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Derivation of Environmental Radiological Protection
Benchmarks
an overview
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Definition of benchmarksNumerical values used to guide risk assessors These values need to be based on scientific reasoning & be transparent. They correspond to screening values when they are used in screening tiers.
Concentration (Bq per unit volume or mass), dose (Gy) or dose rate (Gy per unit time) assumed to be ‘safe’ for the object of protection Based on exposure –response information (e.g. ecotoxicity test endpoints). Referred to as PNEDR (Predicted No-Effects Dose Rate for ecosystems) in ERICA, as ENEV (a set
of Estimated no-effects values wildlife group-specific) in Env.Canada/CNSC…
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How are they used to characterize the risk?
The risk index is expressed as the ratio Predicted environmental dose rate:PNEDR
PNEDR
0
PEDRDeterministic method
pdf1 Uncertainty is introduced only in the exposure
estimate. The risk index is expressed as a probability that the exposure estimate exceeds the PNEDR
Semi-probabilistic method
A statistical distribution is also assigned to PNEDR (e.g. a species sensitivity distribution). This allows calculation of the probability of the risk.0
1pdf
Probabilistic method
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For chemicals, all existing approaches are based on available critical ecotoxicity data, typically EC50 for acute exposure conditions (short-term) and EC10 (preferred to NOEC) for chronic exposure conditions (long-term).
How to derive those « safe levels »?
Effect (%)
Regression model
100 %
50 %
10 %
ED10
EDR10
Dose (Gy)Dose Rate (µGy/h)
ED50
EDR50
Observed data
NOEDR: No observed effect dose rate
LOEDR: Lowest observed effect dose rate
Exposure-response relationship from ecotoxicity tests(stressor, species, endpoint) -> transpose to radioactive substances
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SF
LowestEDRPNEDR 10
2 methods recommended by EC for chemicals (Technical Guidance Document (2003) – easily adaptable to radioactive substances when ED50 or EDR10 are available
The Safety Factor Method
Available ecotoxicity data SF to obtain
chronic PNEDR
At least one short-term ED50 from each of
three trophic levels (plant, invertebrate, vertebrate)
1000
One long-term EDR10 (either vertebrate or invertebrate)
100
Two long-term EDR10 from species representing two trophic levels among (plant, invertebrate, vertebrate)
50
Long-term EDR10 from at least three species (Plant, invertebrate, vertebrate) representing three trophic levels
10
How to derive those « safe levels »?
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Safe levels can be calculated with statistical extrapolation models to address variation between species in their sensitivity to a stressor.
The species for which results are known are representative, in terms of sensitivity, of the totality of the species in the ecosystem.
The endpoints measured in laboratory tests are indicative of effects on populations in the field.
The Species Sensitivity Distribution (SSD) Method
0
20
40
60
80
100
1 10 100 1000 10000
Dose (Gy) or Dose Rate (µGy/h)
PAF (%)
How to derive those « safe levels »?
5%
HDR5%
Calculation of a dose rate that is assumed to protect a given % of species
In the Technical Guidance Document (2003): the agreed concentration is the hazardous concentration affecting 5 % of species with 50% confidence. When its remains other extrapolation issues, the TGD recommends to apply an additionnal SF (1-5)->until now, only used for ERICA screening value
SF
HDRPNEDR 5
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Analysis of the effects data from FREDERICA by ERICA
More than 25,000 data entries - couples (dose, effect) along with info on their acquisition - « raw » data series from experiments – lab or field) - dose-effect relationship not mathematically structured
Acute data (80% - external irradiation) : chronic data (20% - external irradiation)
Mainly γ radiation types some: alpha, beta, neutron, X-ray
Four endpoints: mortality, morbidity, reproductive capacity, mutation
more on reproduction for acute; more on morbidity for chronic, followed by reproduction; too few data on mutation whatever the exposure regime
Only data devoted to effects induced by external irradiation pathway were quantitatively adequate to be mathematically structured in terms of dose-effect relationships.
