Risk Analyses and the Development of Radiological Benchmarks

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


Resulting Dose Rates (mGy y-1)





…a BENCHMARK value

We need a point of reference; a known value to which we can compare…


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


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?



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)


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)


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


Free Radicals (unstable molecule that loses one of its electrons)


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


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



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)



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


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


Predicting radiological effects to wildlife is complicated because:


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


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)

http://www.frederica-online.org/

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



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



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....


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


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


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


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


• 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


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)


…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


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


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.


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


‘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


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


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?


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.

Documents

Risk Analyses and the Development of Radiological Benchmarks