1 protect: numerical benchmarks workshop, may 2008 update of unscear 1996 presented to: workshop on...
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PROTECT: Numerical Benchmarks Workshop, May 2008
Update of UNSCEAR 1996
Presented To:
Workshop onNumerical Benchmarks for Protecting Biota
Against Radiation in the Environment:Proposed Levels and Underlying Reasoning
Aix-en-Provence, May 14, 2008
Presented By:
Dr. Douglas B. Chambers
SENES Consultants Limited28 Years of Environmental Excellence
PROTECTProtection of the Environment from IonizingRadiation in a Regulatory Context
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PROTECT: Numerical Benchmarks Workshop, May 2008
Outline
Background Estimating Doses to Non-Human Biota UNSCEAR 1996 Summary Chernobyl Dose Effects Summary Effects of Radiation on Non-Human Biota
(General Literature) Conclusions
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Background (1) In the past non-human biota have been
considered as part of pathway to humans Over past decades prevailing view on effects
of ionizing radiation on non-human biota was: If humans adequately protected, “then other living
things are also likely to be sufficiently protected” (ICRP 1977) or “other species not put at risk” (ICRP 1991)
UNSCEAR first considered effects of ionizing radiation on biota in its 1996 report
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Background (2)
Increased worldwide concern over sustainability of environment (e.g., UNEP) has resulted in various efforts to assess effects to non-human biota
Due to increased interest in many countries, UNSCEAR decided to revisit its 1996 assessment of the dose rates below which effects on populations of non-human biota are unlikely
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UNSCEAR
Established by UN General Assembly resolution in 1955 Scientists from 21 UN Member States Other States & organizations provide relevant data Holds annual sessions Assess as scientific information on levels and effects of
ionizing radiation Disseminates findings to UN Assembly, UN agencies,
scientific community & public
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Data
Scientific Literature,UN Member States,
organizations & NGOs
General Assembly, public & scientific community
Findings
Levels
Member States
Development Implementation
UNSCEAR- Levels, effects, risks- Scientific independence
ICRP- Protection- Philosophy - Principles & units
EffectsRisks
FAO, IAEA, ILO, WHO, UNEP- Protection- Standards
Recommendations
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UNSCEAR 1996 Update
Draft to be reviewed by Committee in July 2008
(Hopefully) approved and published by year end
Key observations from review draft follow
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Outline
Background Estimating Doses to Non-Human Biota UNSCEAR 1996 Summary Chernobyl Dose Effects Summary Effects of Radiation on Non-Human Biota
(General Literature) Conclusions
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Estimating Doses to Non-Human Biota
Key Issues Include: Transfer from Environment to Organism Internal and External Radiation Exposure Fraction of Radiation Absorbed by Organism Relative Biological Effectiveness (RBE)
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Major Environmental Transfer Routes
Activity in air and rain Activity in water/sediment
Plants Soil/Sediment
Biota
Radiation exposure of biota
Internalexposure
Externalexposure
Externalexposure
Activity in air and rain Activity in water/sediment
Plants Soil/Sediment
Biota
Radiation exposure of biota
Internalexposure
Externalexposure
Externalexposure
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Fraction of Radiation Absorbed by Organism (1)
Radiation absorption is determined by: Activity concentration in organism Size of organism Type of Radiation Energy of Radiation
Key quantity for estimating doses is absorbed fraction [(E)]: Fraction of energy emitted by radiation source that is absorbed
within the target tissue, organ or organism Internal and External dose conversion coefficients (DCC) for
monoenergetic radiation have been calculated In simplest case organism assumed to be in infinite homogenous
medium, have uniformly distributed activity throughout body and densities of medium and organism’s body identical
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DCC for Reference Organisms Living on Soil
Photon source energy (MeV)
0.01 0.1 1 10
DC
C (
Gy
per
phot
on/m
2 )
10-17
10-16
10-15
10-14
cattle roe deer wolf rabbit mole mouse
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DCC for Earthworm at Various Depths in Soil
0.01 0.1 1 1010-15
10-14
10-13
10-12
depth= 0,00 m depth= 0,05 m depth= 0,25 m depth= 0,50 m
DC
C (
Gy
per
phot
on/k
g)
Photon source energy (MeV)
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PROTECT: Numerical Benchmarks Workshop, May 2008Relative Biological Effectiveness
