|RESPONSE OF THE HEALTH PROTECTION AGENCY RADIATION PROTECTION DIVISION TO THE ICRP DRAFT DOCUMENT FOR CONSULTATION ON THE REPRESENTATIVE INDIVIDUAL (42/106/05)
The International Commission on Radiological Protection (ICRP) published a series of documents on its web site for consultation in April 2005. The document considered here is entitled ‘Assessing dose of the representative individual for the purpose of radiation protection of the public’. Staff of the UK Health Protection Agency Radiation Protection Division (HPA-RPD; formerly the National Radiological Protection Board) have considered this document and their comments are given below.
HPA-RPD welcomes the opportunity to comment on the draft document and feels that it covers an important part of the system of radiation protection. HPA-RPD staff are broadly supportive of the major conclusions and judgements made. A number of minor areas for clarification or revision are given below. In developing this response views were sought from all HPA-RPD staff, particularly those who were not directly involved in the work of ICRP. Both general and detailed technical comments are given but not mainly editorial comments as it assumed that they will be picked up in the process of producing the final ICRP publication.
• HPA-RPD staff welcome the publication of a document on assessing doses to individuals for protecting members of the public. This is an important area and it is a good idea to bring together and clarify all previous ICRP guidance on the topic. We support the basic premise of the representative individual and much of the guidance given in this draft document.
• The advice that generally it is sufficient to consider three age groups in assessing doses to the public is welcomed. This reflects what is done in practice in the UK and advice given in both the UK and the EU. We also welcome the advice on taking account of exposure of the fetus.
• We support the recognition of the role of stakeholders in the process of identifying characteristics of the representative individual.
• It would be helpful if the report made the relationship between the old critical group and the representative individual clear from the outset. It would be sufficient to simply state that the representative individual replaces the critical group but that many of the principles associated with critical groups also apply to the representative individual.
• In several places in the document it is stated that retrospective doses are for ‘specific (real) individuals’ (e.g Abstract and paragraphs S4, S18, 28). This is not necessarily the case. For example, a retrospective assessment might be required to consider the significance of measured activity concentrations in the environment and this might be done on the basis of an assessed dose to a hypothetical individual. An assessment of radiation doses from past practices might also be carried out to compare the impact of different sources and again hypothetical individuals might be considered.
• Although the draft document discusses the concept of homogeneity it would be helpful if further guidance could be given. In particular guidance would be welcomed from ICRP on the extent to which homogeneity in habits/characteristics is important as opposed to homogeneity in dose. If a distribution of doses is estimated in a population taking account of the different habits it is possible to identify individuals with similar doses but received in very different ways (e.g for one individual from consuming seafood and for another from living close to the source of an atmospheric discharge and consuming terrestrial foods.) Can such doses be deemed to be homogenous for the purpose of defining a representative individual? Paragraph S11 states that homogeneity in habit data is important while paragraph 14 can be interpreted that it is homogeneity in dose that is important. The view of HPA-RPD is that the need for homogeneity in habits is as important as homogeneity in dose. (Is it appropriate for us to include this view or should we just pose the question to ICRP?)
Paragraph S8 (and third paragraph of abstract). Although in general three age groups are sufficient paragraph 79 does raise the issue of exposure to the fetus. It would be worth adding a caveat to these paragraphs to cover the rare cases when the fetus needs to be considered.
Paragraph S18. It is unlikely to be acceptable to suggest that individuals should change their habits to reduce their doses in routine situations.
Paragraph S16. One problem with defining a percentage probability within a population for compliance is that the population used will affect the value defined. The 5% level within a coastal community will vary depending upon whether, for example, all individuals within a 2 km or 20 km radius are included. This is important and is touched on in paragraph 86 and clearly elucidated in paragraph B44 and such ideas could usefully be brought out in the summary. Guidance on this e.g. some examples would be useful. If only to point out what would not be reasonable. One way would be to only include individuals within a particular area or with doses above particular levels. It may even be necessary to look at different populations to demonstrate that the decision is robust.
Paragraph 9. It is stated here that advice on the protection of future individuals in the case of disposal of long-lived waste is provided in ICRP Publication 81. The report needs to clarify whether the advice in ICRP 81 is still valid and whether the advice in the rest of the document applies to these individuals or not. Also, if the guidance in Publication 81 is still valid then it should be stated that references within Publication 81 to the critical group should be interpreted as referring to the ‘representative individual’ otherwise the system for solid waste disposal will be different to that for other exposure situations. It is also important to note in this context that in ICRP 81 it is said that it is only generally necessary to calculate doses to an adult. This differs from the advice in this document. (ICRP 81 para 46 page 14).
Paragraph 23. It would help to specify here that the representative individual replaces the critical group.
Paragraph 25. Is the last part of this paragraph consistent with the main ICRP recommendations? The draft issued for consultation in 2004 specified going beyond the ALARA principle.
Paragraph 27. The second sentence with its reference to current year is confusing.
