|Australian Radiation Protection Society submission to the ICRP on the 5 June 2006 draft Recommendations
ARPS, in its Position Statement (appended), recognises that there is scientific uncertainty about the dose-effect relationship at low doses and low dose rates, particularly as to whether low levels of exposure cause risk or health benefit. In the face of this uncertainty, ARPS has adopted what it considers to be a rational approach to the assumptions and regulatory controls that should apply to low-level radiation, with distinctions being drawn between different ranges of exposure and different aspects of regulation.
The ICRP (in Clauses 25 to 27 of the Draft Recommendations) states that:
· the aim of an appropriate level of radiological protection cannot be achieved solely on the basis of scientific knowledge concerning the health risks attributable to radiation exposure; and that
· societal and economic aspects of protection also have to be considered, including value judgements about the balancing of these risks and the benefits from endeavours involving radiation exposure.
For the purposes of this balance, the ICRP appears not to countenance that at low doses health benefits may accrue from the radiation exposure itself, viz: that protection might be counterproductive or, at best, pointless except with regard to public perceptions.
ARPS, in its Position Statement, draws attention to controversy in the radiation protection literature on whether ionizing radiation is harmful, beneficial or has no significant health effect at low doses and at low dose rates. ARPS recognises that there is scientific uncertainty about this matter for doses less than about 100 millisieverts in a year, with conflicting scientific evidence and interpretations of this evidence. ARPS does not purport to resolve this controversy but attempts to come to terms with it.
The ICRP recognises the controversy but largely dismisses it. In Clause 12 of its Draft Recommendations, the ICRP says that it has reviewed “the vast body of literature on the health effects of ionising radiation” but sees no need for any fundamental changes to the system of radiological protection. Hence (Clause 29), the use of the “so-called linear, non-threshold hypothesis or LNT, is [still] considered by the Commission to be the best approach to managing risks from radiation exposure” and:
[Clause 56] “…. the assumption of the LNT hypothesis combined with a judged value of a dose and dose rate effectiveness factor (DDREF) provides a prudent basis for the practical purposes of radiological protection, i.e. the management of risks from low level radiation exposure.”
ARPS in this submission is making a plea for the ICRP to more adequately recognise in the Recommendations that risks from dose rates similar to those of average natural background are unknown, but that there is mounting radiobiological evidence that such doses may be without detrimental effect or beneficial.
If such evidence were confirmed by continuing studies it would clearly have significant effects on the practice of radiation protection and in particular on the need for elaborate regulatory controls on doses, particularly of low LET radiation, that are around natural background levels. It would also affect the undue concern about small doses of radiation that has been experienced in contaminated territories. If the ICRP does not at least allow the possibility that the LNT hypothesis, while being a practical guide for radiation protection practice, may not apply at very low dose rates, it could be faced by the accusation in another 15 years that it has grossly overestimated risks from low doses, and thereby given rise to undue concerns about low level radiation.
ARPS proposes that the changes (indicated in red) in paragraphs (29) and (55)-(58) be made to more adequately reflect uncertainty about risks from low doses.
(29) At low doses, of the order of those caused by natural background radiation, the increase in the incidence of stochastic effects is assumed by the Commission for the purpose of prospective radiation protection to occur with a small probability and in proportion to the increase in radiation dose over the background dose. However, it is acknowledged that the magnitude of any effects from radiation at these doses and dose rates is unknown and that beneficial as well as detrimental effects may occur. Despite these uncertainties, the Commission continues to recommend use of the so-called linear, non-threshold hypothesis or LNT as the most practical approach to managing risk from radiation exposure.
(55) Although there are recognised exceptions, for the purposes of radiological protection the Commission judges that the weight of evidence on fundamental cellular processes coupled with dose-response data supports the view that in the low dose range under 100 mSv it is pragmatic to assume that the increase in the incidence of cancer or hereditary effects will rise in direct proportion to an increase in the absorbed dose in the relevant organs and tissues.
