Michael H. Dong MPH, DrPA, PhD
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Transcript of Michael H. Dong MPH, DrPA, PhD
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Michael H. DongMPH, DrPA, PhD
readings
Human Exposure Assessment II(8th of 10 Lectures on
Toxicologic Epidemiology)
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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
Water
Food
Soil
Surface
Dietary
Ingestion
Dermal
Inhalation
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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
IngestionDermal
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Case 1. Child Exposure at the Playground
Inhalation Dietary
IngestionDermal
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Case 1. Child Exposure at the Playground
Inhalation Dietary
IngestionDermal
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Case 2. Swimmer Exposure at the Pool
Inhalation Dietary
IngestionDermal
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Case 3. Harvester Exposure at the Field
Inhalation Dietary
IngestionDermal
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Case 4. Applicator Exposure at the Field
Inhalation Dietary
IngestionDermal
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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. EPA’s 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 = [(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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Today’s 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 lecturer’s 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.