ICRP Draft Report TG81: Radiological Protection against Radon Exposure
Comments on ICRP ref 4829-9671-6554 dated 6-DEC-2011
Comments on key issues
Scope of the report
While the report is entitled “Radiological Protection against Radon Exposure”, the majority of the text refers to protection against radon-222 in existing exposure situations. There is little or no advice in relation to exposure to radon-220 (thoron). Planned exposure situations are referred to throughout the text, but only uranium mining is specifically covered (paras. 168 to 171). In this regard the text is unbalanced and does not fully recognize that other exposures to radon may need to be treated as planned exposure situations. Given that an overall approach to control of radon exposure that is different to that previously outlined by the ICRP is advocated, a more thorough and detailed document is called for.
Consistency with the International Basic Safety Standards
Situations where radon concentrations in workplaces cannot be reduced below the national reference level are covered in section 3.3.6. The proposed approach is inconsistent with the rest of the document and advocates a strategy different to that agreed to by IAEA Member States in the development of the revised International Basic Safety Standards.
Definition of occupational exposure
In paras. 104 and 165, it is noted that the ICRP limits its use of ‘occupational exposures’ to radiation exposures incurred at work as a result of situations that can reasonably be regarded as being the responsibility of the operating management. Occupational exposure is defined differently in the IAEA safety standards, and this will cause logistical problems in the development of guidance.
Choice of reference level
Throughout the text, several references are made to the maximum value of the reference level as being set at an effective dose of 10 mSv. This is the value taken from ICRP Publication 65 and paragraph 294 of ICRP Publication 103. Both these publications advocate an upper value of 10 mSv for the individual dose reference level for exposure to radon. On the other hand, Table 5 of Publication 103 states that appropriate reference levels and dose constraints should be set in the range of 1-20 mSv.
On closer scrutiny, the approach advocated in the ICRP Statement on Radon (issued in November 2009) is in fact based on a reference level of 20 mSv, and not 10 mSv. The reference levels of 300 Bq/m3 for homes and 1000 Bq/m3 for workplaces each correspond to an annual dose of 10 mSv, thereby giving an individual “maximum” value of 20 mSv in any given year. However, this document is advocating that one value of reference level is set for the indoor environment, corresponding to a maximum annual dose of 10 mSv.
It is to be welcomed that, by adopting and implementing this new approach, both the individual and collective dose from radon would be reduced. However, the choice of 10 mSv as a reference level of individual dose seems to be rather arbitrary and is, perhaps conveniently, the mid-point of the range. It is not absolutely clear why the value of 10 mSv has been chosen in preference to any other value and it may not be appropriate for the ICRP to undertake the first step of the optimization process on behalf of national authorities.
Applying one value of reference level to the indoor environment
This is a novel approach that is different to the approach advocated in the recent ICRP Statement on Radon. There is a clear consistency in treating all indoor exposures in the same manner but, in terms of optimization of protection, this approach assumes that the distribution of radon in dwellings is similar to that in other buildings where the exposure is to be treated as an existing exposure situation. This may or may not be the case, and there is generally less information available on radon levels in workplaces compared to dwellings.
The advantage of this approach is that it removes the anomaly of calculating annual radon exposure on the basis of 7000 hours in the home and 2000 hours at work – for those who work in an indoor environment outside the home, there is a clear inconsistency in such an approach. It has the added advantage that it reduces the annual dose from 20 mSv to 10 mSv, as discussed above.
With many national authorities already having adopted a national reference level for dwellings of 200 Bq/m3, the implication of the new approach proposed in this document is that this value should also now be applied to all other indoor environments. The technology certainly exists to support this approach i.e. application of the available corrective actions allows elevated radon concentrations to be reduced to below 200 Bq/m3 in most instances, but the absence of detailed information on the distribution of radon in workplaces and “mixed use” buildings also needs to be recognized . Many countries already apply the reference level for dwellings to exposure in schools and kindergartens, but extending this approach to offices, shops and factories may need further consideration. However, in principle the IAEA recognizes the advantages of this new approach.
Managing exposure in workplaces
This issue is covered in section 3.3.6 and paras. 104 to 112. There seems to be some confusion about which workplaces which should be treated as existing exposure situations and those that should be treated as planned exposure situations. In this regard the IAEA recognizes that the factors that need to be taken into account can be difficult and complex, but there is a responsibility to make the approach as simple and as harmonized as possible.
The basis for choosing a national reference level is that it reflects the situation that applies in the country in question. Therefore, in practice most workplaces are likely to have radon concentrations well below the national reference level. As correctly pointed out, employers should apply the principle of optimization in reducing radon concentrations that are above this value to reduce them below the reference level.
Situations will arise where it is either not possible or not acceptable to reduce radon concentrations below the reference level. The IAEA Safety Standards considers these situations as existing exposure situations to which a graded approach should be applied. The IAEA does not advocate the use of a dose reference level of 10 mSv – if the radon concentration cannot be reduced below the reference level (which, in line with this document, would now be set at a value of 300 Bq/m3 or less and, using an annual occupancy of 2000 hours and an equilibrium factor of 0.4, corresponds to a maximum annual dose 3 mSv), then some additional controls need to be imposed, the stringency of which will depend on the extent to which the radon concentrations exceed the reference level.
In caves and other underground workplaces, it might be appropriate to measure the equilibrium factor and take consideration of occupancy rates in different parts of the workplace, but this should only be to inform the decision of how the graded approach should be applied. In cases where the average radon concentration is only marginally above the reference level, this additional work might indicate that no additional requirements are justified and that it is only necessary to keep the situation under review. In other types of workplaces such as offices, shops and factories, for practical reasons a decision on the application of the graded approach should be made on the measured radon concentration and not the corresponding effective dose.
