|Exposure to natural sources
Many of the editorial comments that the European Commission sent before the deadline of 15 September for web consultation of the current draft of ICRP Recommendations concerned the chapter on natural radiation sources. In part these were comments on substance; many corrections were proposed to improve the background information to this topic. The Working Party of the Article 31 Group of Experts who prepared these comments felt that this chapter was of lower quality than the preceding chapters.
I had the opportunity of explaining our position at the Committee 4 meeting in Madrid, and, as a result of these discussions, the Committee invited me to seek the support of our WP on NORM in editing this chapter. At the meeting of the WP that took place a week later, Jan Van der Steen kindly offered to redraft chapter 7. His draft, with some corrections, was discussed at the meeting of the Group of Experts on 18 October.
I am now pleased to transmit this document, which follows below, to ICRP on behalf of the Group of Experts. While acknowledging the excellent work of Jan Van der Steen, the Group of Experts expressed general views as outlined below.
The Experts are generally in agreement with the text while noting, however, that it has been written in such a way as to be consistent with the main views of ICRP expressed in the original draft. The text proposal is therefore essentially editorial and does not imply endorsement of all such views by the Group of Experts. The Experts also point out that, while the text is broadly in agreement with the graded approach introduced for the regulatory control of NORM industries in the revision of the EURATOM BSS, the Group has not yet discussed more detailed requirements for building materials, the issue of radon in dwellings and workplaces, nor aircrew exposure.
It should also be pointed out that the precise formulation of the threshold of 1 Bqlg for U-Th still raises a lot of discussion, bearing in mind that this level should not be regarded as a universal exclusion level. The earlier comments of the Experts on the document on "scope of radiation protection regulations" thus remain fully valid. A further document discussing this aspect in greater detail will be sent to you separately. There will also be a further contribution on the very important concept of constraints.
While the text proposal for chapter 7 is consistent with the formulations in the overall ICRP document, we feel there is a need for a much sharper definition of constraints, action levels, intervention exemption levels, etc.
Finally, you will also receive the conclusions of the Scientific Seminar held on 17 October. This may give ICRP grounds for further consideration of the dose conversion factors for radon and thoron, allowing also for the values proposed by
UNSCEAR, and their possible impact on the constraints envisaged for radon in dwellings and workplaces.
I hope ICRP will find this contribution useful in further improving the draft Recommendations. I am copying this letter to Annie Sugier and John Cooper for consideration by Committee 4 also.
Yours sincerely, Augustin Janssens
7. EXPOSURE TO NATURAL SOURCES
(277) There are many sources of exposure to natural radiation and each can vary significantly with geography, geology and lifestyle. Natural radiation exposures are broadly grouped as cosmic radiation and radiation from terrestrial radionuclides, which can result in external exposures (both indoors and outdoors) or internal exposures due to inhalation or ingestion. Such sources are the dominant cause of exposure for the vast majority of people.
(278) The majority of the world population incurs doses around the average global exposure of 2.4 mSv per year. However, there are many situations, such as living in high background areas, the use of materials with elevated levels of natural radionuclides for industrial processing or house construction, living in dwellings with high radon concentrations, mining, and aviation, which lead to increased exposure of the public and/or the workforce. An overview of exposure situations and associated occupational and public doses can be found in UNSCEAR 2000.
(279) The Commission's system of radiological protection applies to all radiation sources and exposures including those of natural origin. The exposure situations can be divided into planned situations and existing situations. Emergency situations for natural sources are unlikely, since accidents with natural sources are not likely to result in high exposures or exposure rates.
(280) The industrial use of raw materials with natural radionuclides, often referred to as naturally occurring radioactive materials (NORM), is to be considered as a planned situation. Many ores are enriched in radionuclides from the uranium and thorium decay chains, and the levels of these radionuclides are often further enhanced in the residues. Consequential exposure of members of the public and the workforce can often occur. The difference with artificial sources of radiation is that the radioactivity in NORM industries is an unwanted property. Moreover, in contrast to artificial sources, the planned situations in NORM industries are in many cases characterised by the large volume of the source, i.e. of the order of hundreds of thousands tonnes per year.
