Jordi Vives i Batlle Centre for Ecology and Hydrology, Lancaster, 25 th November 2010
David Copplestone Centre for Ecology & Hydrology - Lancaster October 2011.
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Transcript of David Copplestone Centre for Ecology & Hydrology - Lancaster October 2011.
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David Copplestone
The derivation of benchmarks
Centre for Ecology & Hydrology - Lancaster October 2011
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OBJECTIVESWhat is a benchmark?
Why are benchmarks needed?
How are benchmarks derived?
How are benchmarks used?
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INTRODUCTION
The need for benchmarks...
... a retrospective screening model example
www.ceh.ac.uk/PROTECT
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Fundamental to this approach is the necessity for the dose estimate to be
conservative
A Tier-1 screening model of risk to fish living in a radioactively contaminated
stream during the 1960s
This assures the modeler that the PREDICTED DOSES are LARGER
than the REAL DOSES
www.ceh.ac.uk/PROTECT
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1) SOURCE TERM: used 1964 maximum release as a mean for calculations
2) EXPOSURE: assumed fish were living at point of discharge
3) ABSORPTION: assumed allfish were 30 cm in diameter
which maximized absorbed dose
4) IRRADIATION: behavior offish ignored, assumed theyspent 100% of time on bottom
sediments where > 90% of radionuclides are locatedCONTAMINATED
SEDIMENTS
54 59 64 69 74 79 840
1000
2000
3000
4000
5000To
tal 1
37
-Cs
Re
lea
sed
(G
Bq
)
Year
Conservative Assumptions forScreening Calculations
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Resulting Dose Rates (mGy y-1)
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www.ceh.ac.uk/PROTECT
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…a BENCHMARK value
We need a point of reference; a known value to which we can compare…
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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
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
Benchmarks are numerical values used to guide risk assessors at various decision points in a tiered approach
The derivation of benchmarks needs to be through transparent, scientific reasoning
Benchmarks correspond to screening values when they are used in screening tiers
Definition of benchmarks
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Data on radiation effects for non-human speciesWildlife Group Morbidity Mortality
Reproductive capacity Mutation
Amphibians
Aquatic invertebrates
Aquatic plants
Bacteria
Birds
Crustaceans
Fish
Fungi
Insects
Mammals
Molluscs
Moss/Lichens
Plants
Reptiles
Soil fauna
Zooplankton
No data
To few to draw conclusions
Some data
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Approaches to derive protection criteria
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www.ceh.ac.uk/PROTECT
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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
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A Quantitative approach
Used to derive the ERICA and PROTECT values
Consistent with EC approach for other chemicals
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Effect (%)
Regression model
100 %
50 %
10 %
ContaminantConcentration
Observed data
NOEC: No observed effect concentration
LOEC: Lowest observed effect concentration
Exposure-response relationship from ecotoxicity tests
…based on available ecotoxicity data; (i.e. Effect Concentrations; EC) typically EC50 for acute exposure
conditions and EC10 for chronic exposures
Methods recommended by European Commission for estimating predicted-no-effects-concentrations for chemicals
How to derive « safe levels »
EC10 EC50
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Effect (%)
Regression model
100 %
50 %
10 %
EC10
ED10
EDR10
Concentration (Bq/L or kg)Dose (Gy)Dose Rate (µGy/h)
EC50
ED50
EDR50
Observed data
NOEC: No observed effect concentration
LOEC: Lowest observed effect concentration
Exposure-response relationship from ecotoxicity tests(specific to stressor, species, and endpoint)
....adapted for radiological conditions....
