Risk Analyses and the Development of Radiological Benchmarks
Tom Hinton (IRSN)
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OBJECTIVES What 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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Conservative Assumptions for Screening Calculations
www.ceh.ac.uk/PROTECT
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Resulting Dose Rates (mGy y -1 ) 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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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 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
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www.ceh.ac.uk/PROTECT Knowledge of ionising radiations effect
on wildlife is the basis for the derivation of radiological risk
benchmarks
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What is known about effects from ionising radiation?
www.ceh.ac.uk/PROTECT
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Wilhelm Rontgen (18451923) 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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www.ceh.ac.uk/PROTECT 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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www.ceh.ac.uk/PROTECT base loss base change single stand break
double stand break interstrand crosslinks OOH H H Feinendegen,
Pollycove. J. Nucl. Medicine. 2001. V.42. p. 17N-27N Different
kinds of DNA damage induced by -radiation per 0.01 Gy
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www.ceh.ac.uk/PROTECT Free Radicals (unstable molecule that
loses one of its electrons)
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www.ceh.ac.uk/PROTECT DNA damage and repair
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Fate of Mutations Somatic Cells Somatic Cells Germ Cells Germ
Cells Decrease in number and quality of gametes Increased embryo
lethality Alteration to offspring Cell Death Cell Death Cancer
www.ceh.ac.uk/PROTECT
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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 Deleterious mutations Neutral
mutations Spread in the population Remove from the population
Persist over many generations www.ceh.ac.uk/PROTECT
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Knowledge on Effects of Radiation Exposure on Wildlife
www.ceh.ac.uk/PROTECT
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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
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Increasing SensitivityDecreasing Sensitivity Large nucleusSmall
nucleus Large chromosomesSmall chromosomes Acrocentric
chromosomesMetacentric chromosomes Low chromosome numberHigh
chromosome number Diploid or haploidHigh polypolid Sexual
reproductionAsexual reproduction Long intermitotic timeShort
intermitotic time Long dormant periodShort or no dormant period
Factors Influencing the Sensitivity of Plants to Radiation
(Sparrow, 1961) www.ceh.ac.uk/PROTECT
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Radiation Effects on Non- Human Biota Early Mortality premature
death of organism Early Mortality premature death of organism
Morbidity reduced physical well being including effects on growth
and behavior Morbidity reduced 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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www.ceh.ac.uk/PROTECT Fundamental Differences In Human and
Ecological Risk Analyses Type Unit of Observation Endpoint
Dose-Response Type Unit of Observation Endpoint Dose-Response Human
individual lifetime cancer relationships risk established
Ecological varies varies not established population, community,
ecosystem > mortality, < fecundity, sublethal effects for
chronic, low level exposure to radiation, alone, or mixed with
other contaminants
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Populations are resilient Indirect effects often occur that are
unpredictable Blaylock (1969) studies at Oak Ridge DIRECT EFFECT:
Increased mortality of fish embryos exposed to 4 mGy / d INDIRECT
EFFECT: Fish produced larger brood sizes NET RESULT: No effect to
population Compensating mechanisms exist www.ceh.ac.uk/PROTECT
Predicting radiological effects to wildlife is complicated
because:
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www.ceh.ac.uk/PROTECT Prejevalsky Horses Russian Boar Wolves
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
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www.ceh.ac.uk/PROTECT Data Base of Knowledge on Effects of
Radiation Exposure on Biota FREDERICA
(www.frederica-online.org)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)
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effects data; per ecosystem per exposure pathway (external or
internal irradiation) per duration (acute or chronic)
Acute-external Acute-internal Chronic-external Chronic-internal
Acute-external Acute-internal Chronic - external Chronic - internal
73% of all data FREDERICA Database www.ceh.ac.uk/PROTECT
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Aquatic invertebrates To few to draw conclusions Some data Data
on radiation effects for non-human species Morbidity Mortality
Reproductive capacity Mutation Amphibians Aquatic plants Bacteria
Birds Crustaceans Fish Fungi Insects Mammals Molluscs Moss/Lichens
Plants Reptiles Soil fauna Zooplankton No data Chronic effects and
external irradiation
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Approaches to derive protection criteria
www.ceh.ac.uk/PROTECT
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Effect (%) Regression model 100 % 50 % 10 % Contaminant
Concentration 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 EC 50 for acute
exposure conditions and EC 10 for chronic exposures methods
recommended by European Commission for estimating
predicted-no-effects-concentrations for chemicals How to derive
safe levels EC 10 EC 50
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Effect (%) Regression model 100 % 50 % 10 % EC 10 ED 10 EDR 10
Concentration (Bq/L or kg) Dose (Gy) Dose Rate (Gy/h) EC 50 ED 50
EDR 50 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....
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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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www.ceh.ac.uk/PROTECT 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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www.ceh.ac.uk/PROTECT 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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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
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www.ceh.ac.uk/PROTECT The 5% percentile of the SSD defines HDR
5 (hazardous dose rate giving 10% effect to 5% of species) HDR 5 =
82 Gy/h SSD for generic ecosystem at chronic external -radiation
(ERICA) PNEDR used as the screening value at the ERA should be
highly conservative SF = 5 PNEDR 10 Gy/h PNEDR = HDR 5% / SF
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www.ceh.ac.uk/PROTECT Best-EstimateCentile 5%Centile 95%
VertebratesPlantsInvertebrates 5% HDR 5 = 17 Gy/h [2-211] PNEDR=10
Gy/h (SF of 2) EDR 10 and 95%CI: Minimum value per species Generic
ecosystem and chronic exposure SSD for generic ecosystem at chronic
external -radiation (PROTECT)
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www.ceh.ac.uk/PROTECT a BENCHMARK value We need a point of
reference; a known value to which we can compare 10 Gy/h * 24 h / d
= 240 Gy/d = 0.2 mGy /d
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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 Gy/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
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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 Gy/h for
terrestrial animals - 40 Gy/h Derived using a SSD approach, the
PNEDR of 10 Gy/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.
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www.ceh.ac.uk/PROTECT Background radiation exposure for
wildlife (UNSCEAR, 1996; 2000) terrestrial and aquatic plants 0.02
- 0.7 Gy/h; terrestrial animals (mammals) - 0.01-0.44 Gy/h
freshwater organisms 0.022-0.18 Gy/h terrestrial animals and plants
- 0.069-0.61 Gy/h (Beresford et al., 2008) Derived screening dose
rate (10 Gy/h) is more than 10 times these background values
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www.ceh.ac.uk/PROTECT 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 MODELs pre-defined concentrations
(i.e. media concentrations that result in a PNEDR) ERICA:
environmental media concentration limits EMCLs RESRAD-BIOTA: biota
concentration guidelines BCGs
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www.ceh.ac.uk/PROTECT Estimated assuming: Habitat
characteristics that maximise exposure Probability distributions
associated with the default CR and K d 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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www.ceh.ac.uk/PROTECT Estimated assuming: Infinitely large
(internal) and small (external) geometries for dose calculations
Daughter T 1/2 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
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Is the new benchmark of 10 Gy/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
Distribution based 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.