Figure on Page 9. The wording in the left hand box about dose rates is potentially confusing. It would be better to omit the word "and" and use a list instead. Under external exposure the text in the boxes should be changed to make it clear that it is sometimes possible to directly measure external dose rates.
Paragraph 41. (last sentence). It is said here and in other places (eg S12) that in the absence on site specific data it may be necessary to use regional or national information. The clear interpretation of such statements is that there is a hierarchy of data with site specific at the top followed by regional and then national, ie if you have site specific data this should always be the first choice for using in an assessment. This is too simplistic. In some cases habits are highly site specific, eg the consumption of local seafood by coastal communities, in which case the use of site specific data is important. However, for many foods, eg milk in the UK, the pattern of consumption is not very site specific. Under such circumstances it may be better to use national data, which with large sample sizes may be more statistically robust, rather than limited site specific data. This is of particular importance when considering prospective assessments as these should account for the fact that individuals from other parts of the country may move into the area.
Paragraph 45. (also S7) When it is said that weighting factors and dose coefficients are assumed not to be uncertain isn’t what is really being said that we don’t calculate doses to ‘real individuals’ with their various metabolic rates, mass etc. but that we calculate doses to a set of hypothetical individuals with defined physiological characteristics. This may be easier to understand than to say that the quantities are not uncertain.
Paragraph 62. This paragraph is confusing as it seems to imply that we should be looking at the very low end of the habit distribution as well as the upper
Paragraph 65. There are of course situations in the UK where we have not been able to use national habit data, notably in some coastal locations in regard of fish consumers. This point is picked up here in paragraph 63. Some editing might therefore be helpful so that this section appears more self consistent. This document is aimed primarily but not exclusively at prospective assessments. We agree with the point about not wanting extreme one-off habits to affect things significantly, but on the other hand with site specific data there is also the possibility of using (say) a 5 year rolling average approach to remove annual one-off situations. This possibility has not been mentioned.
Paragraph 74. It is not always the case that a period of 70 years is considered for accumulation of radionuclides in the body. For adults a period of 50 years is used. It would be clearer to say to age 70 y.
Paragraph 78. It would be clearer if in the 4th line ‘0-to-5-year age category’ was replaced by ‘infant category’. Also, 0 to <6 years covers 6 years not 5.
Paragraph 79. The idea that infant doses will also be high for these radionuclides (4th sentence) does not really apply to doses from phosphorus-32 and 33, particularly when considering doses from the ingestion of fish following discharges to freshwater. This sentence could therefore be misleading for these potentially important radionuclides. It would help if this paragraph had more guidance on when the fetus needs to be considered by comparing the dose to the mother to that of the fetus; a list of relevant radionuclides would help.
Paragraph 82. It is not clear why all available age-specific dose coefficients should be used in relation to accidental releases. Assessed doses from accidental releases are particularly uncertain and so the increased sophistication of considering 7 different age groups does not seem worthwhile.
Table 2. Should the last column indicate that habit data for these ages should be used as well as the dose coefficient?
Paragraph 90. This comes back to the comment on paragraph S18. If we have a past practice and it is continuing into the future, then if we exceed the constraint we can do something about the source. If we have an existing situation, we do not really have a source to control - the activity is somewhere in an environment and therefore not totally under our control. We can of course reduce doses via intervention. Is this what is meant by changing habits?? If it is perhaps the scope of the term habits needs to be clearer.
Table 3 page 19. The use of the term physiological characteristics is not necessarily appropriate here. Most physiological characteristics are effectively defined as single values within the modelling of the dose coefficients. The only physiological characteristics that vary within assessments are breathing rates and perhaps skin areas (when considering direct contamination).
Section 4.2 page 20. Solid waste disposal results in potential exposures. It would be useful to reiterate here that for this case advice in ICRP Publication 81 is still valid as ICRP 81 does not go down the risk route in the same manner.
Paragraph 102. A definition of the possible roles for stakeholders would be useful, along the lines given in the document on optimisation.
Paragraph A5. The sentence starting ‘Straightforward comparison of dose coefficients’ is not clear, perhaps the rest if this paragraph could be clarified and shortened.
Paragraph A9. We agree that it is unrealistic to estimate doses from solid foods using the 3-month dose coefficient. Given this it would be better not to make the comparisons in this section. Also f1 appears without explanation. The first bullet point is not clear; the point here is that other exposure pathways will lead to higher doses.
This appendix is 18 pages long, which is comparable with the length of the main text (24 pages). It may be true, as is stated in the opening paragraphs of the Appendix, that probabilistic methods will become more widespread in the coming years, however, they are not common place at the moment. Because of the resource intensive nature of such assessments it is unlikely that they will ever be suitable for use in the assessment of doses from small users, which form the majority of such assessments. Although ICRP may not be directly promoting the use of probabilistic methods as their preferred approach in the main text the fact that such a large fraction of the document is devoted to this issue may be interpreted as implying that this is indeed the case. Much of the information in the Appendix is of a very detailed technical nature and as such is considered not suitable for a general foundation document which is intended to lay down overall principles.
Having said that paragraph B44 in particular is very important and should be brought into the main text.