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Radiation protection in occupational and environmental settings
  1. Linda Walsh
  1. Correspondence to Dr Linda Walsh, Federal Office for Radiation Protection, Department ‘Radiation Protection and Health’, Ingolstaedter Landstr 1, 85764 Oberschleissheim, Germany; lwalsh{at}bfs.de

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The assessment of detrimental health risks for humans, due to exposures from ionising radiation sources such as γ-rays, x-rays and neutrons, which penetrate deeply into the human body, has been an endeavour which has increased in magnitude and effort over the last century. Solid cancer and leukaemia incidence and mortality have emerged as having radiation as an important proven risk factor from the many indicators of cellular damage and health effects that have been investigated to date. Studies on survivors of the World War II atomic bombings over Hiroshima and Nagasaki, who were exposed mainly to γ-rays and neutrons, continue to provide valuable radiation epidemiological data and quantitative assessments of the radiation related solid cancer and leukaemia risks.1 The cohort of the atomic bomb survivors is unique and characterised by: the large number of cohort members (approximately 105 000); the long follow-up period of more than 50 years; a composition that includes males and females, children and adults; whole-body exposures (which are more typical for radiation protection situations than the partial-body exposures associated with many medically exposed cohorts); a large dose range from natural to lethal levels; and an internal control group with negligible doses, that is, those who survived at large distances (>3 km) from the hypocentres. Results from this cohort have formed a basis in the construction of radiation protection guidelines that include the setting of various dose limits to the radiation received by occupationally exposed workers and the general public. Such dose limits come from assessments and recommendations that are issued and updated at regular intervals by international bodies, that is, the International Commission on Radiological Protection2 and the United Nations Scientific Committee on the Effects of Atomic Radiation.3

Some of the major moot issues and sources of uncertainty related to the formulation of dose limit recommendations for radiation protection include the following points: first, the sensitivity of different organs to radiation and the relative tissue damaging effect of the various types of radiation are uncertain; second, the factors by which risks for children and young persons are higher than for adults; third, the shape and statistical significance of the cancer risk measure when plotted as a function of dose are not very well defined, particularly at the lower end of the dose range (where the associated error bars tend to be relatively wider than at higher doses); fourth, whether or not the cancer risks are similar for acute high-dose-rate exposures (as pertinent to the Japanese A-bomb survivours) and protracted low-dose-rate exposures (as relevant to a broad category of nuclear workers and the general population).

An important paper by Daniels and Schubauer-Berigan,4 relevant to the last two of these issues with respect to leukaemia risks appears in this issue of OEM (see page 457). Daniels and Schubauer-Berigan have considered recent epidemiological evidence relevant to leukaemia mortality and incidence risks from protracted low dose and low-dose-rate exposures to γ-rays by making an extensive literature review of many studies on groups of people who were either occupationally or environmentally exposed. The reviewed studies are very interesting and include Chernobyl nuclear-accident clean-up workers, residents of apartments built in Taiwan with accidently contaminated metal support structures and a pooled study of nuclear workers in 15 countries. This initial literature review was narrowed down to 23 (20 occupational and three environmental) studies which fulfilled a predefined set of inclusion criteria related to study design and type of risk estimates reported. The individual risks were then analysed together by Daniels and Schubauer-Berigan in a meta-analysis; this basically involved constructing aggregated risks in the form of weighted means from the individual risk estimates for various subgroups of the 23 studies. The main advantage of conducting a meta-analysis is that the aggregated risks are then usually associated with smaller uncertainties than the uncertainties in the individual studies. Daniels and Schubauer-Berigan also carefully tested their aggregated results to find out if they were unduly influenced by bias or not. Sources of bias tested were: any particular individual study that may have unduly influenced the aggregated measure; an overlap of study participants between two or more studies; and the preferred publication of studies that reported a positive risk over those that found no risk. The main results reported for leukaemia pertain to mixed mortality and incidence effect sizes associated with a total radiation dose of 100 mGy. In order to obtain a perspective on this level of dose, it may be interesting to note that a total dose of 100 mGy is approximately 50–100 times larger than the annual natural background radiation and is roughly equivalent to five times the annual occupational limit that is currently in place in most European countries. The main risk measure value reported in the Daniels and Schubauer-Berigan paper, that is, the Excess Relative Risk, indicated that the spontaneous leukaemia risk (ie, for a group of unexposed persons) was found to be increased by 19% due to a dose of 100 mGy. The 19% increase was reported to agree well with the risk from acute exposure from the Japanese A-bomb survivours and is therefore an indication that leukaemia risks are similar for protracted and acute exposures. Moreover, the 95% CI associated with the 19% was found to range from 7% to 32%, which is reported to be narrower than a comparable interval for the Japanese A-bomb data.5 This latter result is important for radiation protection because it presents narrower uncertainties than those associated with the A-bomb, acute exposure risks at this dose and also narrower uncertainties than those associated with the individual studies that contributed to the aggregated risk. Although Daniels and Schubauer-Berigan did not use the most recent A-bomb leukaemia risks6 7 for comparison, this does not affect their conclusions.

The main conclusions of the Daniels and Schubauer-Berigan paper are that protracted exposure to low-dose gamma radiation is significantly associated with leukaemia and that leukaemia risks are similar for protracted and acute exposures. These outcomes complement similar conclusions drawn for solid cancer in an analogous meta-analysis published in OEM last year8 and add to the weight of evidence that is mounting up to indicate that cancer risks are similar for protracted and acute exposures for both solid cancer and leukaemia. This paper will undoubtedly find its way into international discussions as an important contribution to risk–benefit assessment of medical exposures and radiation protection in general, and specifically for the millions of persons worldwide working in jobs that involve low-level protracted exposure to ionising radiation.

References

Footnotes

  • Linked articles 54684.

  • Competing interests None.

  • Provenance and peer review Commissioned; not externally peer reviewed.

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