(56) Therefore, the practical system of radiological protection recommended by the Commission continues to be based upon the assumption that at doses below around 100 mSv a given increment in dose will produce a directly proportionate increment in the probability of incurring cancer or hereditary effects attributable to radiation, an hypothesis that is generally know as ‘linear non-threshold’ or LNT. This view accords with that given by UNSCEAR (2000) and by NAS/NRC (2006). By contrast, a recent report from the French Academies (2005) argues in support of a practical threshold for radiation cancer risk. However from an analysis conducted by ICRP (Publication 99, ICRP 2006) the Commission considers that the assumption of the LNT hypothesis combined with a judged value of a dose and dose rate effectiveness factor (DDREF) provides a prudent basis for the practical purposes of radiological protection, i.e. the management of risks from low dose radiation exposure. Nevertheless, it recognizes that continuing concerns are being raised about the appropriateness of these assumptions (Tubiana et. al., 2006; Mitchel, 2006).
(57) The Commission emphasises that whilst the LNT hypothesis remains a plausible element in its practical system of radiological protection, biological information that would ambiguously verify the hypothesis is unlikely to be soon forthcoming (see also UNSCEAR 2000). Because of this uncertainty on effects at low doses the Commission judges that it is not appropriate, for the formal purposes of public health, to calculate the hypothetical number of cases of cancer or heritable disease that might be associated with very small radiation doses received by large numbers of people over very long periods of time. On this point, the Commission also emphasises that its estimates of nominal risk coefficients (Table 2 and Annex A) relate to contemporary human populations and depend upon current information on baseline disease rates, disease detriment and associated biological/clinical features. These factors are certain to change substantially over future generations and this adds to the implausibility of attempting to project the magnitude of radiation-associated disease far into the future.
(58) In arriving at its practical judgement on LNT, the Commission has considered potential challenges associated with information on cellular adaptive responses, the relative abundance of spontaneously arising and low dose-induced DNA damage and the existence of the post-irradiation cellular phenomena of induced genomic instability and bystander signalling (Publication 99; ICRP, 2006). The Commission recognises that these biological factors may be components of radiation cancer risk but that current uncertainties on their mechanisms and tumorigenic consequences are too great for the development of practical judgements on low dose risk, other than that at annual doses comparable with those of the average natural background dose the magnitude of any risk is highly uncertain. The Commission also notes that since the estimation of nominal cancer risk coefficients is based upon direct human epidemiological data, any contribution from these cellular phenomena would be included in that estimate. Uncertainty with regard to the role of these processes in cancer risk will remain until the demonstration of not only their relevance to cancer development in vivo but also knowledge of the dose-dependence of the cellular processes involved.
Tubiana, M., Aurengo, A., Averbeck, D. and Masse, R., Recent reports on the effect of low doses of ionizing radiation and its dose-effect relationship. Radiat. Environ. Biophys. 44, 2006, pp.245–251.
Mitchel, R., Cancer and low dose responses in vivo: implications for radiation protection. In Proceedings of the 15th Pacific Basin Nuclear Conference, Sydney, 15-20 October, 2006.
ARPS Position Statement on Risks from exposure to low levels of ionizing radiation
(This was adopted at the AGM held in Melbourne on 16 November 2005)
RISKS FROM EXPOSURE TO LOW LEVELS OF IONIZING RADIATION
Controversy continues in the radiation protection literature on whether or not ionizing radiation is harmful at very low doses. There is scientific uncertainty about the dose-effect relationship below a few tens of millisieverts in a year, and in order to settle what regulatory controls, if any, should apply in this dose region an assumption has to be made relating dose to the possibility of harm or benefit. The assumption made and, more particularly, the way it is applied can have far-reaching effects not only on the scale of regulatory compliance required but also on public perception of risk and therefore on the technological choices made by society. It is important therefore that decisions reached concerning regulation of low doses of ionizing radiation have an ethical basis and derive from rational argument. It is also important that such decisions are neither portrayed nor perceived as resolving the scientific uncertainties: rather they serve merely to facilitate the implementation of appropriate safety measures.
Following a review of available information, the Australasian Radiation Protection Society has adopted the following position. Based on the features observed, the range of exposures has been divided into three broad dose groups, but it should be noted that the boundaries between them are not known with precision.
Doses above about 10 mSv in a year
§ There is strong epidemiological evidence that acute exposure to ionizing radiation of more than about 100 mSv carries a risk of developing fatal cancer that increases with dose, with some limited evidence supporting a risk at slightly lower doses. There are also epidemiological reports of statistically significant risk from long-term cumulative exposures that correspond to doses received at rates down to a few millisieverts in a year, but it is difficult to be confident that the observed effects can be reliably separated from possible confounding factors.
