6. Conclusion: are X-ray security scanners safe?
The SCENIHR opinion states:
This document is prepared in response to the request of the Commission and provides a summary of the scientific knowledge on the potential health effects of X-ray based security scanners for passenger screening. It is not intended to address the issue of justification, which remains a national prerogative, as specified in the relevant EU legislative framework for radiation protection. This framework specifies the requirements for use of all equipment using ionising radiation, including prior justification before a practice is introduced.
The justification for introducing a new practice, particularly outside the medical field, is a complex process and radiation protection considerations are only one aspect. In the use of X-ray security scanners for screening individuals, the benefit accrues primarily to society rather than to the exposed individual. The risk-benefit ratio should be considered in the justification prior to a practice being introduced but this may also need to be revisited when new technologies are introduced or new information becomes available.
Although the doses per scan arising from the use of screening for security purposes are well below the public dose limit, this does not remove the requirement for justification. In addition, for practices that are justified and subsequently authorised, optimisation measures must be taken so that all exposures are as low as reasonably achievable (the ALARA principle) for workers, the general public, and the population as a whole.
The SCENIHR was asked:
1. To assess the potential health effects related to the use of all types of security scanners used for passenger screening which emit ionising radiation.
Although the radiation doses from a single scan are in the range of nano- to microsievert, those from transmission scanners are at least 10 times higher than those from backscatter scanners. This difference could result in significantly higher cumulative doses which may exceed the annual dose limit for members of the public if transmission scanners are used as routine screening devices for frequently exposed individuals (airline crews, frequent flyers, airport personnel, etc.). The higher doses used by transmission scanners must be given an emphasis in the justification process.
2. If any effects are identified under 1, to quantify the risks and, if feasible, to estimate the additional number of cases of diseases that are expected to occur in Europe due to the use of this technology at EU airports, differentiating between the general public and exposed workers as indicated below.
The dose levels from the use of security scanners are well below the range where any health effects are observable. Due to the very low radiation doses to both scanned passengers and the exposed workers, any quantitative estimation of risk would be highly uncertain and rely on non verifiable assumptions about extrapolations to dose levels where health effects cannot be demonstrated. The risk assessment performed here relies on the linear no-threshold model according to which the probability of adverse effects is directly proportional to the radiation dose. The risk estimates are based on theoretical projections from observations at substantially higher exposure levels. There is no sufficient scientific basis for making any quantitative risk estimates such as calculating the additional number of cancer cases induced by the introduction of security scanners at airports, either to the general public or the exposed workers.
In its assessment, the SCENIHR is asked to consider in particular the risk for populations that are regularly exposed to such technologies (e.g. frequent flyers (to be defined), air crew, security workers operating the scanners and other airport staff) and potentially vulnerable groups (e.g. pregnant women, children).
All those screened including frequent flyers and airline crews are subject to the public dose limit of 1 millisievert per year. Annual cumulative effective doses would remain below that level for backscatter technology even with highest plausible scan frequencies (three scans every working day of the year), but with transmission technology such a dose limit could be exceeded for individuals with such high scan frequency. Appropriate dose constraints for members of the public should be set at a substantially lower level than the public dose limit. While a suitable constraint would be very unlikely to be approached for most passengers who are scanned using backscatter scanners, it could potentially be exceeded by those persons who are scanned several times a day throughout the year (e.g. flight crew, ground staff). It would be appropriate to apply a higher constraint value to these groups. However, even in this scenario the annual dose limit for members of the public would not be approached or exceeded. By contrast, the scanning of frequent fliers with X-ray transmission scanners could result in both the constraint and dose limit for members of the public being exceeded.
In view of the low doses from backscatter security scanners there is no scientific basis to separately consider potentially vulnerable groups (e.g. pregnant women, children) in risk assessment. Cumulative doses are very likely to remain below the constraints with backscatter scanners even for frequently scanned individuals.
Use of transmission scanners could result in exceeding dose constraints for frequent flyers and certain occupational groups. An occasional transmission scan does not require separate consideration even for potentially vulnerable groups of the population.
The SCENIHR should compare the relative risk of such security scanners using X-ray based technologies to other security scanner technologies on the market.
The current scientific evidence does not allow for a direct comparison of various technologies because of the different nature of exposure for ionising and non-ionising radiation. There is no scientific basis for predicting stochastic health effects of passenger scanning technologies using non-ionising radiation such as mm wave or THz scanners. Furthermore, the thermal effects of exposure to non-ionising radiation are not cumulative and non thermal health effects are not proven. The use of these technologies has been shown to comply with exposure limits based on thermal effects (ICNRP).
At the levels typical of X-ray based security scanners, only stochastic effects could occur, but the predicted probability of their occurrence is very low and there is no scientific evidence supporting their existence.
Passive devices that do not emit any radiation are not expected to have any adverse health effects.