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Chapter 2
Exposure Assessment Basics

Before there can be a risk assessment, there must be exposure assessment. Assessing how much of a substance people are exposed to is often difficult and based partially on assumptions. The reporter who knows what assumptions have been made can ascertain how much confidence to place in an exposure estimate.

Exposure Assessment Overview

An exposure assessment evaluates how much of a substance people come into contact with, how often, and for how long a period.

Exposure assessment is a major component of risk assessment. *

An exposure assessment evaluates how much of a substance an individual or population ingests, inhales, or contacts through the skin over a period of time.

Exposure may be long-term or short-term and occupational or environmental. Exposure is most frequently assessed by environmental exposure studies. These studies:

estimate how much of a substance is/was present in the environment and how much of it people actually come/came into contact with.

are usually conducted to assess long-term exposures. ** Exposure can also be assessed by per-sonal exposure studies. These studies analyze bodily fluids or tissues to calculate how much of a substance people are exposed to.

* Other components are toxicity assessment ( Chapter 3 ) and problem identification and risk characterization ( Chapter 1 ).

** Ideally, scientists would like to know the amount of chemical that gets to the site in the body where toxicity occurs. Since this is not possible in most cases, the amount that the individual is in contact with is used as the measure of exposure. Exposure can lead to either local effects (burns, rashes, etc.) as a result of direct contact, or to systemic (whole body) effects when it is absorbed into the bloodstream.

Exposure Classifications

Exposures are classified as long-term or short-term and as occupational or environmental.

  • Occupational exposures are generally easier to measure, because they occur in a confined space during known lengths of time.
  • Environmental exposures involve greater uncertainty. The individual may be exposed to a chemical in a variety of locations-home, traffic, shopping-and for varying amounts of time. The great mobility of people in modern society adds to the complexity of this determination.

Examples of exposures:

  • Long-term, occupational exposure: A factory worker inhales a chemical eight hours a day, five days a week, over several years.
  • Short-term, occupational exposure: A plant explosion creates fumes that workers inhale for a few minutes.
  • Long-term, environmental exposure: People in a community drink water from wells contaminated by seepage from an industrialdisposal site.

    Short-term environmental exposure:

    • A child eats 10 aspirin in 5 minutes.
    • People in several city blocks breathe fumes for a few hours after a train tank car spills a hazardous substance.

Measuring Exposure

Exposures are estimated in two ways--directly by measuring body fluids or tissues or indirectly by analyzing environmental levels of contaminants.

Exposure can be estimated in two ways.

  • Personal exposure studies measure the amounts of a substance in the body. This method is usually appropriate after a short-term exposure, when the full amount of the substance taken in may still be in the body.
  • Environmental exposure studies measure amounts of a substance in the environment, determine the route of exposure (inhalation, ingestion, or skin contact), and estimate how much is in contact with the population. This method is usually appropriate for measuring long-term exposures to substances that the human body breaks down and excretes. However, variability over time in the concentration of the substance in the environment may lead to uncertainty in this type of study.

Examples of exposure studies:

  • Personal exposure study: If someone is suffering from the symptoms associated with lead poisoning, the blood can be tested to determine if lead is present and at what level.
  • Environmental exposure study: If an incinerator emits a chemical of concern, calculations can be performed to estimate the amount inhaled by an individual at a particular distance and direction from the incinerator.

Personal Exposure Studies

Personal exposures are usually measured by analyzing bodily fluids or tissues.

Personal exposure is usually measured by analyzing bodily tissues or fluids. Such measures may detect:

Presence of a substance.

  • Recent exposure may be detected through blood or urine tests.
  • Past exposure may be detectedthrough analysis of tissues suchas fat and bone. For example,some organic chemicals, whichare stored in fat, can be detectedthis way.

Bodily changes that indicate a substance is or was present.

  • Some chemicals leave the bodyquickly, but cause physiologicalchanges. For example, certain pesticides change the level of an enzyme in the blood. By measur-ing the magnitude of change,scientists can estimate how much pesticide the person wasexposed to.

Environmental Exposure Studies

In environmental exposure studies, scientists measure the amount of substance present, then estimate how much of it people are in contact with.

When conducting environmental exposure studies, scientists:

  • measure or estimate the amount of the substance present in the environment-air, soil, water, food;
  • then estimate how much of the substance people are exposed to-ingested, inhaled, or in skin contact with-using available data, models, and assumptions.

Both of the above steps include uncertainties because of incomplete knowledge about the properties of chemical substances, their behavior in the environment, how these substances and humans interact, and the variability in personal lifestyles.

AMOUNT OF SUBSTANCE IN ENVIRONMENT

It is most difficult to assess exposures from environmental contaminants when they are from distant or multiple sources or are present over a long time.

Measuring the amount of a substance in the environment is usually straightforward for short-term exposures. Estimating long-term past exposures is usually more complex.

