The derivation of benchmarks

1. The derivation of benchmarks David Copplestone (University of Stirling)

2. OBJECTIVES What is a benchmark? Why are benchmarks needed? How are benchmarks derived? How are benchmarks used?

3. INTRODUCTION The need for benchmarks... ... a retrospective screening model example www.ceh.ac.uk/PROTECT

4. A Tier-1 screening model of risk to fish living in a radioactively contaminated stream during the 1960s Fundamental to this approach is the necessity for the dose estimate to be conservative This assures the modeler that the PREDICTED DOSES areLARGERthan the REAL DOSES www.ceh.ac.uk/PROTECT

5. 5000 4000 1) SOURCE TERM: used 1964 3000 maximum release as a mean Total 137-Cs Released (GBq) 2000 for calculations 1000 2) EXPOSURE: assumed fish 0 were living at point of discharge 54 59 64 69 74 79 84 Year 3) ABSORPTION: assumed all fish were 30 cm in diameter which maximized absorbed dose 4) IRRADIATION: behavior of fish ignored, assumed they spent 100% of time on bottom sediments where > 90% of radionuclides are located CONTAMINATED SEDIMENTS Conservative Assumptions for Screening Calculations www.ceh.ac.uk/PROTECT

6. Resulting Dose Rates (mGy y-1) www.ceh.ac.uk/PROTECT

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10. We need a point of reference; a known value to which we can compare… …a BENCHMARK value

11. Definition of benchmarks Benchmarks are numerical values used to guide risk assessors at various decision points in a tiered approach Benchmarks values are concentrations, doses, or dose rates that are assumed to be safe based on exposure – response information. They represent « safe levels » for the ecosystem The derivation of benchmarks needs to be through transparent, scientific reasoning Benchmarks correspond to screening values when they are used in screening tiers www.ceh.ac.uk/PROTECT

12. Data on radiation effects for non-human species To few to draw conclusions No data Some data www.ceh.ac.uk/PROTECT

13. Approaches to derive protection criteria www.ceh.ac.uk/PROTECT

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15. Historic reviews • From literature reviews • Earlier numbers derived by expert judgement (different levels of transparency) • Later numbers, more quantitative/mathematical • Levels of conservatism? • Often “maximally exposed individual” not population... • NCRP 1991 states use with caution if large number of individuals in a population may be affected

16. A Quantitative approach • Used to derive the ERICA and PROTECT values • Consistent with EC approach for other chemicals

17. Exposure-responserelationshipfromecotoxicity tests Effect (%) 100 % Observed data Regression model 50 % LOEC: Lowest observed effect concentration NOEC: No observed effect concentration 10 % Contaminant Concentration How to derive « safelevels » Methods recommended by European Commission for estimating predicted-no-effects-concentrations for chemicals …based on available ecotoxicity data; (i.e. Effect Concentrations; EC) typically EC50 for acute exposure conditions and EC10 for chronic exposures EC10 EC50 www.ceh.ac.uk/PROTECT

18. How to derive « safelevels » ....adapted for radiological conditions.... Exposure-responserelationshipfrom ecotoxicity tests (specific to stressor, species, and endpoint) Effect (%) 100 % Observed data Regression model 50 % LOEC: Lowest observed effect concentration NOEC: No observedeffect concentration 10 % EC50 ED50 EDR50 Concentration (Bq/L or kg) Dose (Gy) Dose Rate (µGy/h) EC10 ED10 EDR10

19. Deriving benchmarks for radioecological risk assessments i.e. screening values thought to be protective of the structure and function of generic freshwater, marine and terrestrial ecosystems. • Two methods have been developed • Fixed Assessment (Safety) Factors Approach • Species Sensitivity Distribution Approach www.ceh.ac.uk/PROTECT

20. Fixed assessment factor method PNEV = minimal Effect Concentration / Safety Factor www.ceh.ac.uk/PROTECT

21. Fixed assessment factor method PNEV = minimal Effect Concentration / Safety Factor The safety factor method is highly conservative as it implies the multiplication of several worst cases www.ceh.ac.uk/PROTECT

22. The approach used to derive no-effects values www.ceh.ac.uk/PROTECT

23. The predicted no-effect dose rate (PNEDR) evaluation www.ceh.ac.uk/PROTECT

24. SSD for generic ecosystem at chronic external γ-radiation (ERICA) • The 5% percentile of the SSD defines HDR5 (hazardous dose rate giving 10% effect to 5% of species) • HDR5 = 82 μGy/h • PNEDR used as the screening value at the ERA should be highly conservative • SF = 5 • PNEDR ≈ 10 μGy/h PNEDR = HDR 5% / SF www.ceh.ac.uk/PROTECT

25. Percentage of Affected Fraction 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 5% 0% 0.1 1 10 100 1000 10000 100000 1000000 10000000 Dose rate (µGy/h) HDR5 = 17 µGy/h [2-211] Best-Estimate Centile 5% Centile 95% Vertebrates Plants Invertebrates Generic ecosystem SSD for chronic external γ-radiation (PROTECT) EDR10 and 95%CI: Minimum value per species PNEDR=10 µGy/h (SF of 2) www.ceh.ac.uk/PROTECT

26. We need a point of reference; a known value to which we can compare… …a BENCHMARK value 10 μGy/h * 24 h / d = 240 μGy/d = 0.2 mGy /d www.ceh.ac.uk/PROTECT

