Michael H. DongMPH, DrPA, PhD

readings

Human Exposure Assessment II(8th of 10 Lectures on

Toxicologic Epidemiology)

Taken in the early ’90s, when desktop computers were still a luxury.

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.

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.

Simplified Complexity of Human Exposure Assessment (click for larger image)

Air

Water

Food

Soil

Surface

Dietary

Ingestion

Dermal

Inhalation

Basic Algorithm for Calculation of Indirect

Exposure Estimates[human exposure] =

[environmental concentration]

x [human contact]

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).

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.

Case 1. Child Exposure at the Playground

Inhalation Dietary

IngestionDermal

Case 1. Child Exposure at the Playground

Inhalation Dietary

IngestionDermal

Case 1. Child Exposure at the Playground

Inhalation Dietary

IngestionDermal

Case 2. Swimmer Exposure at the Pool

Inhalation Dietary

IngestionDermal

Case 3. Harvester Exposure at the Field

Inhalation Dietary

IngestionDermal

Case 4. Applicator Exposure at the Field

Inhalation Dietary

IngestionDermal

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.

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%.

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. . . .

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.

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.

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.

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.

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.

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.

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.