Defining Exposure Units

Overview

Mother, father and son sit in a wooden boat with oars and feed the fish with bread.

ATSDR has developed specific guidance that teaches health assessors how to identify exposure units for a completed or potential exposure pathway. The guidance also teaches how to compile data for your specific scenarios.

When it comes time to do your public health evaluation, refer to the ATSDR’s Guidance on Identifying Exposure Units for the Public Health Assessment Process. The Associate Director for Science (ADS) group is also a good source of information if you have questions about exposure units.

ATSDR’s Process for Defining and Evaluating Exposure Units

ATSDR developed the following process to define and evaluate exposure units. This flowchart shows how exposure units are defined within the PHA.

Process for Evaluating Exposure Units Chart

This decision tree starts with a text box that states, ”Based on the exposure pathway analysis, do potential or completed pathways exist?” If the answer is no, the tree goes to a text box that states, ”Exposure units do not need to be defined.” If the answer is yes, the tree goes to a text box that states, “Define exposure units per guidance.” This box leads to another text box that asks, “Does the exposure unit have adequate data to evaluate exposure?” If the answer is no, the tree goes to a text box that states, “Identify data gaps and recommend sampling or modeling. Include specifics based on the exposure unit and media in reports.” If the answer is yes, the tree goes to a text box that states, ”Is the maximum concentration of the contaminant in the exposure unit greater than the applicable ATSDR CVs or does the contaminant meet other criteria for inclusion (e.g., a community concern)?” If the answer is no, the tree goes to a decision text box that states, “Exposure in the exposure unit is not a concern.” If the answer is yes, the tree goes to a decision text box that states, “Use the appropriate EPC guidance to calculate EPCs for the exposure pathways of interest for each identified contaminant of concern in the exposure unit. Then calculate doses and continue the health evaluation.”

Factors to consider

You should consider several factors when defining exposure units:

Exposure units can vary greatly in size.
Sites may have multiple exposure units.
Exposure units are defined by people’s activities and not by available environmental data.
An exposure unit might never have been sampled. For example, at sites with widespread groundwater contamination, separate exposure units may need to be identified for different private wells that draw from the contamination plume – including private wells that have never been sampled).
Exposure units can vary by conditions of exposure.
Exposure units may inform environmental sampling.
Exposure units may not be defined at all sites, depending on the nature of the data.

In addition, consider the following information when defining exposure units:

Conceptual site model and exposure pathways analysis
Observations made and pictures taken during site visits
Maps and aerial photographs
Discussions with residents, community groups, and tribal leaders
Institutional controls (such as fishing advisories and hunting regulations)
Physical barriers (such as fences or locked gates)
Discussions with officials from other agencies (like EPA and state, tribal, and local health agencies)

Now, let’s go through some examples to practice defining exposure units.

Examples on how to Define Exposure Units

Example 1

Five homes were built along an area that passes over a groundwater plume. The homes have unfinished basements. Indoor air samples were collected at the homes. The results suggest the potential for intrusion of volatile groundwater contaminants into the homes’ indoor air. Accordingly, the health assessor identified vapor intrusion as a completed exposure pathway that required further evaluation.

Graphic showing the location of five homes. The homes were built along an area that passes over a groundwater plume.

Question: How should you, as the health assessor, define the exposure units shown here?

Answer: You should designate the indoor air in each home as its own exposure unit. Do NOT combine them to make a single exposure unit. The homes should be evaluated separately because

exposure units are defined by locations where people spend their time and not on contamination levels, and
each home may have unique assumptions that need to be factored into the exposure dose calculations.

Example 2

An elementary school has a large field that is completely open for children to play before, during, and after school. The field contains four areas where contaminated fill was previously disposed. Children are known to frequent all parts of the field.

Question: How should you define the exposure units to evaluate?

Answer: You should define one exposure unit for the entire field of the elementary school. Do NOT define different areas within the field as separate exposure units. This decision is based on our knowledge that children play in all parts of the school field, rather than in just certain areas within the field.

However, if you know that children predominantly play in one part of the field, you can treat that portion of the field as a separate exposure unit.

Example 3

Lead and arsenic were emitted from the smokestacks of a former smelter. The contaminated particles were transported via air and settled across the yards of 20 properties in a residential neighborhood.

Question: How should you define the exposure units to evaluate?

Answer: You should define each residential yard as an individual exposure unit. Do NOT define all homes in the neighborhood as a single exposure unit. This decision is based on the knowledge that residents spend time on their own properties.

Now, let’s go to the next “Practice” section to apply what has been discussed about exposure units.

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