27. Reminders… • The PNEDR: • is a basic generic ecosystem screening value • Can be applied to a number of situations requiring environmental and human risk assessment • Be aware of: • PNEDR was derived for use only in Tiers 1 and 2 of the ERICA Integrated Approach • Use for incremental dose rates and not total dose rates which include background

28. Background radiation exposure for ICRP RAPs (weighted dose rates) www.ceh.ac.uk/PROTECT

29. Background radiation exposure for ICRP RAPs Derived screening dose rate (10 μGy/h) is more than 10 times these background values www.ceh.ac.uk/PROTECT

30. Furthermore... • The hazardous dose rate definition means that 95% of species would be protected at a 90% effect However, there may be keystone species among that are unprotected at the 10% level and the effect on the 5% may be > 10% • Some keystone species will be more radiosensitive than others

31. Generic screening dose rate • ERICA (default) and R&D128 assume a single (generic) screening dose rate (i.e. application of predicted no effect dose rate) applicable across all species and ecosystems • Advantage = simple • PROTECT objective to consider scientifically robust determination of (generic) screening dose rate(s) • What are limiting organisms for the 63 radionuclides considered in ERICA?

32. Limiting organisms Marine ecosystem ERICA Tool – generic screening dose rate

33. Limiting organisms Freshwater ecosystem ERICA Tool – generic screening dose rate

34. Limiting organisms Terrestrial ecosystem ERICA Tool – generic screening dose rate

35. Generic screening dose rate • Application of generic screening dose rate: • Identifies the most exposed organism group • Does not (necessarily) identify the most ‘at risk’ (relative radiosensitivity not taken into account) • What does this mean for the assessment • Likely to be conservative • May be overly so • Propose wildlife group specific benchmark dose rates

36. ICRP Approach

37. Effects • As part of ICRP 108, effects considered • No dose ‘limits’ but still need something to compare to • …background • …derived consideration reference levels www.ceh.ac.uk/PROTECT

38. DCRLs • Derived Consideration Reference Levels • “A band of dose rate within which there is likely to be some chance of deleterious effects of ionising radiation occurring to individuals of that type of RAP (derived from a knowledge of expected biological effects for that type of organism) that, when considered together with other relevant information, can be used as a point of reference to optimise the level of effort expended on environmental protection, dependent upon the overall management objectives and the relevant exposure situation.”

39. DCRLs Earthworm Bee Crab mGy/d Frog Trout Flatfish Grass Seaweed Deer Rat Duck Pine tree Backgroundlevel

40. Application • Provision of advice on how to use the RAP framework • Likely to use ‘representative organism’ concept

41. Representative Organism Reference Animals and Plants ‘Derived consideration reference levels’ for environmental protection REPRESENTATIVE ORGANISMS Radionuclide intake and external exposure Planned, emergency and existing exposure situations

42. Integration • Integrating the ICRP systems of protection for humans and non-human species • Consider ethics and values • Consider how principles of justification, optimisation etc apply to both humans and non-human species • Consider the principles used in chemical risk assessment/protection

43. What is a benchmark? Benchmarks are numerical values used to guide risk assessors at various decision points in a tiered approach In radiation protection, usually applied as the incremental dose ABOVE background www.ceh.ac.uk/PROTECT

44. How are benchmarks derived? • Quantitative approach eg chemicals • Safety factor, SSD • ICRP – will use DCRL values • Are they benchmarks? • Currently summarise where biological effects are likely to occur • C5 is working on how the DCRLs can be incorporated into the wider ICRP system of radiological protection

45. Summary • Range of methods for deriving benchmarks • Range of benchmarks proposed • Be careful with the wording around the benchmark • What does it reflect? • Look for clear, well documented benchmark values • Watch this space for further developments! www.ceh.ac.uk/PROTECT

46. Caveats... • Adapted text in the older documents from NCRP (1991), IAEA (1992) and UNSCEAR (1996) is given below: • NCRP Aquatic organisms: it appears that a chronic dose rate of no greater than 0.4 mGy h−1 to the maximally exposed individual in a population of aquatic organisms would ensure protection for the population. If modelling and/or dosimetric measurements indicate a level of 0.1 mGy h−1, then a more detailed evaluation of the potential ecological consequences to the endemic population should be conducted • IAEA Terrestrial organisms: irradiation at chronic dose rates of 10 mGy d−1 and 1 mGy d−1 or less does not appear likely to cause observable changes in terrestrial plant and animal populations respectively. Aquatic organisms: it appears that limitation of the dose rate to the maximally exposed individuals in the population to <10 mGy d−1 would provide adequate protection for the populations • UNSCEAR Terrestrial plants: chronic dose rates less than 400 μGy h−1 (10 mGy d−1) would have effects, although slight, in sensitive plants but would be unlikely to have significant deleterious effects in the wider range of plants present in natural plant communities. Terrestrial animals: for the most sensitive animal species, mammals, there is little indication that dose rates of 400 μGy h−1 to the most exposed individual would seriously affect mortality in the population. For dose rates up to an order of magnitude less (40–100 μGy h−1), the same statement could be made with respect to reproductive effects. Aquatic organisms: for aquatic organisms, the general conclusion was that maximum dose rates of 400 μGy h−1 to a small proportion of the individuals and, therefore, a lower average dose rate to the remaining organisms would not have any detrimental effects at the population level