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What Is OPPT's Approach to Exposure Assessment?

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. The steps in the tiered approach are as follows:

Step 1. Gather Basic Data and Information for a Complete and Transparent Exposure Assessment.

Step 2. Develop a Screening Level Exposure Assessment.

Step 3. If needed, Develop an Advanced Exposure Assessment.

These steps are explained in more detail below.

Gather Basic Data and Information for a Complete and Transparent Exposure Assessment

Manufacturing/Processing/Use: The first step in assessing exposure for a chemical is to identify all of the manufacturing, processing and use activities for the chemical. This would include identifying all industrial, commercial and consumer uses.

Gather Measured Data.
Monitoring or measured data may be available in a variety of resources, such as company records or databases, national databases, studies published in the open literature, references and other resources (e.g., for physical/chemical properties, fate, exposure factors, etc.) When obtaining measured or monitoring data, it is important to obtain all of the needed supporting information. Information on data quality objectives, the sampling plan, use of quality assurance samples, measurement of background levels, establishment and use of quality assurance and quality control measures, and selection and validation of analytical methods are important considerations when evaluating monitoring data or determining a strategy to collect additional monitoring data. The EPA Guidelines for Exposure Assessment includes additional information on these important considerations.

Estimates of Environmental Releases: Environmental release estimates are critical inputs for models that calculate indirect human exposures via the environment such as through ambient air or drinking water. They are also critical to modeling exposures to nonhuman aquatic and terrestrial species. Release estimates may be site-specific or they may be generic for a particular industrial process or industrial use. Releases from consumer and commercial products should also be estimated if applicable.

Potentially Exposed Human Populations: All potentially exposed populations should be identified. The exposed populations should be associated with the activity, task or source of environmental releases that leads to the exposure. Highly exposed or highly susceptible populations should be addressed whenever possible. Include all routes of exposure.

Chemical Properties and Fate: Reliable, 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 the environment.

Mitigation of Exposures: Process and engineering controls which are used to control exposures should be identified. Personal protective equipment (PPE) that will mitigate occupational exposures should be noted and quantitative estimates of exposure with and without the use of PPE should be provided.

Documentation of basic data and information: Document all measured data, environmental release scenarios, exposure scenarios, assumptions and estimation techniques.

Screening Level Exposure Assessment

Purpose of a screening level exposure assessment: Screening level exposure assessments should be used to quickly prioritize exposures for further work.

Approach: A screening level exposure assessment will generate a quantitative conservative estimate of exposure. The screening approach generally involves using readily available measured data, existing release and exposure estimates and other exposure related information. Where conservative estimates of exposure are not available, simple models, which often use generic scenarios and assumptions, may be used to fill in gaps. For example, a screening-level model for ambient air exposure that is using generic assumptions may assume that the exposed populations live near the chemical release locations.

The exposure assessment should include a characterization of the exposure estimates. Guidance for characterizing exposure in EPA exposure assessments can be found in EPA's 1995 "Guidance for Risk Characterization."

Advanced Exposure Assessment

Purpose of an advanced exposure assessment: An advanced assessment will develop more accurate estimates of exposure and will generally focus on the higher priority exposures identified in screening activities.

Approach: An advanced exposure assessment should quantify central tendency (e.g. median, arithmetic mean) and high end (i.e. greater than 90th percentile) exposures. A representative, well designed monitoring study of known quality is the ideal. Information on data quality objectives, the sampling plan, use of quality assurance samples, measurement of background levels, establishment and use of quality assurance and quality control measures, and selection and validation of analytical methods are important considerations when evaluating monitoring data or determining a strategy to collect additional monitoring data. The EPA Guidelines for Exposure Assessment includes additional information on these important considerations. Higher tier exposure models may also be used in advanced assessments. When they are used, every effort should be made to obtain accurate input data. For example, a higher tier model for ambient air exposure may use facility-specific parameters for emission rates, plant parameters such as stack height and exact location of the exposed populations.

The exposure assessment should include a characterization of the exposure estimates. Guidance for characterizing exposure in EPA exposure assessments can be found in EPA's 1995 "Guidance for Risk Characterization.".
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General Notes: The approach described above is tailored to single chemical exposure assessments, although the general process could also be used for other types of hazards (e.g., biological hazards). Sometimes the focus of an exposure assessment will not be an assessment of human and ecological exposures to a single chemical across manufacturing, processing and uses. If the goal of the assessment is to identify safer substitutes for a particular use, the exposure assessment focus will be on all chemicals within that use (e.g., solvents used in a consumer product). In this case the basic data and information collected at the start of the assessment would need to be modified accordingly.

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. OPPT encourages the appropriate use of our screening and higher tier models.

 


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