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.
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
2
Exposure Pathways
Similarities…
• Low concentrations
• Intake (ingestion,
inhalation)
Differences…
• Irradiation at distance
• Hypersensitisation
3
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
4
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
5
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
6
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.
7
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-MDI
If MDI ≥ TDI, TDSI = 50% TDI
8
Setting a LOAEL from
animal data
9
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?
10
11
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)
12
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
13
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’
14
15
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
16
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
17
Common Basis for Comparison?
• Only intended to be an illustration
• Hypothetical site
• Radionuclides assessed using RCLEA
• Non-radionuclides assessed using CLEA UK (beta)
18
Rads vs Non-rads Example
• Substances
include: H-3, Co60, 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
Contaminant
H-3
Co-60
Sr-90
Cs-137
Ra-336
Non-radioactive
Contaminant
Contaminant
As
Cd
Concentration
(Bq kg-1)
50000
400
2000
1000
300
Concentration
(mg kg-1)
50
10
19
Example – SSAC Values
Radioactive
Contaminant
H-3
Co-60
Sr-90
Cs-137
Ra-336
-1
Total Dose (mSv yr )
-1
Criteria Dose (mSv yr )
Total Dose /Criteria Dose
Non-radioactive
Contaminant
Contaminant
As
Cd
RCLEA
SSAC
(Bq kg-1) (Bq kg-1)
50000 4.00E+04
400
3.25E+02
2000
1.62E+03
1000
8.12E+02
300
2.44E+02
Conc
Dose
(mSv yr-1)
1.46E+00
3.25E-01
1.53E+00
1.84E-01
1.94E-01
3.7
3.0
1.2
Conc
SSAC
CLEA UK
-1
(mg kg ) (mg kg-1)
50
10
18.4
1.75
Total Dose/Criteria
Oral+Dermal
(mg kg -1 day-1)
Total Dose/Criteria
Inhalation
(mg kg -1 day-1)
2.63
3.11
0.03
0.013
20
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)
21
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)
22
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)
23