Presentation by Prof. George Gray, Director of the Centre for Risk Science and Public Health, George Washington University, at the Workshop on Risk Assessment in Regulatory Policy Analysis (RIA), Session 12, Mexico, 9-11 June 2014. Further information is available at http://www.oecd.org/gov/regulatory-policy/

1. Center for Risk Science and Public Health
Exposure Assessment
George Gray
Center for Risk Science and Public Health
Department of Environmental and Occupational Health
Milken Institute School of Public Health

2. Center for Risk Science and Public Health
Exposure and Risk
• “Exposure” is used to describe the extent of contact
with the proposed causative agent (or its proxy) in a
risk relationship
• Examples
Risk Exposure Measure
Lung cancer from cigarette
smoking
Pack-years of exposure
Automobile road fatalities Vehicle Miles Traveled
(VMT)
Workplace injuries Working days

3. Center for Risk Science and Public Health
Critical Point – Exposure
Measure Units
• Exposure needs to be measured or modeled in units that
match with risk relationship
• Good:
Exposure = Tcp = annual
minutes on phone while driving
• Bad:
Incremental Cancer Risk = Cancer Slope Factor (mg/kg/day)-1 x Ingestion (mg/kg/day)
Exposure = ppm benzene in air
Cohen, JT and Graham, JD (2003) A Revised Economic Analysis of Restrictions on the Use of Cell Phones While Driving. Risk
Analysis 23: 5-17

4. Center for Risk Science and Public Health
The Risk Assessment
Paradigm
Exposure Assessment asks the questions:
• To how much of the substance of concern are
people exposed?
• What are the sources of exposure?
• What are the pathways of exposure?
• What is the appropriate measure of dose?

5. Center for Risk Science and Public Health
Exposed Populations
• Individuals
• Populations
• Sensitive Subpopulations

6. Center for Risk Science and Public Health
Exposure Assessment

7. Center for Risk Science and Public Health
Exposure Media and Routes
Air
Soil
Water
Diet
Ingestion
Inhalation
Dermal
Contact

8. Center for Risk Science and Public Health
Exposure Pathways
• Examples (not exhaustive!):
• Ingestion of soil, water, food, or particles
• Inhalation of air or particles
• Dermal contact with soil, sediment, water or air
Air
Soil
Water
Diet
Ingestion
Inhalation
Dermal
Contact

9. Center for Risk Science and Public Health
Making Equivalent Doses
X
XXXX
X
500 mg Drug X
Mouse = 0.07 kg Human = 70 kg
500 mg/0.07 kg = 7,143 mg/kg
500 mg/70 kg = 7.143 mg/kg
7,143 mg/kg Drug X
mg/kg is common way to make
doses equivalent across species

10. Center for Risk Science and Public Health
ADD: average daily dose
C: concentration
IR: intake rate
T: time (duration of exposure)
AF: absorption (bioavailability) factor
BW: body weight
AT: averaging time
Units: typically mg pollutant /kgbody weight /d
Generic Average Daily
Dose Equation

11. Center for Risk Science and Public Health
Aggregate Exposure
• Sometimes there may be several sources of exposure to
a single compound
• Ex/ Benzene in air
• Workplace
• Automobile refueling and driving (gasoline)
• Personal habits (cigarette smoke)
• Which belong in (or out) depend on the scope of RIA
and options considered

12. Center for Risk Science and Public Health
Exposure Assessment
Exposures can be either measured or modeled (or
combination of the two)
• Measured
• more precise (sampling!)
• more expensive, not always methods
• can’t be used to predict future risk
• Modeled
• estimate concentrations, exposures or doses
• require many assumptions
• models imprecise and rarely validated
• allow better incorporation of time in exposure
estimates

13. Center for Risk Science and Public Health
Estimating Exposure:
Pesticide Residues on Food
• Goal is understanding of amount of pesticides
“on the dinner plate”
• 3 Ways used to estimate pesticide residues on
food
• theoretical maximum residue contribution
• farm gate data
• residue monitoring
• Residue estimates combined with consumption
data to estimate exposure

