Abstract
One of the first issues to arise in the development of a set of Reference Animals and Plants by the International Commission on Radiological Protection (ICRP) was that of allowing for factors such as relative biological effectiveness (RBE) in considerations of estimates of dose (ICRP, 2003, 2008). The issue was not a new one. The need to consider such factors had been widely recognised for many years, and for several reasons. Firstly, RBE obviously applies to animals other than humans; indeed, much of the RBE information used in human radiological protection had been derived from both in-vivo and in-vitro animal studies. It thus seemed reasonable that allowance should be made for such information in assessments of the relationship between dose and effects for those same animals in the context of their exposures to radiation in an environmental context. Secondly, it was known that many animals and plants can accumulate concentrations of naturally occurring alpha-particle-emitting nuclides within their tissues to very high levels, and thus there had been arguments for the use of ‘weighting factors’ to normalise assessments of comparative radiation background dose rates amongst different types of fauna and flora. Finally, because of the presence of alpha-particle-emitting nuclides (and, in some cases, tritium) in many environmental areas, as a result of different exposure situations, there had been concerns that their potential effects on wildlife would be underestimated if allowances for RBE were not taken into account.
Due to these uncertainties, various attempts to address them, or at least to highlight them, were made for some considerable time. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 1996) suggested that a weighting factor of 5 should be used for internal alpha emitters based on an evaluation of reproductive (tissue reaction) endpoints for all living things other than humans, but no recommendations were made for low-energy beta-particle-emitting nuclides. Subsequently, and in view of the apparent general lack of interest in pursuing the issue further, a number of approaches were proposed by various groups and individuals. One was simply to refer to high- and low-linear energy transfer (LET) components of the dose rate separately; however, this approach was considered to be too clumsy. Another approach was to use a dose equivalent for fauna and flora, being the product of Gy and a ‘qualifying factor’; the starting place for the latter was the LET of radiation in water as it was thought that this could be used until more reliable or relevant information became available (Pentreath, 1999). The term ‘dose equivalent for fauna and flora’ was chosen deliberately because it gave the acronym ‘DEFF’ – a term deemed appropriate at the time because all of the relevant bodies appeared to be turning a deaf ear to various requests to resolve the issue.
However, interest in the problem was growing. At about the same time, Trivedi and Gentner (2000) suggested a somewhat different approach – deriving a weighting factor, the ‘ecodosimetry weighting factor (eR)’, except for stochastic effects at low doses, because it was believed that not all non-human species developed cancer. Yet another suggestion came from a working group of the International Atomic Energy Agency (IAEA, 2000). They suggested that the term ‘(radiation) weighted absorbed dose (rate)’ should be used generally for protection of the environment. This group defined wR as the ‘radiation weighting factor appropriate to the organism, effects endpoint and dose rate’. Kocher and Trabalka (2000) made a case for incorporating relevant biological effectiveness of different radiation types when expressing dose or dose limits for biota. Their work was related to the ongoing effort by the US Department of Energy (2002) to develop and implement a comprehensive system of radiological protection of the environment. At about the same time, Environment Canada (2000) proposed an RBE weighting factor for the ecological assessment of ecologically significant alpha-particle-emitting radionuclides in the environment under Canada’s Priority Substances List 2. This was based on work subsequently published by Thompson et al. (2003). The Canadian Nuclear Safety Commission’s Advisory Committee on Radiological Protection (ACRP, 2002) recommended that such a radiation weighting factor would be appropriate for protecting all biota against ‘ecologically relevant deleterious effects’. They concluded that all studies implying higher RBEs for some endpoints in animals had serious problems, either with alpha dosimetry or with poor statistics, and a Canadian consulting firm also commented on the methodological flaws in the studies that purported to support high RBEs for alpha-particle emitters (SENES, 2005). On a more practical basis, Chambers et al. (2006) identified 66 relevant measurements of alpha RBE and assigned them to one of three broad categories: population-relevant tissue reaction endpoints such as cell mortality, oocyte mortality, and sperm mortality; other tissue reaction endpoints such as haemopoiesis, sperm head abnormality, and lens opacity; and stochastic endpoints such as chromosomal aberration, double-stranded breaks, and mutation.
There had therefore been no shortage of suggestions with regard to how best to allow for RBE in assessing dose rates to non-human biota, and no shortage of suggestions on what any necessary term should be called. The issue became more acute, however, when the framework developed by ICRP required dose rates for Reference Organisms to be compared with bands of derived consideration reference levels (DCRLs) that had been based on radiation effects data for different types of biota (Reference Animals and Plants) because the results of such comparisons were intended to guide what action might be considered in different exposure situations (ICRP, 2008). Quite simply: how, when the principal nuclides of concern were alpha-particle emitters or those of low LET, could one calculate realistic dose rates to biota in order to compare them with the DCRLs?
Unfortunately, none of the studies that had been undertaken contained exhaustive reviews of all of the available data, although there had been various suggestions about what numerical values might be appropriate. There is, of course, a difference between RBE, which is a biological observation in a particular experimental setting, and a weighting factor, which is a practical aggregation of data to inform and assist protective measures. Bearing this in mind, ICRP established a task group to examine the issue with regard to the framework it had developed for non-human biota. This was back in 2007. Since then, there have been many iterations of draft texts; many consultations and views aired, including by way of the ICRP symposia (Higley et al., 2012); and further analyses of databases and suggestions of values made, such as that of 10 for alpha-particle emitters by UNSCEAR (2008). It is therefore to the great credit, and no doubt great relief, of those members of Task Group 72 that this undertaking has at last been brought to a conclusion. There will be many, no doubt, who will still debate the issue, and that is how it should be. However, at least it is now possible to bring some uniformity to the various studies that have related – and future studies that will relate – exposure to dose for biota in different environmental exposure situations, as now set out by ICRP (2014), and it provides a basis for further numerical examination of this issue.