(281) The Commission continues to recommend the application of the principles of justification, optimisation and dose limitation for these planned situations. However, for the justification to operate a NORM industry, the radiological properties will normally have only a minor input, if any.
(282) The process of optimisation is therefore the most important mechanism to reduce the exposure to natural sources, certainly in new facilities where radiological protection requirements can be considered during the planning stage. Radiological protection should be considered in relation to the protection against other chemical or physical contaminants to which the workforce and the public may be exposed. In many cases, conventional measures for occupational health and safety and for environmental protection will be effective in also reducing the exposure of the workforce and members of the public to levels that can be considered as being optimised.
(283) For existing exposure situations to natural sources in the general environment, the Commission continues recommending to apply the procedure of justification of an intervention action to reduce the exposure to below the value of a constraint, down to a level established on the basis of optimisation. The dose limits recommended by the Commission for planned situations do not apply to decisions on intervention in relation to natural sources.
(284) In cases where action to reduce doses is recommended, the process of optimisation below a constraint should be followed. The Commission has previously provided guidance on selecting an action level for protection against radon (see section 7.3), as well as for protection against other natural sources. In ICRP 65 and ICRP 82, the Commission has set bands of generic reference levels, the lower bound being the value below which an intervention would not likely to be justifiable and the upper bound being the value where interventions would be almost always justifiable. Accordingly, action levels should be set through national or local processes of generic optimisation within a defined range. The upper bounds are effectively constraints. They should be considered as levels of aspiration and not a mandatory level, which must be achieved in all circumstances.
(285) In cases where doses exceed the value of constraints, optimised protective actions should be taken to reduce doses. However, there may be situations where actions can be undertaken that are technically easy and not entailing excessive cost to reach an optimised level of exposure, even when the dose is below the value of the constraint. The Commission considers this as existing exposure situations for which an action level may be selected below the upper bound of the band of generic reference levels, i.e. below the value of the constraint. For such situations, which may differ by source and setting, the Commission recommends national authorities to develop guidelines or codes in order to reduce the dose further.
(286) In many cases it will be obvious that action to reduce doses is not warranted. This conclusion will often be based on common sense. The principles for exclusion and exemption of natural sources can be applied in such cases. Natural sources could be excluded from regulatory control on the basis that they are unamenable to be controlled with regulatory instruments. The principle of exemption from regulatory requirements could be applied in situations that are unwarranted to be controlled. This is further discussed in Section 2.4.
7.1. Controllability of sources
(287) Natural radiation sources are ubiquitous and are the largest contributor to human exposure. Logically, it could be supposed that the most stringent radiological protection measures would apply to these sources, but in fact natural radiation exposure has up to now not been dealt with comprehensively in the Commission's radiological protection recommendations.
(288) The Commission's recommendations have been focused on 'man-made' or 'artificial' sources with only a few situations involving 'natural' sources being considered. Artificial sources are generally readily controllable and societal demands in the application of the optimisation principle has led to the situation that in many practices the dose to the workforce and the public is (close to) negligible.
(289) Natural radiation sources are, however, less readily controllable than artificial sources. Due to its ubiquitous nature, the application of the optimisation principle in planned and existing situations will necessarily lead to levels of exposure that are 1 to 2 orders of magnitude higher than the trivial dose, i.e. of the order of (variations in) the natural background dose. In the case of natural sources, a negligible exposure is not quite meaningful, and levels of radioactivity are adequate if they correspond to acceptable levels of exposure. This does not mean that the system of protection for natural sources differs from that for artificial sources; it rather reflects the controllability of the sources.
(290) The Commission recommends that the radiological protection system should apply to all sources of exposure, artificial and natural, in a coherent and consistent way taking into account the feasibility of control, the magnitude of the exposure and the societal demands for protection.