14. Center for Risk Science and Public Health
Theoretical Maximum Residue
Contribution
• Assume every acre of a crop has pesticide
at maximum allowable level (tolerance
level)
• Assume level does not decrease with time,
processing, storage, or cooking
• Considered upper bound on true level of
consumer exposure to pesticide residue on
food

15. Center for Risk Science and Public Health
Farm Gate Data
• Derived from field trials of pesticide use
• Measure level of pesticide on crop at
“farm gate” - after treatment at highest
allowable level with minimum preharvest
interval
• May be adjusted with experimentally
determined processing, washing, or
cooking factors and estimates of
percentage of crop treated

16. Center for Risk Science and Public Health
Residue Monitoring
• Based on measurements of pesticide
residues on food as purchased at grocery
store
• Reflects normal preparation (washing,
cooking, etc.)
• Reflects actual agricultural practices such
as pesticide application rates, different
preharvest intervals, and the effects of
time and storage

17. Center for Risk Science and Public Health
Does The Choice Matter?
Example: Chlorothalonil (Bravo®) on Celery
chlorothalonil (ppm) % of tolerance
• TMRC 15.0 100.0
• Field Data 4.1 27.1
• Residue Monitoring 0.8 0.12
Source: Gary L. Eilrich (1991) Tracking the Fate of Residues from the Farm Gate to the Table, in
Pesticides and Food Safety (Tweedy, B.G., Dishburger, H.J., Ballantine, L.G., and McCarthy, J.
eds.) American Chemical Society, Washington, D.C.

18. Center for Risk Science and Public Health
Quantitative Approaches
• Point Estimates (for either individuals or populations)
• Central Tendency
• Extrema (“Reasonable Maximally Exposed”)
• Bounding Point Estimates
• Interval Estimates
• Distributional Estimates

19. Center for Risk Science and Public Health
Point Estimation
• Example: Inhalation of fine particulate matter (PM) in
ambient air for a city of 1M
• Ambient concentration: 50 µg/m3
• Average inhalation rate: 15 m3/d
• Average body weight: 70 kg
• Average daily dose (ADD): 10.7 µg/kg/d

20. Center for Risk Science and Public Health
Interval Estimation
• Same Example:
• Ambient concentration: 50 µg/m3
• Inhalation rate range: 2 m3/d – 44 m3/d
• Body weight range: 1 kg - 650 kg
• Minimum Possible Dose: 0.2 µg/kg/d
• Maximum Possible Dose: 2,200 µg/kg/d
• Dose based on midpoints: 3.5 µg/kg/d
• Now What?

21. Center for Risk Science and Public Health
Uncertainty & Variability
• Variability
• “…represents heterogeneity or diversity in a well-
characterized population which is usually not reducible
through further measurement or study.”
• Uncertainty
• “…represents ignorance about a poorly characterized
phenomenon which is sometimes reducible through
further measurement or study.”

22. Center for Risk Science and Public Health
Distribution of Body Weight

23. Center for Risk Science and Public Health
Distribution of Inhalation Rate

24. Center for Risk Science and Public Health
Distributional Approach
• Same Example:
• Treat BW, InhR as variable
• BW ~ LogNormal(mean = 70kg, GSD = 1.29)
• InhR ~ LogNormal(mean = 15m3
/d, GSD = 1.9)

25. Center for Risk Science and Public Health
Distributional Result (n=1000)
Median = 7.8
Mean = 10.0
95%ile = 26.2

26. Center for Risk Science and Public Health
Standardized Exposure
Factors
• Examine a regulatory risk assessment and you will
find a large set of standardized assumptions for
estimating exposure.
• Body weight: 70 kg
• Lifetime: 75 yrs
• Daily breathing rate: 20 m3/d
• Water intake: 2 L/d
• Why do these exist?
• Is the use of these factors good or bad?

27. Center for Risk Science and Public Health
Final Thoughts
• To be useful, an estimate of exposure or any
other element of a risk assessment requires a
description of uncertainty
• “Usually, exposure assessments will contain less
uncertainty than other steps in a risk assessment,
especially the dose response portion” -- Dennis
Paustenbach, Human and Ecological Risk Assessment
• Precision required of exposure estimates depends
on precision required in output (risk estimate) and
the precision available in other inputs.