§ In the light of the above, for the purpose of applying regulatory controls to radiation protection when effective doses exceed a few tens of millisieverts in a year, it is reasonable to assume a generalized risk coefficient for fatal cancer of 1 in 20 per sievert for a population of all ages, as recommended by the International Commission on Radiological Protection [ICRP Publication 60]. This assumption is less reliable for exposures below 100 mSv in a year than above.
§ Consistent with this assumption, an effective dose limit for occupational exposure of 20 mSv per year, averaged over 5 years and no more than 50 mSv in any one year, remains appropriate, as does a requirement to optimize protection below this value. Separately, safety measures are required to avoid deterministic effects of radiation at very high doses.
Doses between about 0.1 and about 10 mSv in a year
§ There is insufficient epidemiological evidence to establish a dose-effect relationship for effective doses of less than a few tens of millisieverts in a year above the background level of exposure. It is possible that both an adverse effect, through causation of cancer following radiation damage to DNA, and a beneficial effect, through stimulation of repair mechanisms, may operate. It has also been speculated that such a stimulatory effect might reduce mortality from cancer caused by agents other than radiation, resulting in a net decrease in risk. Consequently, neither harmful nor beneficial effects can be ruled out.
§ To put doses in this range into perspective, it is worth noting that the worldwide average exposure to natural radiation sources is estimated by the United Nations Scientific Committee on the Effects of Atomic Radiation to be 2.4 mSv in a year, with a typical range of 1 to 10 mSv in a year. There are a few areas of the world where much higher doses are received from naturally-occurring sources without causing discernible risks to health.
§ Taking an ethical position of caution in the face of uncertainty, the risk coefficient adopted above for higher doses may be used for the purpose of establishing control measures for exposure to radiation at lower doses. In particular, the use of an effective dose limit of 1mSv in a year for members of the public is appropriate for exposure caused by the conduct of business activities. This limit will ensure that the additional risk of harm, if any, arising from such activities is acceptably small. However, no inference may be drawn concerning the risk to health or risk of fatality of an individual from an effective dose below 10 mSv in a year. For individual doses less than some tens of millisieverts in a year, risk inferences are unreliable and carry a large uncertainty that includes the possibility of zero risk.
Doses below about 0.1 mSv in a year
§ The risk to an individual of doses less than a few hundredths of a millisievert in a year is so small, if it exists at all, that regulatory requirements to control exposure at this level are not warranted. Business activities causing individual effective doses of the order of 0.01 mSv in a year or less should be automatically exempted from regulatory control, provided that the activity is inherently safe: that is, there is little likelihood of accidents leading to significantly higher doses. Activities causing levels of exposure up to 0.1 mSv in a year may also be exempted if the regulatory body determines that the application of controls is not warranted, taking into account all relevant factors. In deciding whether control measures are warranted, or how stringent they should be, regulatory bodies should have in mind, inter alia, the principle that societal resources should not be wasted or freedoms inhibited through mandatory observance of unnecessary regulatory controls.
§ Estimates of collective dose to groups or to populations should be used with caution. In view of the uncertain association between low doses and risk, estimates of collective dose arising from individual doses that are less than some tens of millisieverts in a year should not be used to predict numbers of fatal cancers for the exposed group or population.
§ However, if collective doses to subgroups of an exposed population are each assigned an appropriate weight, they may play a role in making a choice between possible control measures and thus in optimizing protection. The component of collective dose arising from the summation of individual doses that are less than about 1 mSv in a year should be assigned little significance relative to components associated with subgroups receiving higher doses, and the component associated with doses less than some hundredths of a millsievert in a year may be assigned a weight of zero. Various values for this cut-off have been proposed, from 0.01 to 0.1 mSv.
What is ‘safe’?
§ The word ‘safe’ may be used to describe business activities that meet currently prescribed radiation safety standards. While there may be some, as yet uncertain, risk arising from such activities, it is known to be small at most and, through application of the justification principle, to be outweighed by the benefits brought by the activity. It follows that exposures of this order may be described as ‘safe’, understanding that the word is used not in an absolute sense but with the meaning of causing an acceptably small risk, if any.