(291) Not all natural exposure situations can or need be formally controlled by regulatory requirements. The Commission recommends using the concepts of exclusion and exemption to define the scope of regulatory control. Exclusion refers to the process of identifying radiation exposure situations that need not to be covered by radiological protection regulations because they are considered to be unamenable to control by any reasonable means. Exemption refers to the process of identifying situations that are within the scope of the regulations but can be released from specific requirements because their application is not warranted.
(292) Many natural radiation sources are not amenable to control in that they are unavoidable or uncontrollable, at least without inordinate effort. Such sources should be excluded from regulatory control. Examples of unavoidable natural sources are cosmic rays at ground level and 40K in the human body.
(293) The Commission also recommends excluding materials with natural radionuclide concentrations below those prevailing in most parts of the world. A suitable threshold seems to be around 1000 Bq/kg for the heads of the uranium and thorium series and 10,000 Bq/kg for 40K. The values for the natural decay chains are valid for secular equilibrium, i.e. 238U, 235U and 232Th, with the value given being applied to the parent of the decay chain. The values can also be used individually for each decay product in the chains or the head of subsets of the chains, such as 226Ra. The arguments for this recommendation are given below. As explained in (301) there are situations where lower levels are warranted, hence the above values should not be regarded as universal exclusion levels.
(294) There are two approaches to derive exclusion levels for such materials. One approach is based on the consideration of the worldwide distribution of activity concentrations in soil; the other approach is based on the consideration of the average level of exposure of the public due to natural sources and its normal variation. Detailed information on activity concentrations and levels of public exposure can be found in UNSCEAR 2000.
(295) The first approach has been taken by the IAEA. In RS-G-1.7, the worldwide distribution of the activity concentrations of natural radionuclides in soil and in ore has been considered to derive exclusion levels for natural radionuclides. From the data in UNSCEAR 2000, it can be concluded that the concentration of 238U, 226Ra and 232Th in most raw materials is below a value of the order of 500 - 1000 Bq/kg and below a value of some thousands of Bq/kg for 40K. The values of 1000 Bq/kg for the uranium and thorium decay series and 10.000 Bq/kg for 40K represent the upper end of the worldwide distribution of activity concentrations in soil and ores, rounded to the nearest order of magnitude, and these values have been defined as exclusion levels in RS-G-1.7. IAEA concludes that doses to individuals as a consequence of these activity concentrations would be unlikely to exceed 1 mSv per year, excluding the contribution of radon.
(296) The second approach has been taken by the European Commission in Publication RP 122, Part II. Strictly speaking, this document deals with the application of the concepts of exemption and clearance to natural radiation sources, but it is concluded that 'within a scheme for regulatory control (...) there is nevertheless room for excluding (or not including) part of the exposure to natural radiation sources from the total exposure'. The European Commission concludes that a definition of values for regulating natural sources cannot be made on the basis of trivial risk, or dose. If one would impose a restriction of 10 µSv per year, it would in general not be practicable to implement a control scheme for such a small increment to the natural background, in fact below the natural variability.
(297) The European Commission has therefore chosen a dose criterion of 300 µSv per year for establishing exemption and clearance levels for natural sources, which is comparable to regional variations in the natural background (external exposure only). This dose criterion is used to calculate activity concentrations, using enveloping scenarios for public and worker exposures. For the most important radionuclides, these levels are 500 Bq/kg for 238U and 232Th in secular equilibrium, and 5.000 Bq/kg for 40K.
(298) The Commission has recommended in ICRP 75, for materials containing uranium and thorium series radionuclides, that 'regulatory agencies choose activity concentrations of parent nuclides within the range 1–10 Bq/g to determine whether the exposures from these materials should be regarded as occupational', noting that such concentrations 'will lead to an effective dose of about 1–2 mSv in a year'. This recommendation implies that if the effective dose received by a worker from exposure to natural sources does not exceed 1–2 mSv in a year it would be unnecessary to regulate that exposure. Experience with industrial activities involving exposure to natural radionuclides indicates that the dose received by a member of the public living near the industrial facility concerned is generally much lower (exceptionally of the order of 100 µSv per year — see, for instance, UNSCEAR 2000, Annex B, para. 188), and is consequently only a small fraction of the dose that could be received by a worker.
