Dennis Woodhead

Low Doses of Ionizing Radiation: Biological Effects and Regulatory Control

Radiological protection is concerned with the limitation of the consequential risks from exposure to ionising radiation. In recent years, there has been much debate about the validity of one of the fundamental bases of the present system for the limitation of these risks, i.e., at the low dose rates with which we are usually concerned in routine activities, the incremental increase in the risk of stochastic effects (primarily cancer) is linearly related to the additional radiation dose above that from the natural background. This is the linear, no-threshold (LNT) dose-response paradigm adopted by the ICRP in developing its recommendations. Re-analysis and interpretation of existing data, and new data on effects that may (or may not) be of relevance to cancer induction, have led to proposals for contrary supralinear, threshold and hormetic (beneficial) response relationships at low doses. It was the purpose of this conference to provide a forum to examine the latest information and debate the issues. A detailed meeting report has been given in an earlier issue of this journal (Wakeford R and Tawn E J 1998 J. Radiol. Prot. 18 52-6), and the majority of the short papers presented at the conference were issued, at that time, as an IAEA Technical Document (IAEA TECDOC-976, available free of charge from the IAEA in Vienna). This publication provides the keynote papers, summaries of the discussions and the session chairmens summaries for each of the set of ten fora, a special session and a final round-table discussion that constituted the main body of the conference. Also included are the papers presented in two introductory background sessions that provided some context for the conference. All of the keynote papers provide, as might be expected, useful summaries of the state of the art in the respective fields. This is particularly so for the fora 8-10 that introduced the discussion of control measures and criteria for intervention, in which circumstance there is, at present, the potential for confusion (at least in the public mind) resulting from a dose limit of 1 mSv a-1 (with lower constraint values) for prospective practices, and intervention levels in the range 3-10 mSv a-1 for existing situations. The presentations from Gonzales (IAEA) and Clarke (NRPB) are useful in indicating the thinking in progress to resolve this situation (see also Clarke R 1999 J. Radiol. Prot. 19 107-15). In contrast, the discussions appear to be rather anodyne and give no real flavour of the divergent opinions and positions that might have been expected given the reasons for the convening the meeting and some of the participants present (Wakeford and Tawn also noted this point in their meeting report). There appears to have been no advance towards a resolution of the debate over the shape of the low dose-response curve that is appropriate for radiation protection. Overall, this is a useful publication - I have already referred to it for information not easily available elsewhere and expect to do so again - and it will make a helpful addition to your library.

