Michael H. Dong

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Human Exposure Assessment II (8th of 10 Lectures
onToxicologic Epidemiology)

• Michael H. Dong
• MPH, DrPA, PhD

readings
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Taken in the early 90s, when desktop computers
were still a luxury.
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• Learning Objectives
• Learn the basic algorithm for calculation of
  indirect exposure estimates.
• Learn about the computer models and guidance
  documents developed for calculating these
  estimates.
• Appreciate other critical variables not included
  in the algorithm.

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• Performance Objectives
• Able to outline the route-specific basic
  algorithms for calculating indirect exposure
  estimates, and to identify the critical
  variables.
• To give a description on the use of the exposure
  assessment models.
• To highlight the key elements in regulatory
  guidance documents for this type of calculations.

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Simplified Complexity of Human Exposure
Assessment (click for larger image)
Air
Inhalation
Water
Dietary
Food
Ingestion
Soil
Dermal
Surface
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Basic Algorithm for Calculation of Indirect
Exposure Estimates human exposure
environmental concentration x human
contact
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• Practical Approach to Exposure Calculation
• In practice, applied dose is the measurement of
  interest.
• The acquisition or contact rate for applied dose
  varies in form among the three major exposure
  routes (oral, inhalation, and dermal).

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• Intake Portions of Applied Dose
• Applied doses from various routes should be
  summed to estimate the internal aggregate dose.
• Route-specific absorption factors are required to
  account for the intake portions of the applied
  doses.

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Case 1. Child Exposure at the Playground
Inhalation
Dietary


Ingestion
Dermal
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Case 1. Child Exposure at the Playground
Inhalation
Dietary


Ingestion
Dermal
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Case 1. Child Exposure at the Playground
Inhalation
Dietary


Ingestion
Dermal
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Case 2. Swimmer Exposure at the Pool
Inhalation
Dietary


Ingestion
Dermal
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Case 3. Harvester Exposure at the Field
Inhalation
Dietary



Ingestion
Dermal
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Case 4. Applicator Exposure at the Field
Inhalation
Dietary





Ingestion
Dermal
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• Critical Variables Not Included in Algorithm
• Number of days worked in a season clothing
  penetration body surface areas intake of
  drinking water soil ingestion rate, pattern of
  population and occupational mobility, etc.
• U.S. EPAs Exposure Factors Handbook may be used
  as reference.

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• Issues with Calculation of Absorbed Dose
• Dermal absorption is an important variable, as
  skin can be the primary route of exposure.
• Not all of the dose absorbed orally necessarily
  becomes biologically available to induce systemic
  effects.
• Inhalation uptake and intake may each be less
  than 100, or even 50.

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• Aggregate vs. Cumulative Exposure
• The U.S. Food Quality Protection Act of 1996
  mandates the consideration of aggregate and
  cumulative exposure.
• Despite greater attention now given to cumulative
  exposure, it is less compre-hensible than
  aggregate exposure.
• Aggregate exposure is more conceivable since a
  person can receive exposure to the chemical via
  different routes. . . .

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• Two Major Techniques of Exposure Estimation
• Point-estimation is the conventional method in
  which high-end point estimate values are used for
  most parameters in a calculation.
• The probabilistic analysis is a more realistic
  alternative wherein probabilistic distributions
  for the various key factors are used.

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• Residential Exposure
• Residential exposure, especially that of
  children, is as important to regulatory agencies
  as occupational exposure is.
• U.S. EPA has drafted a set of standard operating
  procedures for residential exposure assessments.
• U.S. EPA has also proposed exposure data
  requirements for assessing risks from pesticide
  exposure of children.
• Children are not little adults in some cases,
  their exposures are much higher.

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Complexity of Dietary Exposure Assessment Basic
Algorithm for Daily Intake I S (Uf) x (Rf),
where I total intake of contaminant, Uf daily
consumption rate of food type f, and Rf residue
level in food type f.


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• Air Dispersion Models for Inhalation Exposure
• Inhalation is an important route for exposure to
  air pollutants.
• U.S. EPA has provided a website for download of
  60 air dispersion models.
• Some regulatory models can be useful for
  estimating buffer zones.
• Also gives models for inhalation exposure to
  chemicals in consumer products.

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• Computer Models for Exposure Assessment
• Regulatory agencies and others have developed
  many computer programs and databases for
  occupational exposure, dietary and residential
  exposures, and inhalation exposure.
• Cal/EPA also developed CalTOX? to facilitate the
  numerous complex calculations involved in
  assessing exposures to hazardous wastes.

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• Todays Human Exposure Assessment
• HEA is a science as well as an art.
• Exposure assessors should be well trained in
  toxicology epidemiology human behavior
  environmental chemistry computer modeling, etc.
• HEA is more than this lecturers limited
  experience it has received global interest and
  concern.

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• Overview of Next Lecture Characterization of
  Health Risk
• Health risk assessment is basically the process
  of comparing an estimated exposure to a level
  pre-established as safe.
• Will discuss extensively many of the risk
  characterization schemes commonly used, and the
  uncertainty factors that are often considered in
  risk characterization.