Long-term estimates are difficult when:

  • all the sources are hard to identify.
  • the exact emissions of all sources over time may not be known.
  • movement of the substance is difficult to assess. For example, wind direction, rainfall, or groundwater seepage may be difficult to measure over time.
  • the sources vary over time. For example, if the substance is in a food item, the amount may vary according to maturity, season, or other factors.

Examples of measuring long-term exposures:

  • Substances in drinking water can be measured, and if past measurements are available, scientists will know the amount present over time.
  • If measurements were not made previously, scientists must estimate the amount that was present. Commonly they determine the source of the substance, and estimate how much was emitted and for how long. Then they use mathematical models to calculate how much entered the medium (drinking water, air, soil, food) that people were exposed to.
  • There may be many sources. For mercury in fish, there may be both local and distant sources, such as different industries. Scientists identify the sources and use mathematical models to calculate transport and estimate the amount present in fish over time.

MODELS

Exposure models are based on chemical properties.

The mathematical models used to calculate the movement of chemicals are based on the properties of the chemical in question. These properties include:

  • vapor pressure-how easily it evaporates.
  • solubility-how easily it dissolves in different mediums, such as water or animal fat.
  • adsorption-how strongly it attaches to soil.
  • persistence-how rapidly it is broken down in the environment.

Examples of chemical properties:

  • Benzene evaporates readily and breaks down quickly in the air. However, it is soluble in water and does not adsorb to soil readily. Thus, if it leaks or is poured onto the ground, it is likely to be found in groundwater, where it may reach humans through the water supply.
  • Toxaphene evaporates slowly, but once in the air it is very persistent and is also persistent in animals. Thus, it can be deposited in distant lakes and bioaccumulated through the aquatic food chain, reaching people through the fish.
  • PCBs dissolve only slightly in water, but a much larger amount will dissolve in the fat of living things. Thus, eating fish containing PCBs would expose someone to more PCBs than drinking the water the fish came from.

    AMOUNT OF EXPOSURE

    Scientists have to characterize people's behavior to estimate exposure.

    After the amount of the substance in the environment is assessed, the route of exposure must be determined-inhalation, ingestion, or skin contact-and the amount that people take in is then estimated.

    Incomplete knowledge of human behavior requires that assumptions must be made to estimate how much of the chemical is taken in. Scientists must make assumptions about such things as:

    • how much water or a specific food do people drink or eat each day.
    • whether people filter their water/how they prepare their food.
    • how much time people spend indoors/outdoors.
    • whether behaviors vary with age, socio-economic class, or ethnic group.

    Example of the effect of different behaviors: Subsistence fishers, such as Native Americans of the Great Lakes region and some urban poor, have a higher proportion of fish in their diet than sport fishers or restaurant and fish market customers. As the decline of contaminants in fish continues, the level of safety for subsistence fishers may be achieved later than for other fish consumers.

    Statements of Exposure

    The exposure should be stated as a range of possible exposures.

    An exposure assessment is stated in terms of the likelihood that people are exposed to a given level of a substance over a specified period of time.

    Uncertainty will be intrinsic in the assessment because of the assumptions that were made. Thus the exposure assessment should be reported as a range of exposures.

    In order to protect especially sensitive groups, those responsible for protecting health may base their decisions and statements on the highest feasible exposure in the range.

    This highest exposure value may be quite different from the value that scien-tists believe to be the best estimate of exposure to the hypothetical "average" person.

    Example of an exposure assessment: Groundwater that is used for drinking water is found to have nitrate at 10 ppm (10 mg/liter). A person drinking 2 liters of the water each day will have an exposure of 20 mg/day from this source. Nitrates are also found in food, and the average person consumes about 75 mg of nitrate each day from this source. Thus, if drinking water and food are the only sources of nitrate, the total exposure for this individual would be 95 mg/day. However, daily water consumption varies and the water may come from a variety of sources. In addition, an individual may eat foods that are higher or lower in nitrate than the average. Thus, the total exposure is better described as a range from 50 mg/day to 250 mg/day.

    Role of Exposure Assessment in Risk Assessment

    Once an exposure assessment estimates the level of exposure, scientists can apply the results of toxicity assessments to estimate the degree of harm to the exposed population.

    In the end, an exposure assessment provides information on how much of a substance a population has been or will be exposed to.

    An exposure assessment enables the results of toxicity assessments to be applied to the real world. That is, once the exposure assessment has estimated the amount of a substance the population of interest has actually been exposed to, then the results of the toxicity assessments can be used to estimate the degree of harm to that population.

    This task is conducted by the risk assessor. The risk assessor is most often an individual trained in toxicology, the study of toxic substances. However, risk assessments may be done by scientists with other skills or by teams of scientists-some expert in environmental distribution and fate and others in toxicology.

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