2005 ICRP Recommendation

Draft document: 2005 ICRP Recommendation
Submitted by Staff of the NRPB, National Radiological Protection Board
Commenting on behalf of the organisation

PART 1 Abstract NRPB staff welcome the intention from ICRP to consolidate its previous recommendations and supports the idea that the basic dose limits laid down in ICRP publication 60 still apply. The ICRP document assumes a good knowledge of previous ICRP advice, notably publication 60, and would be strengthened if redrafted to be stand-alone, as the 2005 recommendations are likely to be the principal radiological protection guidance for some time to come. In particular the basic ICRP principles of justification, optimisation and limitation should be clearly stated. The distinction between dose limits and constraints needs to be clarified and explained. In this report comments are presented as general comments on the document as a whole or particular sections, and specific technical comments on the different paragraphs. Comments that are primarily editorial are not included. Foreword from the Director In June 2004 the International Commission on Radiological Protection (ICRP) published on the ICRP website (http://www.icrp.org/icrp_rec_june.asp) its 2005 recommendations in a draft for consultation. I and a number of NRPB senior staff have been involved in the ICRP Committee work that underpins these draft recommendations. Through these Committees we have also had the opportunity to provide our own comments. For these reasons, I requested the formation of an NRPB working group largely composed of staff who had not been involved in the development of the draft. The working group was given the brief to review the draft ICRP recommendations, consult others and develop a report to ICRP that broadly reflects the views of NRPB professional staff. This, I believe, is an open and transparent method of providing an independent view to ICRP and one that has been endorsed by the NRPB Board. ICRP will be publishing all such views on its website (www.icrp.org) I should emphasise that the views expressed in the report of the working group have been subject to only minor editorial change and do not necessarily reflect the views of senior NRPB staff nor those of the NRPB Board. 1. Introduction In June 2004 the International Commission on Radiological Protection (ICRP) published on its website its 2005 recommendations as a draft for consultation. Previously ICRP had been widely discussing the proposed recommendations and has taken views expressed into account in developing the draft. The ICRP recommendations are the basis of radiological protection legislation and practice in the UK and throughout the world and so it is important that they are well founded and widely supported. The National Radiological Protection Board (NRPB), therefore, welcomes the opportunity to contribute to the 2005 recommendations during this consultation. This report contains the response of the NRPB staff to the draft 2005 recommendations. In developing this response a working group was established and views were sought from all members of staff, particularly those who are not involved directly in the work of ICRP. This report gives comments on all aspects of the draft ICRP recommendations and is divided into general comments on the draft as a whole and then comments on the different sections of the ICRP document. For each section both general and detailed technical comments by paragraph are given. This report does not give mainly editorial comments as it is assumed that they will be picked up in the process of producing the final ICRP publication. 2. General comments NRPB staff welcome the intention from ICRP to consolidate its previous recommendations and to clarify its objectives. We also support the idea that the basic dose limits laid down in ICRP publication 60 still apply. There should be no need for fundamental changes to radiation protection legislation guidance in the UK. In parts the draft recommendations are difficult to read, as they are too long and repetitive with differing levels of detail provided. Some parts contain lengthy supporting/explanatory text (often repeated from earlier ICRP publications), while other parts have very little detail. Therefore, some parts appear “stand-alone”, while others must be read in conjunction with previous ICRP publications. It would be helpful if ICRP could clarify the role of this document and its relationship with other documents, such as the foundation reports to be published by the ICRP Working Groups. It would be better if this document was written as top level advice clearly stating ICRP’s recommendations, with the scientific basis for the advice given in supporting documents or annexes. The document as written assumes a good knowledge of previous ICRP documents, including ICRP 60. The 2005 recommendations will be the principal advice document for some time to come and so should stand-alone. The basic aims and objectives of the document should be clearly presented. In particular: The document needs to have a clear statement of the basic ICRP principles of justification, optimisation and limitation. These are implied and discussed throughout the document but not clearly stated. As the dose limits from ICRP 60 are to be retained these should be included in the summary and stated earlier in the document than currently (paragraph 185). It needs to be clear which earlier ICRP documents are still valid and if parts of documents only are valid, which parts. A table in an Annex listing publications and their status would be very helpful. Basic recommendations from previous or supporting documents should be given in the 2005 recommendations but the other documents can be referred to for details and the reasoning behind the recommendations. The document places great emphasis on the future use of dose constraints but it fails to make clear what a dose constraint actually represents, and provides almost no practical guidance on how to determine its value. The recommended constraints appear to match (or even exceed) existing dose limits. Overall, the practical difference between a constraint and a limit is less clear than previously. The recommended maximum values of the dose constraints appear to be solely based on multiples of natural background radiation. This seems insufficient and there needs to be further consideration of the risks in a more quantifiable