Comparison of Risk Assessment for Radioactive and Chemical Contaminants

Similarities, Differences and Scope for Comparison

BRMF/SAFESPUR Workshop, 30 September 2008

Dr James Wilson, Quintessa, Ltd.

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Structure of this Presentation

• Exposure and Response– Pathways, mechanisms and responses

• Comparison of criteria for the protection of human health for radionuclides and non-radioactive contaminants in soil

• Comparison of exposure assessment tools for radiological and non-radiological soil contamination

• An example of rad vs non-rad contamination assessment

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Exposure Pathways

Similarities…• Low concentrations• Intake (ingestion,

inhalation)Differences…• Irradiation at

distance• Hypersensitisation

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Exposure-Response: Radionuclides

• Most evidence from A-bombs• Same mechanism for all radionuclides

– Direct ionisation or free radicals• Depends on intensity, duration, organs

– Deterministic: threshold– Stochastic: non-threshold

• Important uncertainties remain– Progression from damage to cancer– Response at low dose rates– Extrapolation from animals

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Exposure-Response:Chemicals

• Observations are key (animal experiments, human epidemiology, occupational exposures)

• Threshold and non-threshold effects

• Uncertainties include:– Extrapolation from animal experiments– Exposure route extrapolation and bio-

availability– Inter and intra-species variability

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Dose Criteria: Radionuclides

• Recommendations of ICRP• Dose proportional to absorbed energy• Weightings: type of radiation, organ• Risk proportional to dose (< thresholds)• Standards and Criteria (IAEA, Euratom)

– Risks and thresholds– Can be compared with natural background– Independent of particular radionuclides– UK legislation reflects ICRP recommendations

Radiation Dose Criteria for Contaminated Land

• HPA (2006): Annual dose approaching 10 mSv justifies intervention (considers ICRP recommendations)

• HPA: health criteria for determination = 3 mSv yr-1 (to protect from non-threshold effects, lifetime cancer risk of ~1/100)

- or annual equivalent dose to lens of eye > 15 mSv or annual equivalent dose to skin > 50 mSv

• Dose criteria apply to incremental dose from contamination (i.e. total dose – background), average UK background dose is ~2.2 mSv yr-1 therefore additional dose of 3 mSv yr-1 is increase between 2 and 3 times UK annual average.

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Health Criteria Values for Chemicals- threshold effects

• Defra/Environment Agency CLR Toxicological reports• Assess international and other national guidance (e.g.

WHO)• HCV = ‘Tolerable Daily Soil Intake’ (TDSI)

– TDI (from all sources) identified from NOAELs (or LOAELs) often from animal experiments - multiplied by uncertainty factor(s) e.g. 10 for interspecies and 10 for intra-species variation

– Mean Daily Intake (MDI) set by considering other (non-soil) routes of exposure

– TDSI = TDI - MDI– Old approach: If MDI > 80% TDI, then TDSI = 20% TDI.– New approach:

If MDI < 50% TDI, then TDSI = TDI-MDIIf MDI ≥ TDI, TDSI = 50% TDI

Setting a LOAEL from animal data

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Health Criteria Values for Chemicals- non threshold effects

– HCV = ‘Index Dose’ (ID)– ID is daily intake that represents a very low to negligible

risk to human health (i.e. set to be protective)– Provision that all exposures (inc. soil) should be ALARP,

therefore intakes from sources other than soil not included

– In UK not directly based on a fixed level of risk (issue of animal to human extrapolation for quantitative risk assessment)

– Excess lifetime cancer risks generally ~1/10 000 to 1/ 100 000 depending on substance and exposure route

– Possible future use of Benchmark Dose (BMD) data?

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Soil Assessment Criteria: Chemicals

• Soil Guideline Values / Site Specific Assessment Criteria• SGVs are concentrations of contaminant in soil such

that Health Criteria Values should not be exceeded (based on CLEA exposure model assumptions)

• Generic non-statutory guidance• Difficulties reported by Local Authorities in use of SGVs

for determination under Part IIA of the EPA (1990), and uncertainty in how great HCV has to be exceeded to represent ‘significant possibility of significant harm’

– triggered ‘Way Forward’ consultation (Defra)

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Non-threshold risk:Chemical vs. Radiation

• Chemicals: - Index Dose not set at specific risk - excess lifetime cancer risks

generally range from ~1/10 000 to 1 000 000 (ID oral for As ~1/1000)

- UK Expert Medical Committee (CoC) does not endorse quantitative cancer risk models based on high-dose animal data