Protection of the environment from the effects of ionising radiation

It will not have escaped the notice of the readers of this journal that the subject of radiological protection of the environment - explicitly of wild plants and animals from radiation exposure, as opposed to the more frequently accepted interpretation in terms of the possible resultant impacts on humans arising from contamination by radionuclides - has an increased profile on the scientific/political agenda (see, e.g., [1-3]). In reality, this is not a fundamentally new concern; it was apparent in the programmes of research instituted in the United States when the plutonium production reactors were built at Hanford during the Second World War, and by the United Kingdom Atomic Energy Authority when the Windscale facility was established in the early 1950s. It achieved greater international prominence from the outcome of the first UN Conference on the Human Environment at Stockholm in 1972. In this forum, it was suggested that, in order to protect and enhance human well-being, the control of pollution should have as a principal objective the protection of populations of non-human organisms. In the context of radiological protection of the environment, this position had already been taken up by the IAEA and the resulting activities produced several reports [4-6] in the period 1976-92. These have considered the incremental radiation exposures of wild organisms as a consequence of the managed disposal of radioactive wastes to several different environments, and reviewed the available information on the effects of radiation on plants and animals so that the possible environmental impact of such radiation exposures could be assessed. This work has been continued by national authorities (e.g., by the NCRP in the USA [7] and the Environment Agency in the UK [8]) and in the international arena (most recently by UNSCEAR [9]). The outcome of the UN Conference on Environment and Development in Rio de Janeiro in 1992 reinforced the concern in the context of sustainable development for which environmental protection was seen to be an indispensable component. This was made explicit for the management of radioactive wastes in the resulting UN Action Plan (Agenda 21 [10]). Within the European arena, these developments have fed through to a requirement for assessments of possible impact on the environment in, for example, the Habitat Directive and the Water Framework Directive [11, 12]. What has been missing from all these efforts, however, has been a framework within which the existing information could be developed and integrated into an internationally acceptable system that would provide: a sound basis for an assessment of possible impacts; defined objectives for environmental protection so that appropriate criteria can be developed; the management tools to achieve the desired outcomes; and guidance on their application, including the issue of determining compliance. In the absence of such a system, a number of national authorities (e.g., Canada [13], Sweden (see the article by Holm et al on p 235 of this issue), the USA [14] and the UK [8]) have taken interim steps, or have begun to develop their own regulatory frameworks, to provide for the radiological protection of the environment. While these have much in common, there are substantive differences in detail that may make it more difficult to develop a coherent international consensus. The ICRP has recently instituted a Task Group to develop its own position on the protection of the environment with the intention of producing a consultation document this year, and to publish a final report in 2003. It is to be hoped that this will identify sufficient common ground to allow the establishment of an internationally accepted framework for progress that is compatible with the proposed developments of the existing system for human radiological protection. In parallel, the European Commission is part-funding two related projects within the 5th Framework Programme that have a bearing on this subject. These are FASSET (a Framework for the Assessment of Environmental Impact that could be applied across the geographical area of the EU) and EPIC (Environmental Protection from Ionizing Contaminants with a more restricted focus on the European Arctic environment). Both projects are adopting the same general approach - the development of a framework in which: the radionuclides are traced from the source through the environment (pathway analysis); the resulting concentrations within, and external to, the organisms are the source terms for dosimetry models allowing the estimation of the radiation exposure of reference organisms; and the available information on the effects of radiation is summarised so that the levels of dose rate can be determined at which different degrees of damage might be induced in the four broad categories of endpoint (morbidity, mortality, reproductive capacity and mutation for the reference organism types). The application of this systematic approach will demonstrate a commitment to environmental protection and permit the transparent regulation of waste management practices including the development of relevant standards against which to test compliance. Two particular features of FASSET have been the open dissemination of progress on a web site (www.fasset.org) and the conduct of an open forum at which interested parties were given the opportunity to provide constructive comment on, and influence, the future direction of the project. It will have been noted that there has been no mention of the development of dose rate limits for the environment. Indeed, given the wide diversity of environments, ecosystems and organisms that will probably need to be considered, such an approach is likely to be impractical. Rather, the intention has been to make a start on providing all the necessary tools within the framework to complete a sound scientific assessment of the situation on a case by case basis. This would allow the management options to achieve environmental protection to be considered in the wider arena that includes the socio-economic and political inputs, and in parallel with the separate, but related, requirements for the protection of humans. Waste producers, and particularly the nuclear industry, would probably argue that the current ICRP position [15] (paraphrased as: if man is sufficiently protected, then so is the environment) is adequate, and that any additional effort is, therefore, unnecessary. Given that the pathway analysis component of the process is common to the assessment of the human radiological situation, the extra effort is likely to be small. The gain in terms of a credible and transparent commitment to environmental protection, that would stand comparison with the existing regimes for non-radioactive contaminants, would be great. Environmentalists would also consider the assessment exercise as unnecessary because the application of the precautionary principle leads to the demand for zero release. This, however, ignores the practical advantages to be gained for both humans and the environment from the application of the ALARA principle and the best available technology (BAT). The frameworks that are under development will help to illuminate this ongoing debate.

Standards for Intakes of Radionuclides

The NRPB is mandated by the UK Government to provide guidance on the application of the ICRP Recommendations, including their expression in International and European Basic Safety Standards, to the practice of radiological protection in the UK. This NRPB Document fulfils this mandate following the issue of the revised recommendations in ICRP Publication 60, and their further development in terms of internal exposures in a number of subsequent publications. It endorses the new approaches for application in the UK through the revised Ionising Radiations Regulations that are soon to be implemented. This summary and guidance are especially useful in identifying the significant changes that have been made, since the issue of ICRP Publication 26 (and the dependent Publication 30), in the physiological and dosimetric models that underlie the revised dose coefficients (dose per unit intake) provided in ICRP Publications 68 and 72. This is an attribute that will be particularly helpful in interpreting the origin of actual or potential changes over time in the assessed exposures from internal sources for either workers or members of the general public. This document is an essential complement to the various ICRP Publications for any practising health physicist in the UK.