way. Although there appear to be no fundamental changes in the recommendations for protecting people from medical exposures, the small amount of attention given to this topic appears to be disproportionate to its importance as a major source of ‘controllable’ population exposure. In particular, the optimisation of protection for patients requires further consideration. NRPB supports the change in terminology to radiation weighted dose as this aids clarity. On balance making additional changes to the unit name (from “sievert”) seems unnecessary, and is likely to cause confusion at the working level. In view of the changes found to be necessary to the approach adopted in Publication 60, a clearer explanation would be useful of the basis for the equations giving the radiation weighting factor for neutrons. This could be given in an Annex. The restrictions on the use of effective dose seem too strong. It is valid to use effective dose retrospectively, for example when demonstrating compliance with a dose limit or when assessing doses associated with measured values. Important considerations are the level of the dose (is it in the range over which stochastic effects predominate?) and the purpose of the assessment but not whether it is a prospective or retrospective dose assessment. 3. Summary of the recommendations The summary should include a clear statement of the fundamental principles of radiation protection (justification, limitation and optimisation). The distinction between the dose limit and constraint should be clearly made. If the ICRP 60 dose limits still apply then they should be given in the summary. There is nothing in the summary on medical exposures and these should at least be mentioned in the section on optimisation. It would also be helpful if Table 6, containing the revised nominal probability coefficients for stochastic effects, was included in the summary. The following Table gives specific technical comments on the Summary. TABLE 1 Technical comments on the Summary of the Recommendations S3 The argument presented here seems weak. If there is “stability” and “no major problem”, then a more persuasive argument for “strengthening” is required. The conclusion drawn from the wording is that there are minor (unspecified) weaknesses. However, nowhere in the document is there an explanation of how constraints are the cure (indeed, in countries such as the UK, they seem largely irrelevant in practice). And the last sentence is disingenuous in two ways – the recommendations contain a lot more than dose constraints, and the quantification of dose constraints in real situations is not really described. S4 The role of justification, optimisation and dose limits should be stated here. Dose constraints as previously recommended were intended to constrain optimisation but the overall control for normal situations was through the dose limit. This gave overall flexibility. If the current dose constraint used in the UK were now to be viewed as an “obligatory” level which if not reached would be regarded as “a failure” then the implication would be a change to the law. This moving away from the primary control being limits on all sources to an individual to individual sources is potentially very significant. These constraints also apply to dose levels at which countermeasures should be imposed following an accident. Whilst such levels may be useful for planning purposes under accident conditions it is not possible to ensure that all individuals would necessarily get doses below such levels and as such the use of terms obligatory and failure are too extreme. S5 If these recommendations are a “natural evolution of” the 1990 recommendations as stated in paragraph S4 then the dose constraint is not the most fundamental level of protection. Dose constraints (as previously defined by ICRP), are merely an optimisation tool, and must be less fundamental than the optimisation principle itself (which, in fact, is only briefly addressed in this document). The dose constraint is defined as “the basic level of protection”, or the level of protection for the “most exposed individuals”. These do not necessarily mean the same thing – and the latter definition is almost indistinguishable in practice from a dose limit. Also does the reference to the “most exposed individuals” conflict with later guidance to use the critical group concept for members of the public? S6 Optimisation of protection includes, “but is more comprehensive than …’’. This is a departure from the previous ICRP definition, and if you have taken social and economic factors into account and gone as low as reasonably achievable it is not clear how you can further optimise? If there are additional factors (other than economic and social), then they should be mentioned. Of course, it mostly requires judgement, rather than a quantitative analysis, but “social factors” (which cannot be quantified meaningfully) are still the issue. It seems misleading to say that it is assumed that there is “some probability of health effects even at small increments of exposure to radiation above the natural background.” There is assumed to be risk associated with increments of natural background as well; the control only of the additional doses above background is a separate point. S7 and Table S1 Strictly Table S1 does not contain “maximum dose constraints” as three of the values are maxima but one is a minimum value. It is undeniably logical to set maximum dose constraints at the level of dose limits but this needs to be explicitly recognised and discussed. However, it is the “actual” (not maximum) dose constraint that is important. The last sentence of S7 suggests that it should be lower than the maximum by up to a factor of ten. However, this falls far short of the practical guidance needed for such a fundamental change. ICRP should at least recommend the factors that should be considered when determining the value of a dose constraint. Also, as the distinction between limits and constraints is not clear this paragraph could be used to justify a reduction in dose limits by up to a factor of ten. The text for the 20 mSv level states that it covers situations where there is direct or indirect benefit for exposed individuals who receive information, etc. Whilst this applies to occupational exposures and comforters and carers it does not seem to apply to the accident situations which are also included in this