(299) The approach taken by the IAEA and the approach taken by the European Commission converge and lead to the same conclusion, namely that the activity concentrations of natural radionuclides up to the levels as recommended now by the Commission will in general not lead to exposures of members of the public and the workforce exceeding 1 mSv per year. This recommendation is coherent with the Commission's recommendation in ICRP 75.
(300) The recommended values for exclusion are also pragmatic, in the sense that lower levels would lead to regulatory control of a considerable part of materials in the world. The Commission believes that regulatory control of these materials would generally not lead to an improved level of protection.
(301) There are, however, situations for which exposures from materials with activity concentrations below those recommended by the Commission that would necessitate consideration by the regulatory authority for some types of regulatory control. This accounts for commodities such as building materials, foodstuffs, drinking water and animal feed. The Commission reaffirms its recommendation in ICRP 82 that a generic intervention exemption level of around 1 mSv should be used for the individual annual dose expected from a dominant type of commodity. It is recommended that the exposure to natural sources in these commodities be controlled by separate regulatory control measures, in order to ensure that public exposures to natural sources remain below 1 mSv per year in any foreseeable situation. On the basis of this recommendation, concerned national and, as appropriate, relevant international organisations should derive generic, and radionuclide specific, intervention exemption levels for individual commodities, in particular for specific building materials. For drinking water, the recommendations of WHO would apply. With regard to foodstuffs the Codex Alimentarius Commission has recently adopted new guidelines, which however apply only after a radiological emergency. In general there will be no need to control food with regard to naturally occurring radionuclides.
7.1.2. Exclusion of radon exposures
(302) As explained in the foundation document The Scope of Radiological Protection Regulations (ICRP, 2006), the Commission considers that it would be convenient to define some concentration value of radon below which the exposure could be considered to be excluded from regulation. Typical outdoor long-term average radon concentrations have been reported to range from 1 - 100 Bq/m3 (UNSCEAR 1993). Typical concentrations are several tens of Bq/m3, with an arithmetic mean of the worldwide distribution of 10 Bq/m3 outdoors and 40 Bq/m3 indoors (UNSCEAR 2000).
(303) Following a reasoning parallel to the case of the exclusion levels for materials, an exclusion level could be set for radon concentrations leading to doses in the order of 1 - 2 mSv per year, i.e. 40 - 100 Bq/m3. Such radon concentrations may be regarded as unamenable to control in that they are unavoidable or uncontrollable, at least without inordinate effort. However, since exposure to radon is regarded as an existing situation for which intervention levels would apply (see Section 7.3), the derivation of an exclusion level has little practical significance.
(304) Materials with activity concentrations exceeding the recommended values should enter the system of radiological protection. There are situations, however, that the level of exposure to such materials will still be below 1 mSv per year. The Commission recommends that regulatory authorities consider the application of the concept of exemption for such situations. This may be done on a case-by-case basis, or generically for a group of comparable exposure situations.
(305) There may be also non-excluded situations where the level of radiological protection may be regarded as being optimised, even if the exposure level exceeds 1 mSv per year. Exercising regulatory control for such situations would not lead to an improved level of radiological protection. The Commission recommends that such exposure situations be exempted from specific regulatory requirements because their application is not warranted.
7.1.4. Graded approach of regulatory control
(306) In cases where regulatory control is considered necessary, a graded approach should be used, taking account of the potential risks to workers and members of the public. The application of the requirements for practices shall be commensurate with the characteristics of the practice or source and with the magnitude and likelihood of the exposures. The graded approach applies to all sources of radiation subject to regulation. However, it is particularly relevant to practices involving exposure to natural sources, because the exposures are generally (but not always) moderate, with little or no likelihood of extreme radiological consequences from accidents, and because occupational health and safety measures already in place to control other (non-radiological) hazards in the workplace may provide protection against radiological hazards as well.