- Non-soil intakes not considered (ALARP principle assumed to have been applied to all sources of exposure)

• Radiation:- UK effective dose criteria set at 3 mSv yr-1 (lifetime fatal

cancer risk of ~1/100)- Background doses were considered in setting effective dose

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CLEA Approach

• CLEA UK (now in redevelopment) and RCLEA• Tiered approach• Contaminant-specific guideline values• Produces soil concentrations SGVs/RSGVs, or Site

Specific Assessment Criteria (SSACs) • Generic land-use scenarios: residential both with and

without plant uptake, allotments, commerical/industrial

• ‘Critical receptors’

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Differences between CLEA UK and RCLEA

• CLEA SGVs are contaminant specific, RCLEA allows additive effective dose to be calculated

• CLEA UK had 18 age groups, RCLEA has 3 (infant, adult, child)• RCLEA has 2 additional exposure pathways: (1) whole

body external irradiation from contamination at a distance; (2) irradiation of skin from direct contact with contaminated material

• Adsorption through skin not applicable in RCLEA (tritium exposure should be considered separately)

• RCLEA has only 1 soil type due to uncertainties in solid:liquid Kd values

• Volatilisation is excluded from RCLEA – considered insignificant for historic contamination

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Similarities, Differences• Exposure-response

– Threshold and non-threshold effects– Intake pathways/other pathways– Dependence on contaminant

• Standards– Epidemiological/toxicological studies– Protection from threshold effects– Dependence on contaminant/pathway– Regulatory approach to non-threshold risks

• Assessments– Exposure pathways and groups

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Common Basis for Comparison?

• Only intended to be an illustration• Hypothetical site

• Radionuclides assessed using RCLEA• Non-radionuclides assessed using CLEA UK

(beta)

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Rads vs Non-rads Example

• Substances include: H-3, Co-60, Sr-90, Ra-226, As, Cd.

• Ingestion, inhalation, ext. irradiation, dermal

• SSAC values for residential land use (with plant uptake and female infant receptor

Radioactive ConcentrationContaminant

(Bq kg-1)H-3 50000

Co-60 400Sr-90 2000

Cs-137 1000Ra-336 300

Non-radioactive ConcentrationContaminant

Contaminant (mg kg-1)As 50Cd 10

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Example – SSAC ValuesRadioactive Conc RCLEA DoseContaminant SSAC

(Bq kg-1) (Bq kg-1) (mSv yr-1)

H-3 50000 4.00E+04 1.46E+00Co-60 400 3.25E+02 3.25E-01Sr-90 2000 1.62E+03 1.53E+00

Cs-137 1000 8.12E+02 1.84E-01Ra-336 300 2.44E+02 1.94E-01

Total Dose (mSv yr-1) 3.7

Criteria Dose (mSv yr-1) 3.0Total Dose /Criteria Dose 1.2

Non-radioactive Conc SSAC Total Dose/Criteria Total Dose/CriteriaContaminant CLEA UK Oral+Dermal Inhalation

Contaminant (mg kg-1) (mg kg-1) (mg kg -1 day-1) (mg kg -1 day-1)

As 50 18.4 2.63 0.03Cd 10 1.75 3.11 0.013

Example – risk comparison

• Arsenic oral intake is 2.6 times oral Index Dose (ID oral = 0.3 μg/kgbw/day: lifetime risk of developing skin cancer ~ 1/1000, assuming 1% mortality rate of those who develop skin cancer, risk of death ~1/10 000)

• Cadium oral intake is 3.1 times TDSI • (TDSI = 0.77 μg/kg bw/day – set to protect against kidney

damage, based on studies of proteinuria in humans)

• Total effective radiation dose is 1.2 times criteria dose (3 mSv yr-1 lifetime fatal cancer risk of ~1/100)

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Conclusions

• Comparisons of risk can be made• Differences in risk assessment approaches

are significant (especially with regard to availability of toxicological data and regulatory approaches for using cancer risk models)

References

Defra/Environment Agency. (2002) Contaminants in soil: collation of toxicological data and intake values for humans (CLR 9).

HPA (2006) Dose criteria for the designation of radioactively contaminated land. Report RCE-2.

Defra/Environment Agency. (2002) The Contaminated Land Exposure Assessment (CLEA) model: technical basis and algorithms (CLR 10).

Defra (2005) CLAN briefing note 2/05: Soil Guideline Values and the Determination of Land as Contaminated Land under Part IIA.

Defra (2007) The radioactively contaminated land exposure assessment methodology - technical report (CLR 14)

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