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Risk Analyses and the Development of Radiological Benchmarks. Tom Hinton (IRSN). 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. - PowerPoint PPT Presentation
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Risk Analyses and the Development of Radiological
Benchmarks
Tom Hinton (IRSN)
OBJECTIVESWhat is a benchmark?
Why are benchmarks needed?
How are benchmarks derived?
How are benchmarks used?
INTRODUCTION
The need for benchmarks...
... a retrospective screening model
examplewww.ceh.ac.uk/PROTECT
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
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
www.ceh.ac.uk/PROTECT
Resulting Dose Rates (mGy y-1)
www.ceh.ac.uk/PROTECT
www.ceh.ac.uk/PROTECT
www.ceh.ac.uk/PROTECT
www.ceh.ac.uk/PROTECT
…a BENCHMARK value
We need a point of reference; a known value to which we can compare…
www.ceh.ac.uk/PROTECT
Definition of Benchmarks
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
www.ceh.ac.uk/PROTECT
Knowledge of ionising radiation’s effect on wildlife is the basis for the derivation
of radiological risk benchmarks
What is known about effects from
ionising radiation?
www.ceh.ac.uk/PROTECT
www.ceh.ac.uk/PROTECT
Wilhelm Rontgen
(1845—1923)
First roentgenogram, 1895
Henri Becquerel
(1852-1908)
Becquerel plate, 1896
Discoverer of radioactivity, 1903 Nobel
Prize in Physics
First Nobel Prize in Physics, 1901
Marie Curie (1867-1934)
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DNA is the primary target for the induction of biological effects from radiation in ALL living organisms
Broad similarities in radiation responses for different organisms……and yet, wide differences in radiation sensitivity
(Whicker and Schultz, 1982)
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base lossbase change
single stand break
double stand break
interstrand crosslinks
O OHH
H
Feinendegen, Pollycove. J. Nucl. Medicine. 2001. V.42. p. 17N-27N
Different kinds of DNA damage induced by γ-radiation per 0.01 Gy
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Free Radicals (unstable molecule that loses one of its electrons)
www.ceh.ac.uk/PROTECT
DNA damage and repair
Fate of MutationsFate of Mutations
SomaticCells
SomaticCells
GermCells
GermCells
Decrease in number
and quality of gametes
Decrease in number
and quality of gametes
Increased
embryo lethality
Increased
embryo lethality
Alteration to
offspring
Alteration to
offspringCell
DeathCell
DeathCancerCancer
www.ceh.ac.uk/PROTECT
For humans, risk of hereditary effects in offspring of exposed individuals is about 10% of the cancer risk to the exposed
parents (UNSCEAR, 2001)
For non-human biota the risk of hereditary effects is unknown
Fate of mutations in non-human biota
Mutation
Cell
Confer a selective
advantage
Confer a selective
advantage
Deleterious mutations
Deleterious mutations
Neutral mutationsNeutral
mutations
Spread in the
population
Spread in the
population
Remove from the
population
Remove from the
population
Persist over many
generations
Persist over many
generationswww.ceh.ac.uk/PROTECT
Knowledge on Effects of Radiation Exposure on
Wildlife
www.ceh.ac.uk/PROTECT
www.ceh.ac.uk/PROTECT
early data came from…• laboratory exposures• accidents (Kyshtym, 1957)• areas of naturally high background• nuclear weapons fallout • large-scale field irradiators
early data came from…• laboratory exposures• accidents (Kyshtym, 1957)• areas of naturally high background• nuclear weapons fallout • large-scale field irradiators
wealth of data about the
biological effects of radiation on
plants and animals
wealth of data about the
biological effects of radiation on
plants and animals
Increasing Sensitivity Decreasing SensitivityLarge nucleus Small nucleus
Large chromosomes Small chromosomes
Acrocentric chromosomes Metacentric chromosomes
Low chromosome number High chromosome number
Diploid or haploid High polypolid
Sexual reproduction Asexual reproduction
Long intermitotic time Short intermitotic time
Long dormant period Short or no dormant period
Factors Influencing the Sensitivity of Plants to Radiation
(Sparrow, 1961)
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www.ceh.ac.uk/PROTECT
Radiation Effects on Non-Human Biota
Early Mortalitypremature death of
organism
Early Mortalitypremature death of
organism
Morbidityreduced physical well being including effects
on growth and behavior
Morbidityreduced physical well being including effects
on growth and behavior
Reproductive Success
reduced fertility and fecundity
Reproductive Success
reduced fertility and fecundity
These categories of radiation effects are similar to the endpoints that are often used for risk assessments of other environmental stressors, and are relevant to the needs of nature conservation and other forms of environmental protection
Reproduction is thought to be a more sensitive effect than mortality
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Fundamental Differences In Human and Ecological Risk Analyses
Type Unit of Observation Endpoint Dose-Response Human individual lifetime cancer relationships risk established
Ecological varies varies not established population,
community,ecosystem
> mortality,
< fecundity,sublethaleffects
for chronic,low level exposure
to radiation, alone, ormixed with other
contaminants
Populations are resilient
Indirect effects often occur that are unpredictable
Blaylock (1969) studies at Oak RidgeDIRECT EFFECT: Increased mortality of fish embryos exposed to 4 mGy / dINDIRECT EFFECT: Fish produced larger brood sizesNET RESULT: No effect to population
Compensating mechanisms exist
www.ceh.ac.uk/PROTECT
Predicting radiological effects to wildlife is complicated because:
www.ceh.ac.uk/PROTECT
Prejevalsky Horses
Russian Boar
Wolves
With the removal of humans, wildlife around Chernobyl are flourishing
With the removal of humans, wildlife around Chernobyl are flourishing
48 endangered species listed in the international Red Book of protected animals and plants are now thriving in the Chernobyl Exclusion Zone
