Transcript David Copplestone (University of Stirling) OBJECTIVES What is a benchmark? Why are benchmarks needed? How are benchmarks derived? How are benchmarks used? INTRODUCTION The need for benchmarks... ...

David Copplestone
(University of Stirling)
OBJECTIVES
What is a benchmark?
Why are benchmarks needed?
How are
benchmarks
derived?
How are
benchmarks
used?
INTRODUCTION
The need for
benchmarks...
... a retrospective
screening model example
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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 are LARGER
than the REAL DOSES
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Total 137-Cs Released (GBq)
Conservative Assumptions for
Screening Calculations
5000
4000
1) SOURCE TERM: used1964
maximum release as a mean
for calculations
3000
2000
1000
0
54 59 64
69 74 79 84
2) EXPOSURE: assumed fish
were living at point of discharge
Year
3) ABSORPTION: assumed all
fish were 30 cm in diameter
which maximized absorbed dose
CONTAMINATED
SEDIMENTS
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4) IRRADIATION: behavior of
fish ignored, assumed they
spent 100% of time on bottom
sediments where > 90% of
radionuclides are located
Resulting Dose Rates (mGy y-1)
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We need a point of reference; a
known value to which we can
compare…
…a BENCHMARK value
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
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Data on radiation effects for nonhuman species
Wildlife Group
Morbidity
Mortality
Reproductive
capacity
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
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Some data
Mutation
Approaches to derive
protection criteria
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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

A Quantitative approach
Used to derive the ERICA and PROTECT
values
 Consistent with EC approach for other
chemicals

How to derive « safe levels »
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
Exposure-response relationship from ecotoxicity tests
Effect (%)
100 %
Observed data
Regression model
50 %
LOEC: Lowest observed effect concentration
NOEC: No observed effect concentration
10 %
EC10
EC50
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Contaminant
Concentration
How to derive « safe levels »
....adapted for radiological conditions....
Effect (%)
Exposure-response relationship from ecotoxicity tests
(specific to stressor, species, and endpoint)
100 %
Observed data
Regression model
50 %
LOEC: Lowest observed effect concentration
NOEC: No observed effect concentration
10 %
EC50
EC10
ED50
ED10
EDR10 EDR50
Concentration (Bq/L or kg)
Dose (Gy)
Dose Rate (µGy/h)
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
PNEV = minimal Effect Concentration / Safety Factor
Main underlying assumptions
In the frame of this approach,
extrapolations are made from:
•The ecosystem response
depends on the most sensitive
species
•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
•Protecting ecosystem structure
protects community function
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Fixed assessment factor
method
PNEV = minimal Effect Concentration / Safety Factor
Main underlying assumptions
In the frame of this approach,
extrapolations are made from:
The safety factor method is highly
•The ecosystem
response
•Acute
to chronic the
conservative
as
it implies
depends on the most sensitive
•One life stage to the whole life
multiplication of several
worst cases
species
cycle
•Protecting ecosystem structure
protects community function
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•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
The approach used to derive noeffects 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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The predicted no-effect dose rate
(PNEDR) evaluation
STEP 3 –
giving 10%
The
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The hazardous dose rate (HDR5)
effect to 5% of species is estimated
final PNEDR is then obtained by
applying an additional safety factor
(typically from 1 to 5)
to take into account remaining
extrapolation uncertainties
SSD for generic ecosystem at
chronic external γ-radiation (ERICA)
•
PNEDR = HDR 5% / SF
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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
PNEDR used as the screening value at the
ERA should be highly conservative
SF = 5
PNEDR ≈ 10 μGy/h
Generic ecosystem SSD for chronic
external γ-radiation (PROTECT)
Percentage of Affected Fraction
100%
90%
80%
70%
60%
50%
EDR10 and 95%CI:
Minimum value
per species
40%
30%
20%
10%
5%
0%
0.1
1
10
100
1000
10000
100000
1000000 10000000
Dose rate (µGy/h)
PNEDR=10 µGy/h
HDR5 = 17 µGy/h [2-211]
(SF of 2)
Best-Estimate
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Vertebrates
Centile 5%
Centile 95%
Plants
Invertebrates
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
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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

Background radiation exposure for
ICRP RAPs (weighted dose rates)
Freshwater organisms –
0.4 – 0.5 μGy/h (Hosseini et al., 2010)
Marine organisms –
0.6 - 0.9 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
Freshwater organisms –
0.4 – 0.5 μGy/h (Hosseini et al., 2010)
Derived screening doseMarine
rate organisms
(10 μGy/h)
– is more than
10 times 0.6
these
background
- 0.9 μGy/h
(Hosseini et values
al., 2010)
Terrestrial animals and plants –
0.07-0.6 μGy/h (Beresford et al., 2008)
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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
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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Tool – generic screening dose rate
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ERICA Tool – generic screening dose rate
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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
ICRP Approach
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.”
DCRLs
1000
mGy/d
100
10
Bee
1
Frog
Trout
Flatfish Grass Seaweed
0.1
Deer
0.01
0.001
Rat
Duck Pine tree
Background level
Crab Earthworm
Application
Provision of advice on how to use the RAP
framework
 Likely to use ‘representative organism’
concept

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
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 nonhuman species
 Consider the principles used in chemical risk
assessment/protection

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
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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

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