(Alpha) Number of authors have reported nominal
values for alpha RBE ranging from 5 to 40 As noted by FASSET, difficult to develop a
generally valid radiation weighting factor for use in environmental risk assessment
Updated UNSCEAR document recommends a nominal (generic) RBE of 10 for internally deposited alpha radiation
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PROTECT: Numerical Benchmarks Workshop, May 2008Relative Biological Effectiveness
(Beta) Number of studies suggest that low-energy beta
radiation with energies below 10 keV have higher biological effectiveness than beta radiation with energies above 10 keV (depends on reference radiation)
Updated UNSCEAR document continues to recommend a nominal (generic) RBE value of 1 for beta radiation but acknowledges the most appropriate RBE for low energy (<10 keV) beta radiation remains “open question”
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Outline
Background Estimating Doses to Non-Human Biota UNSCEAR 1996 Summary Chernobyl Dose Effects Summary Effects of Radiation on Non-Human Biota
(General Literature) Conclusions
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UNSCEAR 1996 Summary (1) Unlikely that radiation exposures causing
minor effects in most exposed individual would have significant effects on population
Individual responses to radiation exposure likely to be significant to population level: Reproduction Endpoints Mortality
Reproductive changes more sensitive indicator of radiation effects than mortality
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UNSCEAR 1996 Summary (2)
Mammals most sensitive animal organism Dose rates that are unlikely to result in
significant effects on population: Chronic dose rates of less than 100 uGy/h to most
exposed individual in terrestrial animal population Maximum dose rates of 400 uGy/h to small
proportion of individuals in aquatic populations Notional range of 1 to 10 Gy acute exposure
unlikely to result in effects on populations of non-human biota
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Outline
Background Estimating Doses to Non-Human Biota UNSCEAR 1996 Summary Chernobyl Dose Effects Summary Effects of Radiation on Non-Human Biota
(General Literature) Conclusions
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Chernobyl Dose EffectsSummary (1)
Chernobyl Forum important consolidation of data
Chernobyl Forum identifies 3 Distinct Exposure Phases:
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Chernobyl Dose EffectsSummary (2)
Phase 1: First 20 days, acute exposures due to large quantities of short-lived radionuclides Gamma irradiation up to ~20 Gy/d deposited onto
plant & ground surfaces Additional dose rate from deposited
radionuclides to surface tissues and small biological targets (e.g., mature needles)
High doses to thyroids of vertebrate animals
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Chernobyl Dose EffectsSummary (3)
Phase 2: Summer and Autumn of 1986, short-lived radionuclides decayed and longer-lived radionuclides transported to different environmental components Dose rates at soil surface declined to <10% of
initial values ~80% of total radiation accumulated on plants and
animals was received within first 3 months and 95% of this was from beta radiation
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Chernobyl Dose EffectsSummary (4)
Phase 3: Continuing Phase, decay of short-lived radionuclides and migration of remaining Cs-137 into soil Chronic dose rates less than 1% of initial values Migration of Cs-137 has led to total radiation
exposure from beta and gamma radiation more comparable
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Chernobyl Dose EffectsSummary (5)
Main Observations from Chernobyl Forum : Numerous acute adverse effects in biota located
in areas of higher exposure No adverse radiation-induced effects reported in
plants and animals to doses <0.3 Gy in first month after accident (i.e., <10 mGy/d)
By next growing season, population viability of plants and animals substantially recovered
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Population Effects Around Chernobyl
(Geras’kin et al. 2008)
Summarized effects data for: Scots pine Spruce Herbaceous plants Soil fauna Amphibians Hydrobiants Small mammals cattle
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Population Effects Around Chernobyl (Geras’kin et al. 2008)
Species effectEstimated minimum doses
(or dose rate) at which effect was observed
Estimated maximum doses at which effect
was not observed
Scots pine. Death of weakened trees 8–12 Gy 5 Gy
Mass death of young cones and anthers 10–12 Gy 5 Gy
Mass yellowing of needles, Scots pine 35–40 years old 8–12 Gy 5 Gy
Inhibition of reproductive capacity (reduced number of seeds per cone and increased fraction of hollow seeds)
1–5 Gy 0.5 Gy
Morphological disturbances one year after the accident 0.1–1.0 Gy 0.05 Gy
