Presentation on theme: "Environmental Protection: the Concept and Use of Reference Animals and Plants ICRP Committee 5."— Presentation transcript:
Environmental Protection: the Concept and Use of Reference Animals and Plants ICRP Committee 5
Report structure Preface Introduction Reference plants and animals Pathways of exposure Calculation of dose conversion factors for RAPs The effects of radiation and its relevance for RAPs Assessing effects in terms of derived consideration levels Applications and extrapolations Conclusions References Appendices
Preface Timeline Developments at ICRP within the field of environmental protection from ionising radiation –Task Group set up in 2000 –Framework for Assessing the Impact of Ionising Radiation on Non-Human Species, ICRP Publication 91, in 2003. –June 2005 – Committee 5 established –Task Group on Dosimetric modelling - 2006 C5 members –R. J. Pentreath (Chairman), C-M. Larsson (Vice-chairman), K. A. Higley (Secretary), F. Brechignac; M. Doi (to 2006), G. Proehl, A. Johnston (to 2007), A. Real, K. Sakai (from 2007), P. Strand.
Introduction I New recommendations –Planned, existing and emergency situations –all of the environment needs to be considered, including areas where humans are absent. Aims of environmental protection now include –Preventing or reducing the frequency of deleterious radiation effects to a level where they would have a negligible impact on the maintenance of biological diversity, the conservation of species, or the health and status of natural habitats, communities, and ecosystems.
Introduction II ICRPs approach to environmental protection –Provide ”high level”guidance for demonstration of compliance corresponding with existing/emerging national and international legislation –Radiation one factor among many –Compatible with other approaches to protect the environment –Group biota effects in terms of early mortality, or morbidity, or reduced reproductive success. –Provide a framework for more applied and specific numerical approaches Report - concept and use of Reference Animals and Plants, serves as an introduction to this complex subject
Reference animals and plants Reference man of great utility – use similar approach for environment. Limited group of biota for relating exposure to dose and dose to effect for environmental situations –Employ derived consideration levels –Consequences for individuals or relevant populations Points of reference for drawing comparisons with sets of information on other organisms Not necessarily the direct objects of protection –Allows more site-specific information (e.g. secondary sets) to be compared and examined.
Criteria for selection of RAPs Requirements –To meet existing or expected legislation vertebrates, wetland habitats; –For environmental impact assessments animals and plants relevant to practices such as fisheries, agriculture, forestry; –To achieve consistency in regulatory approaches reasonable coverage of the major ecological compartments of terrestrial and aquatic ecosystems. Pragmatism in selecting RAPs –radiobiological information available; amenable to future research; typical of particular ecosystems; likely to be exposed to radiation; exposure can be modelled and life-cycle relevant for evaluating total dose and dose-effect responses; reasonable chance of identiying effects in individuals; political and public ”resonance”
Appropriate level of generalization Animal kingdom = Phyla, Classes, Orders, Families (which share ‘typical’ traits and features), Genera, species. no internationally accepted ‘rules’ on classification above Family (or ‘Super Family’) level, and this has therefore been suggested as the most suitable level of generalisation
RAPs Definition A Reference Animal or Plant is a hypothetical entity, with the assumed basic biological characteristics of a particular type of animal or plant, as described to the generality of the taxonomic level of Family, with defined anatomical, physiological, and life-history properties, that can be used for the purposes of relating exposure to dose, and dose to effects, for that type of living organism.
Further notes on RAPs The set is essentially one of ‘wild’ animals and plants rather than domesticated ones What RAPs are NOT intended to be –Objects of protection –”Sentinel” biota, i.e. such types are protected then other types will also be protected: –Biota that ICRP considers should be particularly protected; –Representatives of key links in food chains –Representatives of key links in ecosystem functioning
The Individual Reference Animals and Plants Additional general information relating to RAP biology and ecology provided in Appendix A, + general discussion on populations Brief introduction and description of each RAP type provided in main report –Which family, e.g. Deer = Cervidae –How many species make up the family –Habitat –Use as human resource –Legislation –Average life-span and information on reproduction (how many offspring etc.)
