What is An Exposure Assessment ?
An exposure assessment attempts to answer the following questions
for a particular substance or chemical:
. Who or what is exposed (e.g.,people, aquatic ecosystems) ?
. Does the exposure occur through breathing air, drinking water, skin contact or any other routes?
. How much exposure occurs?
. How often and for how long does exposure occur, that is, what is its frequency and duration ?
Exposure occurs through contact with a chemical. Such contact can occur by inhaling air, drinking water, eating food, or touching a variety of products that contain the chemical. The concentration of the chemical and the extent of the contact are important components of exposure assessment. The results of an exposure assessment are often considered with a hazard assessment of the chemical. A hazard assessment provides an understanding of the potential for the chemical to cause adverse effects to humans and plant and animal life. Together, the exposure assessment and the hazard assessment can be combined into a risk assessment, which reaches conclusions about the likelihood of adverse effects in the exposed population.
Exposure assessments in OPPT typically include occupational exposures in the workplace, exposures to the general population from chemicals in the air and drinking water, consumer exposure through the household use of products, and environmental exposure to aquatic life. Representative measured exposure data obtained under realistic conditions are generally more accurate than modeled estimates. However, models can provide useful exposure estimates. OPPT's models and tools are intended to be used by scientists and engineers familiar with exposure assessment principles.
OPPT uses a tiered approach to exposure assessment: Exposure assessments may use measured data or model estimates. Representative measured data of known quality are preferred over model estimates and are needed to validate and improve models. The EPA Guidelines for Exposure Assessment includes guidance on collecting and using monitoring data for exposure assessments. One of the goals in selecting the approach should include developing an estimate having an acceptable amount of uncertainty. In general, estimates based on quality-assured measurement data, gathered to directly answer the questions of the assessment, are likely to have less uncertainty than estimates based on indirect information (e.g., modeling or estimation approaches). For risk assessment purposes, a quantitative exposure assessment approach is needed and exposure information must be clearly linked to the hazard identification and dose-response relationship.
. Screening-level assessments that allow one to quickly prioritize exposures for further work; these assessments are based primarily on readily available monitoring data and other data, conservative assumptions and simple models.
. Advanced assessments which focus on higher priority exposures that attempt to represent actual environmental conditions and exposures; these assessments require more data and make use of more sophisticated models or ideally, a well-designed monitoring study.
Chemical Properties and Fate: Representative measured values
are preferred, and should be used when available. Measured values
or estimates of water solubility and vapor pressure are important
in evaluating whether a chemical will dissolve in water or exist
as a vapor at ambient temperature, and are used to estimate worker
and consumer exposures. Measured data or estimates of biodegradation,
sorption, and volatilization potential are used to predict removal
in wastewater treatment. Information on decay rates in the atmosphere,
surface water, soil, and ground water are important in evaluating
how long it takes a chemical to break down in the environment,
and are used to estimate exposures to the general population and
Estimating Concentrations in the Environment Using Models:
Before environmental concentrations can be predicted, releases
to the environment need to be estimated. Chemicals can be released
to air, water, or landfill. Release estimates are generated using
industrial data, engineering expertise and information on the
production process. Manufacturing and processing operations are
reviewed to determine potential releases in the work place (e.g.,
vapors from processing equipment, etc.,) that could result in
worker exposure and releases to the environment. Releases from
consumer products should also be considered.
A number of databases and tools allow the user to gather information about the environment into which the chemical is discharged, and to estimate chemical concentrations in air and water. These tools range from mathematical equations to predict simple dilution in a room to complex computer models which estimate the path of the chemical through the environment over time. Some of these computer models can account for chemical decay in the environment and estimate overlapping concentrations from multiple chemical releases.
Assessing Exposures: The last step in an exposure assessment
involves estimating the level of contact of the exposed population
with the chemical. For people who live near a discharge location,
the level of contact involves two factors: the location of nearby
populations and the daily human activities that influence how
often people come in contact with the chemical. For consumer product
exposures and worker exposures, the frequency of use, duration
of exposure and use conditions are important factors.
Assessing Exposures Using Monitoring Data: The most accurate way to obtain environmental concentrations and human exposures is usually to conduct a well-designed exposure monitoring study. Elements of a well-designed exposure monitoring study include: establishing quality assurance objectives that will allow exposure assessors to make estimates of average and high end exposures with a known level of reliability; where possible, using sampling and analytical chemistry methods that have been found acceptable by an independent authoritative body (e.g. ASTM, NIOSH, etc); and ensuring that quality control procedures have been employed and documented.
Assessing Exposures Using Models: The screening level tools often make simplifying assumptions which are protective by design (for example, assuming that people live near chemical discharge locations). Higher tier tools are more complex and allow for more realistic exposure assessments, such as using census data and a measure of the distance between the location of the chemical release and the populations living nearby. Daily activities include the amount of time people spend at home as well as the amount of air they breathe and the amount of water they drink. For workers, daily activities include the amount of time they spend handling the chemical during the day. The amount of chemical that an individual breathes, comes into contact via the skin, or drinks via water, is the final product of an exposure assessment. Often, a report describing the exposure assessment is prepared. Depending on the complexity of the assessment, the report can be a few pages or it can be quite lengthy.
Generally, the data and information needed to fully assess exposure is not available, and assumptions and default values are used to fill these data gaps. A clear understanding of the predictive model, and of the assumptions and default values used in the absence of data are critical when making decisions to manage potential risks from exposure to chemicals. The exposure assessment models in this website include standard default values which can be changed by the user. Where possible, language which describes the impact of the standard assumptions and default values on the exposure result is included. The default values were carefully selected but often reflect general rather than specific exposure conditions. If the standard assumptions and default values are applied when using the model, the descriptive language should be incorporated into the results. Using specific exposure related data in the models will enable you to tailor the model to the exposure scenario of interest.