Meta-analysis of effect data to reconstruct dose-effect relationships and to estimate comparable critical ecotoxicity endpoints ie ED50 for acute exposure and EDR10 from chronic exposure
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Sp = weighted; TW: none
0%
20%
40%
60%
80%
100%
1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07
Dose Rate (µGy/h)
Best-Estimate Centile 5% Centile 95%
Vertebrates Plants Invertebrates
R² = 0.9513
KSpvalue = 0.500wm.lg = 3.71wsd.lg = 1.09
Log Normal – Generic Ecosystem (SW+FW+TER)
Cum
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HDR5 = 81.8 µGy/h
CI95% = [23.8-336] µGy/h
Number of data = 24Number of species = 18
Sp = weighted; TW: none
0%
20%
40%
60%
80%
100%
1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07
Dose Rate (µGy/h)
Best-Estimate Centile 5% Centile 95%
Vertebrates Plants Invertebrates
R² = 0.9513
KSpvalue = 0.500wm.lg = 3.71wsd.lg = 1.09
Log Normal – Generic Ecosystem (SW+FW+TER)
Cum
ula
tive
we
ight
ed
pro
bab
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HDR5 = 81.8 µGy/h
CI95% = [23.8-336] µGy/h
Number of data = 24Number of species = 18
Data set adequate to apply SSD-based method to derive the PNEDRThe application of the SF method came out with more stringent value.ERICA adopted SSD-based methodology to derive its screening value.
HDR5=81.8
SF = 5rounded downand keeping 1 digit
PNEDR 10 µGy/h
Chronic external exposure conditions: SSD
SSD-based PNEDR
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How to use properly the ERICA PNEDR?
PNEDR does not apply for any other ecological object to be protected (than generic ecosystem)
PNEDR was derived to be used in the first tiers of the ERICA tiered approach
PNEDR is the benchmark value to screen against incremental dose rates and not to total dose rates including background (similar to the added risk method (Struijs et al., 1997).
■ Implicit assumption is that the BG has resulted in the biodiversity of ecosystems. No deleterious potential effect originating from the
background
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Existing values: a brief review
Environment Canada Review, SF on the lowest Critical radioToxicity Value Terrestrial Plants 110 Terrestrial Invertebrates 220 Small mammals 110 Aquatic algae/macrophytes 110 Amphibians/Reptiles 110 Benthic invertebrates 220 Fish 20 USDoE Review based on NCRP 1991; IAEA 1992; UNSCEAR 1996 Terrestrial Plants or Aquatic animals 400 Terrestrial Animals 40 Environment Agency (UK) Critical review for screening purpose from IAEA 1992 Terrestrial Plants 400 Terrestrial Animals 40 Freshwater and coastal marine organisms 400 Marine mammals 40 Deep ocean organisms 1000 5 % of guideline values from AIEA at initial screening tier fSu Review Contaminated environments Vertebrates and cytogenetic effects 4 – 20 Vertebrates and effects on morbidity 20 – 80 Vertebrates and effects on reproduction 80 – 200
Method used and derived values (in µGy/h)
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Objectives within PROTECT WP3 “To derive an extended set of numerical target values and their
associated derivation methods, designed to assure compliance to environmental protection goals that resonate with protective goals set up for releases of hazardous substances in general, and to assess the implications for society at large.”
On the basis of the target for protection (& the level of protection) [WP1-WP2], explore the possibility for the application of advanced statistical methods (classical or Bayesian) that:
(1) allow to make the best use of the available knowledge when this is represented by small data sets,
(2) allow to quantify the associated uncertainty,
(3) easily allow to revise the outcoming values when new knowledge becomes available
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News from UNSCEARUNSCEAR literature review (1996) is currently under
revision
Some of the key components of the « effects » sections:New knowledge on effects since 1996 presented, starting with a
review on effects mechanisms from subcell to higher organisational level
Review of existing approaches (methods used to assess the radiological impact or the risk to ecological objects & their associated benchmarks).
Case studies for illustration purpose
Too early to say whether this would result in a major revision of the former well- known statement