How to derive « safe levels »
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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
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Fixed assessment factor method
Main underlying assumptions In the frame of this approach, extrapolations are made from:
•The ecosystem response depends on the most sensitive species
•Protecting ecosystem structure protects community function
•Acute to chronic•One life stage to the whole life cycle•Individual effects to effects at the population level•One species to many species•One exposure route to another•Direct to indirect effects•One ecosystem to another•Different time and spatial scales
PNEV = minimal Effect Concentration / Safety Factor
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www.ceh.ac.uk/PROTECT
Fixed assessment factor method
Main underlying assumptions In the frame of this approach, extrapolations are made from:
•The ecosystem response depends on the most sensitive species
•Protecting ecosystem structure protects community function
•Acute to chronic•One life stage to the whole life cycle•Individual effects to effects at the population level•One species to many species•One exposure route to another•Direct to indirect effects•One ecosystem to another•Different time and spatial scales
PNEV = minimal Effect Concentration / Safety Factor
The safety factor method is highly conservative as it implies the
multiplication of several worst cases
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The approach used to derive no-effects values
STEP 1 – quality assessed data are extracted from the FREDERICA database
STEP 2 – A systematic mathematical treatment is applied to reconstruct dose-effect relationships and derive critical toxicity endpoints. For chronic exposure, the critical toxicity data are the EDR10
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STEP 3 –
The hazardous dose rate (HDR5) giving 10% effect to 5% of species is estimated The final PNEDR is then obtained by applying an additional safety factor (typically from 1 to 5) to take into account remaining extrapolation uncertainties
The predicted no-effect dose rate (PNEDR) evaluation
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• The 5% percentile of the SSD defines HDR5 (hazardous dose rate giving 10% effect to 5% of species)
• HDR5 = 82 μGy/h
SSD for generic ecosystem at chronic external γ-radiation (ERICA)
• PNEDR used as the screening value at the ERA should be highly conservative
• SF = 5 • PNEDR ≈ 10 μGy/h
PNEDR = HDR 5% / SF
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www.ceh.ac.uk/PROTECTBest-Estimate Centile 5% Centile 95%
Vertebrates Plants Invertebrates
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0.1 1 10 100 1000 10000 100000 1000000 10000000
Dose rate (µGy/h)
Percentage of Affected Fraction
5%
HDR5 = 17 µGy/h [2-211] PNEDR=10 µGy/h
(SF of 2)
EDR10 and 95%CI: Minimum value per species
Generic ecosystem SSD for chronic external γ-radiation (PROTECT)
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…a BENCHMARK value
We need a point of reference; a known value to which we can compare…
10 μGy/h * 24 h / d = 240 μGy/d = 0.2 mGy /d
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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
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Background radiation exposure for ICRP RAPs (weighted dose rates)
Marine organisms – 0.6 - 0.9 μGy/h (Hosseini et al., 2010)
Freshwater organisms – 0.4 – 0.5 μGy/h (Hosseini et al., 2010)
Terrestrial animals and plants – 0.07-0.6 μGy/h (Beresford et al., 2008)
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Background radiation exposure for ICRP RAPs
Marine organisms – 0.6 - 0.9 μGy/h (Hosseini et al., 2010)
Freshwater organisms – 0.4 – 0.5 μGy/h (Hosseini et al., 2010)
Terrestrial animals and plants – 0.07-0.6 μGy/h (Beresford et al., 2008)
Derived screening dose rate (10 μGy/h) is more than 10 times these background values
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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
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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?
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Limiting organisms Marine ecosystem ERICA Tool – generic screening dose rate
0
2
4
6
8
10
12
14
16
18
20
Phyto
plan
kton
Zooplan
kton
Vascu
lar p
lant
Mac
roalg
ae
Anem
one/
cora
l
Moll
usc
Polyc
haet
e wor
mBird
Mam
mal
Reptile
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Limiting organisms Freshwater ecosystem ERICA Tool – generic screening dose rate
0
5
10
15
20
25
30
Phyto
plan
kton
Moll
usc
Gat
ropo
d
Inse
ct la
rvae
Vascu
lar p
lant
Zooplan
kton
Amph
ibian Bird
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Limiting organisms Terrestrial ecosystem ERICA Tool – generic screening dose rate
0
5
10
15
20
25
Soil/D
etrit
ivoro
us in
vert
Flying
inse
ct
Gas
tropo
d
Lich
en
Gra
ss/h
erb
Shrub
Tree
Mam
mal
Bird
Reptile
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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
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ICRP Approach
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Effects
As part of ICRP 108, effects considered No dose ‘limits’ but still need something to
compare to …background …derived consideration reference levels
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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.”
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DCRLs
Series1
0.001
0.01
0.1
1
10
100
1000
Deer Rat Duck
Frog Trout Flatfish
Bee Crab Earthworm
Pine tree
Grass Seaweed
mG
y/d
Background level
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Application
Provision of advice on how to use the RAP framework
Likely to use ‘representative organism’ concept
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Representative OrganismReference Animals and Plants
‘Derived consideration reference levels’ for environmental protection
REPRESENTATIVE ORGANISMS
Radionuclide intake and external exposure
Planned, emergency and existing exposure situations
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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
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What is a benchmark?
www.ceh.ac.uk/PROTECT
In radiation protection, usually applied as the incremental dose ABOVE background
Benchmarks are numerical values used to guide risk assessors at various decision points
in a tiered approach
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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
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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!
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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