(307) The regulatory instruments to be used in a graded approach are notification and authorisation. Where the regulatory authority decides that there is a need to apply regulatory requirements to a particular type of operation or process, the first instrument to be used is notification, i.e. the requirement for the legal person to formally submit a notification to the regulatory authority. Notification alone could be sufficient where exposures are unlikely to exceed a small fraction of the relevant constraints and where the source or practice is inherently safe. In practical terms, this is similar to exemption, but with the important difference that the regulatory authority is kept informed of all such operations or processes.
(308) Where the nature of the hazard is such that further requirements need to be placed on the legal person, he should apply to the regulatory authority for an authorization. The authorization may take the form of either a registration or a licence, the difference being essentially in the level of stringency of regulation. Registration, which typically places only limited requirements on the legal person, may provide a sufficient level of control in many operations. In situations where optimized protection can only be achieved through the enforcement of specific exposure control measures, licensing may be the more appropriate form of authorization.
7.2. Existing NORM industries
(309) For historical reasons, many existing NORM industries have not always operated within the Commission's system of protection. Where necessary, steps should be taken to bring these facilities under regulatory control. A decision should be made on whether it is justified to reduce exposures. This decision should also consider whether continued operation of the facility is justified.
(310) When optimising protection, the constraint for occupational exposure should comply with the corresponding occupational dose limit, because occupational exposures are normally relatively straightforward to control.
(311) There may, however, be a number of issues surrounding public exposure, particularly to the accumulated waste residues from historical operations. Application of a dose constraint that complies with the Commission's public dose limit, which is intended to be used in circumstances where radiological protection has been planned in advance, may lead to protective actions which are inappropriate for the health benefits obtained. Such public exposure situations may be considered as existing exposure situations with the value of the constraint being selected accordingly.
7.3. Constraints for radon in dwellings and workplaces
(312) The Commission's view on radon risk assessment has, up to now, been that it should be based on epidemiological studies of miners. Given the wealth of data on domestic exposure to radon, the Commission now recommends that the estimation of risk from domestic radon exposure be based on the results of pooled residential case control studies. The currently available epidemiological evidence indicates that risks other than lung cancer from exposure to radon (and decay products) are likely to be small.
(313) The Commission has issued specific constraints for 222Rn at home and at work. There are several reasons to treat radon in this separate manner. The exposure route differs from that of other natural sources, and there are dosimetric and epidemiological issues peculiar to radon. For many individuals 222Rn is an important source of exposure that, in principle, can be controlled. The Commission issued the current recommendations for protection against 222Rn at home and at work in ICRP 65. The policy has found wide acceptance and the present recommendations broadly continue the same policy.
(314) Because most people exposed to radon will also be exposed to other sources of radiation, it is helpful to provide a conversion from radon exposure to effective dose. The Commission has not used a dosimetric approach for radon in this conversion, but a direct comparison of the detriment associated with a unit effective dose and a unit radon exposure. In terms of detriment, an exposure to radon progeny of 1 mJ.h/m3 is equivalent to an effective dose of 1.4 mSv for workers or 1.1 mSv for members of the public (ICRP 65). The corresponding figures for 1 working level month (WLM) are 5.1 mSv for workers and 3.9 mSv for members of the public. The difference is due to the different detriments per mSv for workers and members of the public. The conversions obtained in this way are called conversion conventions, and they are based on detriment, not on dosimetry.
(315) In ICRP 65, the policy was based upon first setting a level of effective dose from 222Rn where action would certainly be warranted to reduce the exposure. This was an effective dose of 10 mSv per year. The effective dose was converted into a value of radon concentration, which was different between homes and workplaces largely because of the relative number of hours spent at each. National regulatory authorities were expected to apply the optimisation process to find a lower level at which to act. The optimisation presumption thus led to a suggested range, within which so-called action levels were expected to be set. The result of the optimisation was to set action levels above which action was required to reduce the dose. For practical application the Commission used activity concentrations for these levels, rather than dosimetric quantities. For dwellings this range was a radon concentration between 200 - 600 Bq/m3, while the corresponding range for workplaces was 500 - 1500 Bq/m3. The Commission, however, accepted that an action level might be set below this suggested range, where national considerations so indicated. In particular, this may be the case where action levels are set for new buildings where radon remedial measures are far more cost effective than for existing buildings (see also paragraph 285).