group. It is technically incorrect to state in Table S1 that there can be no benefit above the top (100 mSv) dose constraint. There surely can be societal benefit, and even individual benefit. Surely the point is that the risks are considered unacceptable (to the individual - for the scenarios described) and must always take precedence over any benefits. In Table S1 the “situations to which it applies” could be made clearer. For example, what is meant by “high levels of controllable existing exposures”? It is assumed that the excluded emergency situations (saving lives, etc) can involve higher doses – this should be made clear and there could usefully be another entry for this situation. Alternatively it could state that it is the responsibility of Governments to determine their own constraints. S8 If the Commission continues to recommend the dose limits the numerical values should be given here. Also it is not clear what is meant by “in normal situations only”. S9 This paragraph uses the word “practical” rather than “practicable”. This may be semantics, but may cause problems in the UK, ie, because “practicable” has legal precedent. The phrase “tend to” (lower radiation doses) seems very strange. It is weak, and implies that all options that offer some indirect and uncertain dose-reduction effect should be evaluated. It should be stated in the second sentence that optimisation should be applied to the medical exposure of patients as well as to members of the public and workers. S10 Again the ideas expressed here (“Have I done all I reasonably can to reduce doses?”) are likely to give weight to calls to reduce limits. It also implies that optimisation is not really about getting the optimum level of protection but the aim is to achieve the lowest possible exposure. S11 The involvement of stakeholders is strongly supported. Although the text is laudable, it merely states current wisdom. It would be more useful if the document could suggest a mechanism for increasing stakeholder involvement. S12 and Table S2 It would be helpful to clarify that these are exclusion levels and that exemption levels could be higher (as in paragraph 25). There is the scope for confusion between the two concepts. There are a number of omissions from Table S2, notably tritium and carbon-14, which occur naturally but can also be produced artificially, and uranium-235. It is also not clear what the adoption of a exclusion activity concentration for 40K will achieve. Pure natural potassium contains about 30 Bq/g and does not pose any significant hazard. This is because internal exposures to 40K are homeostatically controlled so that the activity concentration is irrelevant, and the external dose from pure potassium is very low and is unlikely to warrant any controls. The use of such exclusion levels implies a de minimus dose level below which there is no concern and this has implications for the collective dose concept. S13 As written this assumes a great deal of prior knowledge. It should be possible in a couple of paragraphs to describe the stages from absorbed dose to effective dose and the role of weighting factors – rather than beginning with weighting factors and their inadequacies. S14 The change to “radiation weighted dose” is seen as a useful clarification. Although the arguments for a change are recognised, on balance making additional changes to the unit name (from “Sievert”) seems unnecessary, and is likely to cause confusion at the working level. A suggested change to the first sentence to “When, as is usual, the whole body is not uniformly exposed, it is necessary to use the tissue weighting factor.” (This is to allow the inclusion of the case where only one organ or tissue is exposed, for which case it would still be necessary to use a tissue weighting factor to calculate E). Table S3 If muons are included, perhaps positrons should also be included (unless it is assumed that the term electrons applies to e- and e+), and pions? Table S3, figure S1, equation S1 All the physics of high energy interactions indicates that above a few GeV (important energies for fields near high energy research accelerators, and, to lesser extent, the cosmic radiation exposure of aircraft crew) the radiation weighting factors for all hadrons (specifically in this context protons, neutrons and pions) should become the same (a value of 2 is appropriate). Table S4 What is the rationale for inclusion of adipose tissue and connective tissue in the list of remainder tissues? Are they identified in terms of cancer risk? Is it possible to calculate a specific dose in phantoms? Perhaps the footnote about remainder tissues could explain that the average dose is the simple arithmetic average of the individual tissue doses, not the mass-weighted average that was recommended in Publication 60. S18 and S19 In a recommendations document it seems strange to have sections that talk about what ICRP will do. 4. ICRP Draft Section 1 - Introduction The history of the Commission is useful background but it would be helpful if the way the Commission operates and the status of its recommendations were also clarified. Table 2 gives technical comments on the Introduction section to the ICRP draft recommendations. TABLE 2 Technical comments on the Introduction Table 1 This lists publications since Publication 60 which give policy guidance. Are these publications (or parts of them) still valid? If the 2005 recommendations are to consolidate the previous advice as stated in paragraph 8, then they need to be very clear on the status of these reports. 8 The difference between a dose limit and constraint appears to be solely that the constraint is for a single source and the dose limit is for multiple sources. Previous ICRP guidance was that the dose limit was the fundamental level for protection while the constraint was a restraint on optimisation. It appears now that the constraint is more like a limit – was this the intention? 6th bullet: Implies that the patient dose should rise as the expected benefit increases – surely it is the benefit that should rise as the dose increases? 10 Gives a perspective from a user benefiting in some way rather than a person inadvertently exposed. Few of the “general public” would in reality be as well informed as it is assumed here. Some “softening” of approach to general public exposures might be useful in explaining why people are exposed due to the activities of others.