Center for Risk Science and Public Health
Exposure Assessment
George Gray
Center for Risk Science and Public Health
Depart...
Center for Risk Science and Public Health
Exposure and Risk
•  “Exposure” is used to describe the extent of contact
with t...
Center for Risk Science and Public Health
Critical Point – Exposure
Measure Units
•  Exposure needs to be measured or mode...
Center for Risk Science and Public Health
The Risk Assessment
Paradigm
Exposure Assessment asks the questions:
•  To how m...
Center for Risk Science and Public Health
Exposed Populations
•  Individuals
•  Populations
•  Sensitive Subpopulations
Center for Risk Science and Public Health
Exposure Assessment
Center for Risk Science and Public Health
Exposure Media and Routes
Air
Soil
Water
Diet
Ingestion
Inhalation
Dermal
Contact
Center for Risk Science and Public Health
Exposure Pathways
•  Examples (not exhaustive!):
•  Ingestion of soil, water, fo...
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...
Center for Risk Science and Public Health
ADD: average daily dose
C: concentration
IR: intake rate
T: time (duration of ex...
Center for Risk Science and Public Health
Aggregate Exposure
•  Sometimes there may be several sources of exposure to
a si...
Center for Risk Science and Public Health
Exposure Assessment
Exposures can be either measured or modeled (or
combination ...
Center for Risk Science and Public Health
Estimating Exposure:
Pesticide Residues on Food
•  Goal is understanding of amou...
Center for Risk Science and Public Health
Theoretical Maximum Residue
Contribution
•  Assume every acre of a crop has pest...
Center for Risk Science and Public Health
Farm Gate Data
•  Derived from field trials of pesticide use
•  Measure level of...
Center for Risk Science and Public Health
Residue Monitoring
•  Based on measurements of pesticide
residues on food as pur...
Center for Risk Science and Public Health
Does The Choice Matter?
Example: Chlorothalonil (Bravo®) on Celery
chlorothaloni...
Center for Risk Science and Public Health
Quantitative Approaches
•  Point Estimates (for either individuals or population...
Center for Risk Science and Public Health
Point Estimation
•  Example: Inhalation of fine particulate matter (PM) in
ambie...
Center for Risk Science and Public Health
Interval Estimation
•  Same Example:
•  Ambient concentration: 50 µg/m3
•  Inhal...
Center for Risk Science and Public Health
Uncertainty & Variability
•  Variability
•  “…represents heterogeneity or divers...
Center for Risk Science and Public Health
Distribution of Body Weight
Center for Risk Science and Public Health
Distribution of Inhalation Rate
Center for Risk Science and Public Health
Distributional Approach
•  Same Example:
•  Treat BW, InhR as variable
•  BW ~ L...
Center for Risk Science and Public Health
Distributional Result (n=1000)
Median = 7.8
Mean = 10.0
95%ile = 26.2
Center for Risk Science and Public Health
Standardized Exposure
Factors
•  Examine a regulatory risk assessment and you wi...
Center for Risk Science and Public Health
Final Thoughts
•  To be useful, an estimate of exposure or any
other element of ...
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Exposure Assessment, George Gray

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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/

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Exposure Assessment, George Gray

  1. 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. 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. 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. 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. 5. Center for Risk Science and Public Health Exposed Populations •  Individuals •  Populations •  Sensitive Subpopulations
  6. 6. Center for Risk Science and Public Health Exposure Assessment
  7. 7. Center for Risk Science and Public Health Exposure Media and Routes Air Soil Water Diet Ingestion Inhalation Dermal Contact
  8. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 22. Center for Risk Science and Public Health Distribution of Body Weight
  23. 23. Center for Risk Science and Public Health Distribution of Inhalation Rate
  24. 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. 25. Center for Risk Science and Public Health Distributional Result (n=1000) Median = 7.8 Mean = 10.0 95%ile = 26.2
  26. 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. 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.

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