This document goes some way towards standardizing the approach to controlling exposure to both natural and artificial sources of radiation in the workplace, but having gone so far it seems to be a backward step to introduce a dose reference level of 10 mSv for a small number of workplaces with unusual characteristics. A different approach, based on the national reference level, has been agreed by Member States in the drafting process for the International Basic Safety Standards. Once a national reference level is established, this should be the starting point for deciding if some of the measures used to control a planned exposure situation need to be applied, using a graded approach.
Homes with exceptionally high radon concentrations
A useful addition to the document would be to explain the philosophy and approach to be adopted when exceptionally high radon concentrations are identified and the homeowner is unwilling to take corrective actions voluntarily. ICRP Publication 103 recognizes that doses above 100 mSv, received either acutely or in a year, are justified only under extreme circumstances. In the case of radon, the corresponding concentration in a dwelling is 3000 Bq/m3. There are probably several thousand dwellings in Europe alone with radon concentrations above this value where the occupants receive annual doses exceeding 100 mSv. Does the ICRP consider that corrective action should be mandatory in such situations, that the dwelling should be declared unfit for habitation or is some other form of action advocated?
General The term “radon” is used as an inclusive term for radon-222, radon-220 and radon-219 in para (6), but subsequently in the text it is used to refer solely to radon-222. Terminology needs to be made consistent;
General The report deals almost exclusively with radon-222. Radon-220 is briefly discussed only in paras. (21) and (136). It might be better to deal with all radon-220 issues in one section, and in greater detail. The remainder of the document would then deal exclusively with the control of radon-222;
General: Throughout the text, the terms “radon action plan”, “radon protection strategy” and “national radon strategy” are used. It is not clear if these mean different things or if they are intended to mean the same thing;
General: While reference is made to the WHO Radon Handbook, no references are made to IAEA safety standards;
General: It would be helpful to clarify if the ICRP regards the reduction of high radon concentrations in existing buildings as primarily a public health (societal/population risk) or a radiation protection (individual risk) issue;
General: The document uses the terms “remediation”, “mitigation” and “corrective actions” interchangeably in referring to reducing high radon concentrations in existing buildings. The term “remediation” could be confusing and should not be used;
General: The document uses the term “dose reference level”. The definition of “reference level” states that it is a level of dose or risk, so it is unnecessary to use the term “dose reference level”.
Executive Summary (c) It should be made clear that it is the inhalation of radon daughters present in indoor air that gives rise to the exposure and not the radioactive decay of inhaled radon;
Executive summary (r) The expression “upper value of the tolerable risk” is not used in ICRP 103, and it is recommended that it should not be used in this document. This also applies to para (112) and para (167);
Para (7) The reference to “patients” might be confusing. In this context, patients are members of the public. This also applies to para. 44;
Para (21) Some reference should be made to the potential for high thoron concentrations in homes built on exposed soil or with mud bricks made from local soil. This may not be a typical construction type in Europe, but it is in other parts of the world;
Para (56) Is it absolutely true that exposure to outdoor radon cannot be controlled? If the radon exposure is due to mining wastes, these can be removed, subject of course to other principles of radiation protection being complied with;
Figure1 to Figure 3 The half-lives of isotopes need correction for consistency with the latest nuclear data information. Also, the half-lives values should not be rounded as that results in error. This is important when a series of isotopes are described with half-lives varying from nanoseconds to billion years. (For instance, the authors are referred to Appendix 1 of the IAEA Safety Report No.68, 2011 which provides U-238 and Th-232 series);
Figure 5 This figure uses the terms “averted dose”, “residual dose” and “planned added dose”. Apart from the figure being very confusing, these terms are not used elsewhere in the document. If they are to continue to be used, it is recommended that they should be discussed in para 88. It is also considered that the term “planned added dose” not be used: it is unclear what is meant and its use may be confused with “planned exposure situation”;
Para (96) The expression “…prevailing country – or region – specific….” suggests that different values for the reference level could be adopted in different regions within the same country. It is difficult to see that this would be practical and could be interpreted as different levels of protection being applied to different groups of the general public in a given State;
Para (132) It is recognized that the construction of energy-efficient homes and the retrofitting of existing homes to improve energy efficiency may impact – either positively or negatively – on indoor radon concentrations. However, the IAEA does not agree that there is sufficient scientific evidence to support the statements made in this paragraph. In addition, Figure 8 seems to be based on measurements in only five dwellings – it seems highly inappropriate to reproduce this figure and use it as the basis for far-reaching statements in the text. It would be more appropriate to highlight this as an issue requiring consideration as part of any national radon strategy;
Para (145) The examples given at the end of the paragraph are not always associated with high indoor radon concentrations and therefore the text may be misleading;
Para (146) The use of soil gas measurements as a predictor of indoor radon concentrations has not always been successful and is used, with varying degrees of success, in only a small number of countries;
Para (158) It is not clear why “consideration” needs to be given to buildings with public access when the thrust of this document is that the same reference level should apply to all indoor environments. This is not “consideration”, but rather treating all indoor radon exposure equally. Note: previous drafts used the term “special consideration”, but the concept of “consideration” is still inappropriate;
Para (159) Same point as that made for para 158;
Para (171) The text should be modified to recognize that the contribution of radon to the total individual dose can be highly variable both within and between sites. Particularly in the case of modern mines with good ventilation systems, gamma radiation rather than radon is normally the principal source of radiation exposure while in certain parts of the world, radon will be the predominant source. This variability needs to be captured in the text.