Significant increase in cytogenetic effects in seedlings and needles 0.5 Gy 0.05 Gy
Frequency of mutations of enzyme loci in seed endosperm 0.07 Gy 0.01 Gy
Spruce, 10–15 years old. Death of trees 4–5 Gy 1 Gy
Spruce, 25 years old. Dying-off of young sprouts. Mortality of much of the trees within 2–3 years
8–10 Gy 5 Gy
Spruce, 40 years old. Noticeable reduction in sprout mass 2.5–3 Gy 1 Gy
ETC
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Outline
Background Estimating Doses to Non-Human Biota UNSCEAR 1996 Summary Chernobyl Dose Effects Summary Effects of Radiation on Non-Human Biota
(General Literature) Conclusions
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Previous Generic Reference Dose Rates
Biota IAEA 332 NCRP 109 UNSCEAR 1996
Terrestrial Plants
10 mGy/d
(4 Gy/a)
- 10 mGy/d
(4 Gy/a)
Terrestrial Animals
-Mortality-Reproductive
1 mGy/d
(0.4 Gy/a)
-
-
-
-
-
-
10 mGy/d (4 Gy/a)
1 mGy/d (0.4 Gy/a)
Aquatic Organisms
- 10 mGy/d
(4 Gy/a)
10 mGy/d
(4 Gy/a)
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Canada ENEVs
Environment Canada and Health Canada’s approach used in ecological risk assessment is using Estimated No Effect Values (ENEVs)
Application (safety) factor of 1 was used to estimate ENEVs for radiation
ENEVs based on detailed evaluations of literature
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ENEVs Used For Assessment Near Canadian Nuclear Facilities
Taxa ENEV (Gy a-1)
Fish 0.2 (0.5 mGy/d)
Benthic invertebrates 2 (5 mGy/d)
Algae 1 (3 mGy/d)
Macrophytes 1 (3 mGy/d)
Mammals 1 (3 mGy/d)
Terrestrial plants 1 (3 mGy/d)
Terrestrial invertebrates 2 (5 mGy/d)
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Summary for Chronic Effects Data Based on FRED
Wildlifegroup
Morbidity MortalityReproductive
capacityMutation
Plant
Plant growth begins to be affected at >100 μGy h-1.Continued exposure at 21 μGy h-1 for 8 years increases the sensitivity in pines
50% mortality at 8 years at ~103 μGy h-1 in pines
A field study indicated a decrease in seed weight of a herb at 5.5 μGy h-1
The mutation rate in microsatellite DNA increased at ~40 μGy h-1
Fish
One experiment, but not another, indicates effects on immune system at 8.3 μGy h-
1
Too few data to draw conclusions
One study showing effects on gametogenesis at 230 μGy h-1.Otherwise effects at >103 μGy h-1
Radiation exposure increases the mutation rate
Mammals
Rat growth not affected at 16 μGy h-1 but affected at>3 x 103 μGy h-1
Some blood parameters affected at 180–850 μGy h-1. No effect on thyroid function at 8 x 103 μGy h‑1
No effect on mouse lifespan at 460 μGy h-
1, but significant reductions above ~103 μGy h-1 in the mouse, goat and dog
Threshold for effects at ~100 μGy h-1, with clear effects at >103 μGy h-1
Too few data to draw conclusions.One of nine references gives an LOEDR of 420 μGy h-1 for mice.
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Dose Rates Proposed Basedon ERICA Data
Targeted protected level as described in the
sourceMethod/justification of the value
Dose rate (µGy h‑1)
Reference
Terrestrial ecosystems
Generic ecosystems SSD-95% species protected plus SF of 5SSD giving an HDR5 of 81.8 Gy h‑1 divided by an SF of 5 and rounded down
10 [E9]
Generic ecosystems SF method: SF of 10 applied to the lowest critical radiotoxicity value EDR10
0.6 [E9]
Plants Background 0.02–0.7 [U3]
Plants Review, SF on the lowest critical radiotoxicity value
110 [B31, E5]
Plants Review based on NCRP 1991; IAEA 1992; UNSCEAR 1996
400 [O1, U16]
ETC
Aquatic and terrestrial flora and fauna
Review concluded that few indications for readily observable effects at chronic dose rates below
<100 [F5]
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ERICA SSD (1) Species Sensitivity Distribution (SSD) developed for
chronic and acute exposures to derive Predicted No Effect Dose Rate (PNEDR)
Chronic SSD approximated the dose rates where 95% of species in
aquatic/terrestrial ecosystem protected HDR5 which results in 10% effect to 5% species No statistical justification to derive ecosystem specific
screening dose rates HDR5 was 81.8 uGy/h Derivation of PNEDR used safety factor of 5 Screening dose rate of 10 uGy/h
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ERICA SSD(2) Acute
Same SSD method applied for acute exposure Statistical difference between marine ecosystems compared
to terrestrial and freshwater ecosystems Varied from about 1 to 5.5 Gy, according to ecosystem type To derive PNED, safety factor of 5 was applied PNEDs of 900 mGy for marine ecosystem and 300 mGy for
terrestrial and freshwater ecosystems
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Conclusion
Overall conclusion is that population level effects on non-human biota are unlikely to be observed at chronic dose rates below (about) 100Gy/h, unchanged from 1996
Recommend further work on mechanisms
SENES Consultants Limited28 Years of Environmental Excellence
PROTECTProtection of the Environment from IonizingRadiation in a Regulatory Context