Populations In some cases individual exposure is important but in other cases, populations need to be considered A population –group of individuals of the same species that live in the same place at the same time –area for population = sufficient for the organisms to carry out their normal functions –group of genetically similar individuals that can be characterised in terms of e.g. birth rate, death rate, age structure etc. Effects of radiation at a population level requires information on: –characteristics of the population being considered, –the fraction of the population known or assumed to be exposed to different dose-rates ( total dose) –stages in the life cycle receiving the relevant dose
Pathways of exposure Useful to consider ‘sources’ –indicates any physical entity or procedure that results in a potentially quantifiable radiation dose. Types of exposure situation –Planned –Emergency –Existing
Data needs with regard to different exposure situations Direct measurements sometimes available Otherwise modelling approaches required (notably in planned + emergency situations) –Steady state (transfer factors, CFs relevant) or Dynamic –Data often pertain to parts consumed by humans –Data often lacking on parts of life-cycle Factors that may need consideration –external exposure from contaminated soil, sediment, or water; –contamination of fur, feathers and skin; –inhalation of (re)suspended contaminated particles or gaseous radionuclides; –ingestion of radionuclides; and –the direct uptake from the water in the case of aquatic organisms. Subsequent report in relation to transfer in RAPs
Dose conversion factors for RAPs Simplification – whole organisms represented by simple shapes Uniform isotropic models, or simplified analytical or semi-analytical methods sufficient for aquatic environments Large density differences require radiation transport models (e.g. using Monte Carlo)
Dosimetry : dose-concept Basic unit = absorbed dose (Gy) but –Different types of radiation are known to produce different degrees of effect in the same biological tissue, for the same absorbed doses, for many types of organisms. Key quantity = Absorbed fraction energy emitted by a radiation source that is absorbed within the target
Dosimetric modelling assumptions Based on task group intercomparisons exercise (details provided in Appendix B of report) –EPIC (Doses-3D), EA R&D128, EDEN, RESRAD-BIOTA and FASSET-ERICA. FASSET-ERICA selected for reference DCF derivation because of flexibility Units of μGy day −1 per Bq kg −1.
Dosimetry - Selected method A practical method to estimate absorbed fractions for a wide range of ellipsoids and spheres has been developed by Ulanovsky and Pröhl (2006) : –computed using Monte Carlo code MCNP4C –body composition = four-component composition from ICRU and body density of 1.0 g cm −3. –The organisms are assumed to be in an infinite water medium. –The transport of electrons and photons simulated to energy cut- offs of 1 keV for photons and 10 keV for electrons; –The mass of the organisms considered covered a range from 10 -3 g to 10 6 g in steps of an order of magnitude. –For both photons and electrons the energies ranged from 10 keV to 5 MeV. re-scaling of the absorbed fraction for tissues in water –A ‘non-sphericity’ parameter η is derived, to adjust the absorbed fraction for organism shape. Ulanovksy, A & Pröhl, G., 2006. A practical method for assessment of dose conversion coefficients for aquatic biota. Radiation and Environmental Biophysics, 45, 203-214.
DCF derivation Internal DCF External DCF (aquatic) For external terrestrial DCFs – explicit MC simulations for selected ”target-source configurations” see Taranenko et al. (2004). To enable the use of specific weighting factors absorbed dose that are due to different types of radiation are also given in Appendix C. Split into components of , low (< 10 keV) beta and . Ei is the energy of component of emitted radiation (MeV); yi is the yield of emitted radiation of energy Ei (dis-1); T(Ei) is the absorbed fraction in the target for energy Ei; 1.384x10 -2 is the factor to account for conversions of MeV to Joules and seconds to days Taranenko, V., Pröhl, G. Gómez-Ros, J.M.(2004) Absorbed dose rate conversion coefficients for reference biota for external photon and internal exposures. J. Radiol. Prot. 24: A35–A62.
Special note on reference Deer DCFs some preliminary considerations were also given to the relative dosimetry of internal organs, such as the liver and gonad, but essentially for illustrative purposes rather than as definitive models.
Effects of radiation and its relevance to RAPs Large data base on the effects of radiation on plants and animal – regularly reviewed (e.g. UNSCEAR) More systematic approach FREDERICA database. For individual studies, enormous variation in –range of individual species studied –mode of exposure, –dose rates and –selection of ‘biological effects’ recorded.
Current understanding of radiation effects in general, and within the context of the human animal. The principal cellular target for biological effects = chromosomal DNA Effects at a sub-cellular level –a high proportion of radiation induced damage in DNA is represented by the occurrence of complex clusters of chemical alterations –Frequency and complexity of clusters depends on LET Error-prone repair of double strand breaks best explains chromosome aberrations, gene mutation and cell killing
Current understanding of radiation effects II Compelling evidence that changes in DNA damage response/repair and apoptotic/cell cycle control are closely associated with tumor development Recent radiobiological work on –Induction of gene and chromosomal mutations at low doses. –Genomic instability : consequences expressed after many post irradiation cell cycles
Tissue and organ effects Stochastic effects –Single cell death = no consequences for tissues, but mutation in a single cell tumorogenesis cancer –No threshold, frequency related to dose Early or late tissue or organ reactions –larger doses substantial amount of cell killing detectable tissue reactions –Structure of organs and tissue plays a role in response –Reserve capacity in organs high tolerance to partial irradiation Radiation tumorogenesis –Weak ‘promoter’ of tumour development, likely acts in earliest phase Mutations causing heritable diseases –Principal genetic effects in humans = multi-system developmental abnormalities rather than single gene diseases.
