Element 3: Exposure Points

This section discusses the third exposure pathway element: the exposure point. It provides guidance on how to identify exposure points by environmental medium, how to define exposure units, and other factors to consider.

The exposure point is the specific location where people may come in contact with site contaminants. Health assessors can use the following to identify exposure points:

Considering different environmental media for exposure points.
Defining exposure units.
Examining other factors, such as conditions that might limit or eliminate contact with contaminated media.

Considering Possible Exposure Points by Medium

Using the resources identified in Getting Familiar with the Site, pinpoint exposure points that could be relevant to a particular site by considering different environmental media (see table below). The exposure points shown in the table represent the most likely exposure scenarios for contact with each specific medium.

Possible Exposure Points by Environmental Medium

Possible Exposure Points by Environmental Medium
Medium	Exposure Point
Groundwater	Source for public water supplies. Wells used for municipal, domestic, industrial, and agricultural purposes. Groundwater used as a water supply source for swimming pools and other recreational water activities. Natural springs used for recreation or municipal, domestic, industrial, and agricultural purposes. Off-gassing of vapor-forming contaminants.
Soil	People can come into contact with contaminated soil in different ways. Direct contact with surface soils (i.e., soil less than 3 inches deep) is the most common exposure pathway, while subsurface soil (i.e., soil deeper than 3 inches) contact can occur under certain scenarios (see subsections below). At some sites, contaminated on-site soils and other waste materials may be used as fill at off-site locations.
Surface Soil	Surface soil at various locations, such as gardens, playgrounds, and campsites. Transport from soiled clothes, dirty shoes, pets, and other means into residences. Ingestion of soil by children exhibiting soil-pica behavior. Dust stirred up from surface soil disturbances in contaminated areas. Transport of contaminants from contaminated soil areas to locations where exposures can occur.
Subsurface Soil	Digging (e.g., gardening), excavating, drilling, and other activities that bring subsurface soils to the surface. Migration of contaminants from subsurface soils through groundwater elevations/variations, vapor intrusion, or other events that can bring contamination to the surface (e.g., wildlife digging, washout events from extreme weather).
Surface Water	Source for public water supplies. Irrigation and public, industrial, and livestock water supplies. It is particularly important to identify the location of water supply intakes downstream of a site and note the outfalls and direction of surface water flow based on topography. Surface water used for recreational activities such as swimming, fishing, and boating. Note that recreational use of surface waters is not limited to parks and public beaches; some residents (particularly children ages 6 to 12) may wade, swim, play, and even fish in stormwater drainages, local streams, and ponds (e.g., private ponds and creeks passing through rural residential settings).
Sediment	Submerged or exposed sediment can serve as an exposure point for swimmers, workers, and others. Sediments exposed when the water in a stream or creek running through residential yards recedes. At some sites, beaches along rivers may be important exposure points, as the sediment on the beach may have originated from upstream locations. Sand bars, overbank flood deposits, and other sandy areas along streams and in drainage ditches, which are often attractive play areas for young children. Sediments excavated and transported to other areas and used as topsoil. Maintenance of ditches, drainage channels, canals, and other watercourses throughout the United States commonly results in sediments being placed in a variety of areas. However, current environmental regulations require handling of highly contaminated sediments as hazardous waste and prohibit their transport to public use areas. Sediments stirred up from the bottom of water wells during extreme drought conditions or high-water usage periods.
Air	Contaminants that are volatile or adsorbed to airborne particulates can occur outdoors or indoors. Ambient air in areas downwind of a site might become contaminated because of volatilization or entrainment of contaminants in dust particles. Vapor intrusion may occur from a groundwater plume downgradient of a release. The air inside buildings near a contaminated site can become contaminated from migrating soil gases. Specifically, buildings on or adjacent to landfills should be evaluated for the presence of flammable (methane) and asphyxiating (carbon dioxide) conditions from migrating landfill gas.
Food Chain	Consumable plants, animals, or other food products that have contacted contaminated soil, sediment, waste materials, groundwater, surface water, or air. Contamination may be on the food surface or absorbed into the plant or fruit during growth. This may include fruits and vegetables grown in home gardens, orchard produce, plants used for medicinal or cosmetic purposes, livestock, game, and other terrestrial or aquatic organisms. In some areas, plants, animals, and fish can constitute a significant portion of the diet of local residents, possibly at the subsistence level.
Other	Contaminated materials at commercial or industrial sites (e.g., raw materials, sludge from treatment processes, waste pilings, radioactive materials) can provide a direct point of contact for on-site workers, visitors, or trespassers.

Defining Exposure Units

An exposure unit is a geographically defined point or area where a person is expected to contact an environmental medium, such as soil, surface water, groundwater, air, or food items (e.g., fruits, vegetables, fish, game). Health assessors define exposure units for each potential contaminant of concern identified in a completed exposure pathway or potential exposure pathway.

Based on the nature of the data set, you can define exposure units during the exposure pathway evaluation and either before or after the screening analysis as long as you do so before estimating EPCs.

ATSDR has developed a process (see diagram below) to help health assessors define exposure units. Ask the Associate Director for Science (ADS) group if you have questions about exposure units.

Refer to ATSDR’s EPC Guidance Documents and the EPC and Exposure Calculations section for more information.

ATSDR’s Process for Defining and Evaluating Exposure Units

Appendix B: Process for Evaluating Exposure Units Chart

For image description, click here.

Examples of Exposure Units

Three children play on a treehouse outside

_{CDC/Dawn Arlotta; photographer: Cade Martin}

Soil
A resident’s yard or playground.

A water pump in a field full of brown dirt

_CDC

Water
A well system that serves a neighborhood or a private well.

ATSDR developed this process to help health assessors define and evaluate exposure units for a completed or potential exposure pathway and compile the necessary data for all environmental media to evaluate the exposure units for specific site scenarios.

ATSDR’s Guidance on Identifying Exposure Units

Refer to ATSDR’s Guidance on Identifying Exposure Units prior to defining exposure units at your site. This guidance describes the various factors that health assessors need to consider when completing this step and provides examples and case studies on how to define exposure units for various pathways.

PHAT Exposure Unit Mini-Module

The PHAT Exposure Unit Mini-Module summarizes ATSDR’s guidance for defining exposure units and provides examples and case exercises for health assessors to practice.

More [PDF – 1,217 KB]

Considering Other Factors

Consider other factors that might limit or eliminate exposure to the contaminated media, such as engineering controls that prevent site accessibility and exposure (e.g., physical controls and barriers like fences, effective treatment systems), as well as institutional controls that prevent exposure (e.g., deed restrictions, zoning rules). However, keep in mind: controls are not always effective or well-maintained. For example, you might see evidence of trespassers at a fenced site or hear accounts of residents catching fish at a site with a fishing advisory. Thus, even if controls are in place, you cannot assume there is no exposure. You must examine these other factors and recommend ways to amend controls in situations possibly resulting in exposures. Consider the usefulness of supporting, reinforcing, or encouraging additional controls.