(316) The Commission reaffirms the basic principles for controlling radon exposure as set out in ICRP 65. Even though the nominal risk per Sv (and the risk per unit exposure to radon) has changed slightly, the Commission, for the sake of continuity and practicality, retains the relationship between the constraint of 10 mSv given in ICRP 65 and the recommended corresponding activity concentration. This means that the upper bounds of the range of action levels remain at 600 Bq/m3 for homes and 1500 Bq/m3 for workplaces. In the terminology of these recommendations, these upper bounds are now called constraints (Table 6).
Table 6. Constraints for 222Rn†
Domestic dwellings 600 Bq/m3
Workplaces 1500 Bq/m3
†Head or initial radionuclide of the decay chain activity level
(317) It is the responsibility of the appropriate national authorities, as with other sources, to establish their own constraints and then to apply the process of optimisation of protection in their country. All reasonable efforts should be made to reduce radon exposures at home and at work to below the constraints that were set. It is important that the action taken should be intended to produce substantial reduction in radon exposures. It is not sufficient to adopt marginal improvements aimed only at reducing the radon concentrations to a value just below the constraints.
(318) Once optimisation has resulted in concentration activities well below the action levels, no further action is required, apart from perhaps sporadically monitoring activity concentrations to ensure that levels remain low.
7.4. Thoron (220Rn)
(319) Thoron is much different from radon due to its very short half-life (T½ = 55.6 s). Generally, the indoor thoron concentration is low, except in some building materials and soil with high concentrations of 232Th and high porosity. Due to the very short half-life of thoron, it can be rarely detected at some distance from the source.
(320) The problems posed by thoron are much less widespread, and generally more tractable, than those posed by radon. However, people at sleep generally breathe air close to walls, where in the above-mentioned situations the presence of thoron may expose members of the public to a non-negligible dose.
(321) Measurement of the thoron concentration is generally of low significance. However, the thoron decay product 212Pb has a half-life of 10.6 hours and is therefore generally quite homogeneously mixed in an indoor environment. Since the correlation between thoron and its decay products (mainly 212Pb) cannot always be ensured, a correct estimate of the dose may even require measurement of thoron itself. In any case, for meaningful thoron measurements it is necessary to use a measurement protocol, which includes a precise statement on the distance from the source where the thoron concentration has been measured.
(322) The current conversion convention suggested in ICRP 65 for radon decay products, which is based on detriment and not on dosimetry, is not applicable to thoron decay products due to the lack of epidemiological studies. The Commission is aware of new developments in deriving dose conversion factors using modern respiratory tract models. However, for this moment, the Commission continues to recommend a dose conversion factor of 0.52 Sv per J.h/m3, which has been derived in ICRP 50 based on the available dosimetric models at that time.
7.5 Cosmic rays above ground level
(323) In ICRP 75, the Commission indicated that exposure to cosmic rays at altitudes above ground level may be considered as a candidate for either exclusion from regulations or generic exemption from most regulatory requirements, depending on the regulatory system and national arrangements in place. The Commission noted that the only practical regulatory measures were controlling flying time and route selection. Some national regulations require monitoring of aircrew exposures and others set a limit of 6 mSv per year above which their work schedule should be adjusted, but they are generally mute for non-crew flyers. The Commission continues its recommendation that only aircrew should be considered, and that it is not necessary to treat the exposure of frequent-flyer business passengers as occupationally exposed for the purpose of control.
(324) The Commission considers that there seems to be no obvious reason to introduce regulatory controls for common exposure situations to cosmic rays above ground level but recognizes that regulatory authorities may wish to monitor these situations. The need for monitoring will often be prompted by the legitimate demand of aircrew to be informed of their exposure. Precautionary measures for pregnant aircrew may also be warranted. Exceptional cases of cosmic ray exposure, such as exposure during space travel, where doses may be significant and some type of control warranted, should be dealt with separately taking into account the special types of situations that can give rise to this type of exposure.