A special note on RBEs An approach to account for RBE is needed because –RBE phenomenon exists in animals other than man (most RBE studies are in fact for non-human biota) –Naturally occuring alpha emitters ubiquitous in bodies of plants and animals. Therefore useful to apply weighing factors in attempting to normalise radiation doses. –Many environmental problems concern releases of alpha emitters – therefore more damaging aspect of these needs to be accounted for. Subsequent report on issues relating to RBEs and RAPs
Radiation Effects in RAPs Effects of radiation take place at the level of the individual BUT –useful to consider such effects in terms of how they might effect populations – early mortality, reduced reproductive success, some forms of morbidity and ‘scorable’ cytogenetic effects No attempt to interpret such effects at a ‘population’ level
Summarised effects data In the report, pages 48-87 summarise effects under the headings Mortality, morbidity, reduced reproductive success and chromosomal damage for each RAP group. –Mammals fish and conifers – comprehensive data coverage –Much fewer data for other categories, e.g. birds, amphibians, insects and grasses –No data for macroalgae
Discussion on Effects data Miscellaneous data sets Many data but little guidance on reliability, consistency, interpretability, or utility. Most data for acute exposure therefore of limited relevance to environmental situations Data often difficult to use because dose rates averaged (arbitrarily) over periods of hours, days, or years. Data often organised in terms of exposure pathway as oppose to phylogeny or biology Many data on stochastic effects in mammals used for improving human radiological protection have been omitted.
General observations – effects data For the higher vertebrates, –there is little difference in response across a range of dose rates for mammals, –Similar response for birds (although data insufficient in this case to draw conclusions) For the lower vertebrates, –generalisations are difficult because allowance has not usually been made for their lower metabolic rates –if this fact accounted for, differences between higher and lower vertebrates may be less than it appears to be. Invertebrates more radioresistant than invertebrates –Mechanistic understanding missing –eggs and larvae have usually been found to be more radiosensitive, Trees and plants –Long time scales required for study (for effects to appear) –Few controlled experiments and little data on which tissues have received dose –No clear information on differences in effects of radiation on plant and animal cells WHAT CAN BE DONE WITH THIS INFORMATION IN A STRUCTURED WAY ?
Derived Consideration Levels Practical means required to make environmental management decisions and judgements based on knowledge of effects of radiation on different types of biota Useful comparator might be natural background –Additional doses that were e.g. fractions of normal background dose rates might be unlikely to cause concern, whereas dose rates that were very much higher, and in the region of expected effects, would need to be considered further –bands of dose rates based on natural background = Derived consideration levels –Point of reference to summarise what is know about effects on RAPs –Used in conjunction with other relevant information, e.g. area affected Information on natural background –Typical background dose-rates cited from published works
Preliminary DCL values Extreme simplification of existing data Start point to stimulate development Derived Consideration Levels highlighted in yellow Dose-rates > 1 Gy d -1 not relevant for environmental management but considered for completeness
DCLs - Matters for consideration DCLs NOT intended to be dose limits –Values greater than DCLs not necessarily to be considered as environmentally damaging –Values less than DCLs not necessarily to be considered safe. –DCLs are the starting points to consider such conclusions in the light of the local legislation and local situation. Management – use other information to justify action, e.g. –Exposure situation (existing, planned, emergency) –Area where dose-rates occur –Time over which dose-rates occur etc. etc. Issues –Not considered appropriate to simplify tables –Precautionary factors (e.g. safety factors) might be applied but at least can be related to tables –Link between protection of individuals versus populations : still very uncertain
Applications Environmental management requirements – pollution control, nature conservation, EIA, e.g. : –compliance with national pollution control licensing requirements –compliance with the requirements of specific national wildlife and habitat protection legislation. Process of assessment key in all cases. Assessment models exist : –Exposure : adaptation of existing human rad. protection models –Consequences : more problematic because ”open-ended” Major requirement to make evaluations at population or ecosystem level BUT focus on radiation effects on the individual for purpose of developing a framework –Consistent with assessment method for other contaminants –Effects mediated via individuals Secondary sets of references organisms may require development
Extrapolations 3 extrapolation issues: –biology, –dosimetry and –effects
Extrapolation – differences in biology Awareness that biological objects of interest may be different to RAPs and that –Differences in biology could make large differences to estimates of exposure to certain radionuclides via different pathways ”Appendix A – Biological background to reference animals and plants” cited as a supporting document to assist in adapting approach to specific case.
Extrapolation – differences in dosimetry Effects of changing various parameters considered –Mass, energy, shape –Configuration of target to source Task Group on ”more realistic dosimetry” –Explore above issue, including inhomogeneity of contamination
Extrapolation – differences in effects High acute dose rates (low LET γ- and X-rays) to lower doses accumulated at lower dose-rates –Very few data on environmentally relevant dose-rates over life-span of organisms From one organism type to another –Variation in radiosensitivity between and within taxonomic groups + lifestages From individuals to populations and communities –Including extrapolation from laboratory to field
Conclusions Environmental protection complex and difficult to articulate –Any approach should be compatible with other approaches ICRP will provide high-level guidance and advice upon which regulators and operators may draw in order to demonstrate compliance Development of framework central –Key feature = Reference Animals and Plants –RAP report serves as an introduction to this