Exposure Units

Introduction

Brother and sister playing on the playground

Defining exposure units is a step in the public health evaluation process. This primer will provide general information on how to identify exposure units and compile data for them.

Health assessors identify exposure units by determining a geographically defined point or area where a person is expected to contact an environmental medium, such as soil, surface water, and air. Within this area, the concentration of a contaminant in the environmental medium is used to characterize short-term and long-term exposure. Depending upon different factors, such as number of samples and non-detects, the media concentration used to estimate exposure can be an average, a 95th upper confidence limit of the mean, or a maximum.

Health assessors define exposure units for each potential contaminant of concern identified in a completed or potential exposure pathway. Health assessors also compile data for each contaminant in each completed and potential exposure pathway (by exposure unit, if appropriate). In a completed exposure pathway, all the five exposure pathway elements (contaminant source, environmental fate and transport, exposure point, exposure route, and potentially exposed population) are present. In contrast, in a potential exposure pathway one or more of the five exposure pathway elements are missing.

A complete exposure pathway means that exposure occurred in the past, is occurring presently, or could occur in the future.

A potential exposure pathway indicates that exposure to a contaminant could have occurred in the past, could be occurring currently, or could occur in the future.

Let’s look at the public health evaluation process, and see where the topic of this primer fits in. As you will see, the parts of the process that are discussed in this primer are bolded in the following list.

Public health evaluation process highlighting the exposure units steps

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Identify completed, potential and eliminated exposure pathways
Define exposure units to evaluate, as appropriate
Compile data for each contaminant in each completed and potential exposure pathway (by exposure unit, if appropriate)
Perform a screening analysis to identify potential contaminants of concern requiring further evaluation
Estimate exposure point concentration (EPC) for each potential contaminant in each completed and potential exposure pathway
Calculate doses using the EPC or adjust air concentrations by duration
Calculate hazard quotients (HQs) for noncancer hazards and see if >1
Calculate cancer risks (CRs) for carcinogens and see if >1.0E-6
Perform tox evaluation for contaminants with HQ >1 or CR >1.0E-6, no health guidelines, no cancer risk values for known or suspected carcinogens, community concern, or other factors that warrant it
Evaluate health outcome data, if available and appropriate for site
Summarize health effects evaluation findings, including discussing uncertainty

When to Define Exposure Units

While the public health assessment process has many steps, ATSDR considers four primary technical components involving evaluations of exposure pathways: screening analysis, exposure point concentrations (EPCs), and in-depth toxicological effects.

The sequence of steps laid out in the previous figure may differ slightly based on site-specific factors. For instance, based on site-specific information, health assessors may define an exposure unit before or after the screening analysis. With large data sets, for example, the health assessor may decide to complete the media-specific screening on the entire data set to identify potential contaminants of concern at the site, and then define appropriate exposure units for further analysis. On the other hand, for smaller or more defined data sets, the health assessor may determine it is most appropriate to define the exposure units prior to conducting the screening analysis.

In summary, health assessors will use professional judgment to determine when to define exposure units depending on the nature of the data set; however, they must define them before determining exposure point concentrations (EPCs). The next primer will explain how to estimate EPCs.