www.ceh.ac.uk/PROTECT
Data Base of Knowledge on Effects of Radiation Exposure on Biota
FREDERICA (www.frederica-online.org)
An online database of literature data to help summarise dose-effect relationships
FREDERICA can be used on its own; or in conjunction with the ERICA assessment tool (for conducting risk assessments of wildlife exposed to ionising radiation)
(> 1500 references; 26 000 data entries)
effects data; per ecosystemper exposure pathway (external or internal irradiation)per duration (acute or chronic)
288
344
97 20
4058
970
3449
11564
milieu terrestre exposition aiguë externe milieu terrestre exposition aiguë interne
milieu terrestre exposition chronique externe milieu terrestre exposition chronique interne
milieu aquatique exposition aiguë externe milieu aquatique exposition aiguë interne
milieu aquatique exposition chronique externe milieu aquatique exposition chronique interne
Acute-externalAcute-internalChronic-external
Chronic-internal
Acute-external
Acute-internal
Chronic - external
Chronic - internal
73% of all data
FREDERICA Database
www.ceh.ac.uk/PROTECT
www.ceh.ac.uk/PROTECT
Aquatic invertebrates
To few to draw conclusions
Some data
Data on radiation effects for non-human species
Morbidity MortalityReproductive
capacity MutationAmphibians
Aquatic plants
BacteriaBirds
CrustaceansFish
Fungi
InsectsMammalsMolluscs
Moss/LichensPlants
ReptilesSoil fauna
Zooplankton
No data
Chronic effects and γ external irradiation
Approaches to derive protection criteria
www.ceh.ac.uk/PROTECT
www.ceh.ac.uk/PROTECT
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
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)
How to derive « safe levels »
....adapted for radiological conditions....
www.ceh.ac.uk/PROTECT
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
The safety factor method is highly conservative as it implies the
multiplication of several worst cases
PNEV = minimal Effect Concentration / Safety Factor
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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
www.ceh.ac.uk/PROTECT
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
www.ceh.ac.uk/PROTECT
• 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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Best-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 and chronic g exposure
SSD for generic ecosystem at chronic external γ-radiation (PROTECT)
www.ceh.ac.uk/PROTECT
…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
www.ceh.ac.uk/PROTECT
Reminders… The PNEDR is a basic generic ecosystem screening value to
benchmark where additional work is needed The derived PNEDR equal to 10 μGy/h can be applied to a number of
situations for which environmental and human risk assessment are carried out
The risk assessor needs to be aware of the following rules while using the ERICA tool: the PNEDR does not apply for any other ecological object to be
protected besides the generic ecosystem the PNEDR was derived for use only in the first two tiers of the
ERICA Integrated Approach the PNEDR is the benchmark value for screening against
incremental dose rates, and not the total dose rates including background
www.ceh.ac.uk/PROTECT
IAEA (1992) and UNSCEAR (1996) suggested the following no-effect values for populations of non-human biota:
• for aquatic animals and terrestrial plants - 400 μGy/h • for terrestrial animals - 40 μGy/h
Derived using a SSD approach, the PNEDR of 10 μGy/h is consistent with these previously recommended values
• The hazardous dose rate definition means that 95% of species would be protected. However, there may be keystone species among the 5% that are unprotected.
www.ceh.ac.uk/PROTECT
Background radiation exposure for wildlife (UNSCEAR, 1996; 2000)
terrestrial and aquatic plants –
0.02 - 0.7 μGy/h;terrestrial animals
(mammals) - 0.01-0.44 μGy/h
freshwater organisms – 0.022-0.18 μGy/h
terrestrial animals and plants -
0.069-0.61μGy/h (Beresford et al., 2008)
Derived screening dose rate (10 μGy/h) is more than 10 times these background
values
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‘Concentration limits’
Both ERICA Tool & RESRAD-BIOTA use ‘tiered assessments’ with initial assessment (Tier-1) being very simple (minimal input---conservative output)
YOUR media concentrations compared to the MODEL’s pre-defined concentrations (i.e. media
concentrations that result in a PNEDR) ERICA: ‘environmental media concentration limits’
EMCLs RESRAD-BIOTA: ‘biota concentration guidelines’ BCGs
www.ceh.ac.uk/PROTECT
ERICA Tool – EMCLs
Estimated assuming: Habitat characteristics that maximise exposure Probability distributions associated with the default
CR and Kd databases were used to determine 5th percentile EMCL
No conservatism applied to dosimetry
For aquatic ecosystems EMCL for water includes consideration of external dose from sediment and that for sediment includes external dose from water and biota-water transfer
Environmental Media Concentration Limits
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RESRAD-BIOTA - BCGs
Estimated assuming: Infinitely large (internal) and small (external)
geometries for dose calculations Daughter T1/2’s up to 100 y included All terrestrial organisms 100% in soil; aquatic
100% water-sediment interface ‘Maximum’ CR values or 95th percentile CR
values predicted using a kinetic-allometric approach
Answers
Is the new benchmark of 10 µGy/h final?
www.ceh.ac.uk/PROTECT
How are benchmarks derived?
Safety Factor Method stringent method as the PNEC value is obtained by dividing the lowest critical data by an appropriate SF ranging from 10 to 1000.
Species Sensitivity Distributionbased on a statistical extrapolation model to address variation between species in their sensitivity to a stressor.
What is a benchmark?
Benchmarks are numerical values used to guide risk assessors at various decision points in a tiered approach.
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