Exposure Assessment Tools by Media - Soil and Dust
Overview
Contaminated media to which people might be exposed include air, water and sediment, soil and dust, food, aquatic biota, and consumer products. Soil can become contaminated as a result of direct or indirect discharges, deposition of contaminants from the atmosphere, erosion of soil and runoff flow, and other processes.
Sources of contaminants to indoor dust can include use of consumer products or emissions from building materials or furnishings. Outdoor soil and dust that has been tracked or has migrated indoors are other possible contributors to indoor dust. Soil and dust can serve as reservoirs of historic sources of contamination.
Direct exposure to contaminants in soil and dust can occur by intentional ingestion (e.g., PICA or GEOPHAGY), incidental ingestion (e.g., from hand-to-mouth contact), or via dermal contact. Indirect exposure to soil or dust contaminants could also result from transfer and subsequent contact with other media (e.g., uptake from soil into food crops and subsequent ingestion).
Indoor or outdoor dust that is airborne represents a potential inhalation exposure. This potential exposure pathway is discussed in the Air Module of the Media Tool Set in EPA ExpoBox.
The concentrations of contaminants in soil and dust at the point of exposure may differ from the concentration at the source as a result of fate and transport processes (e.g., dispersion, biodegradation). Various tools are available for evaluating sources and releases of contaminants to soil and dust, fate and transport processes, and potential exposure concentrations. Exposure factors, calculation tools, and guidance for assessing exposure to contaminants in soil and dust are also provided.
Information about EPA’s land, waste, and clean-up programs can be found at:
Land, Waste and Cleanup Topics
Links to state hazardous waste programs are available here:
Hazardous Waste Generators
The Department of Interior’s Bureau of Land Management website at:
The Bureau of Land Management provides useful information on land resources.
Sources and Releases
Soil can become contaminated as a result of direct or indirect discharges from industrial, commercial, or residential sites; deposition of contaminants from the atmosphere to soil; erosion of soil and runoff flow; and other processes. Sources of contaminants to indoor dust can include use of consumer products or emissions from building materials or furnishings. Contaminants in outdoor soil could also be tracked indoors, contributing to the level of contaminants in household dust.
Contamination of soil and dust can occur from anthropogenicResulting from human activity. sources or natural sources.
- