or “backscatter” technologies.
Millimetre radio wave systems scan travellers by bombarding them with radio waves and collecting the reflected radio waves via antennae to generate an image.1 This technology does not use x rays. In contrast, backscatter systems use low intensity x rays to scan the body. The x rays do not penetrate the body but bounce off the skin, and are then captured by detectors to create images. These x rays are useful for detecting objects hidden under clothing and taped on the skin but not for detecting objects hidden inside the body.2 For this, transmission x ray systems are needed.2 The table⇓ lists the doses of radiation produced by backscatter systems and the number of backscatter scans needed to yield an equivalent dose to that of a chest x ray and other radiation sources.3 4 5 6 7 8
A typical backscatter scan takes about eight to 15 seconds to perform and provides two images—front and back.2 For the past few years, full body scanners have been in use as secondary screening devices at various airports, including Heathrow Airport in London, and travellers have been allowed to opt-out. With increased emphasis on airport security, however, mandatory screening of travellers with full body scanners may soon become routine.
Several important concerns exist regarding full body scanners—biological risks to travellers and concerns about privacy, the longevity of images, and the stability of scanners. In this context, it is recommended that radiation doses from backscatter systems should not exceed 0.1 µSv, and the doses measured have been reported to be between 0.05 μSv and 0.1 μSv per scan.2 3
A person would therefore have to undergo 1000-2000 backscatter scans before receiving a dose equivalent to a medical chest x ray (100 μSv).4 The dose of radiation from a single backscatter scan is equivalent to that received from less than 30 minutes of background radiation and two to 10 minutes of average air travel.5 6 9
The National Council of Radiation Protection and Measurements (NCRP), an advisory body to the United States government, uses the concept of “negligible individual dose (NID),” which is, “an effective dose corresponding to the level of average annual excess risk of fatal health effects attributable to radiation exposure below which effort to further reduce the exposure to an individual is not warranted.” NID is set at an annual effective dose of 10 μSv per source or practice.7 A person would have to undergo 100-200 backscatter scans before receiving a dose equivalent to NID.
The Nuclear Regulatory Commission in the United States recommends an annual limit on doses to the public of 1000 μSv, and 250 μSv a year from any single source or practice.8 To exceed 250 μSv a year at a dose of 0.1 or 0.05 μSv per scan, a traveller would need to have 2500-5000 scans, which is highly unlikely in one year.
Another concern about backscatter systems is the ability of scanners to deliver a low radiation dose but yield images of sufficient quality. This is especially pertinent in countries where poor or non-existent infrastructure means that periodic checks are not guaranteed. It is therefore essential to establish routine maintenance and quality assurance programmes and involve trained professionals, such as health physicists or medical physicists, to verify the radiation dose delivered by the backscatter systems.10 11 Even though the radiation exposure to operators is negligible, they should undergo radiation safety training to avoid any inadvertent exposure to radiation.2 10 11
Infringement of personal privacy is the final major concern because of the detailed images produced. This has led to use of the term “virtual strip search.” These concerns can be mitigated by having the image viewing stations at remote locations, not next to the scanners,12 and also ensuring that the system cannot save the images in the long term. In addition, many software programs have been developed to modify the backscatter image to make the image appear more like a “chalk outline,” with less personal detail.
Currently, the use of full body scanners is optional, but when it becomes mandatory, the alternative measures for people who decline to go through these scanners are complete physical pat-downs and other technologies that may be even more intrusive and cumbersome.
Current calculations indicate that backscatter systems are safe for general use, even in infants and children, pregnant woman, and people with genetically based hypersensitivity to radiation. When considered in the context of a potential increase in security, the benefits outweigh the potential for harm.
Cite this as: BMJ 2010;340:c993
Competing interests: The author has completed the Unified Competing Interest form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declares: (1) No financial support for the submitted work from anyone other than his employer; (2) No financial relationships with commercial entities that might have an interest in the submitted work; (3) No spouse, partner, or children with relationships with commercial entities that might have an interest in the submitted work; (4) No non-financial interests that may be relevant to the submitted work.
Provenance and peer review: Commissioned; not externally peer reviewed.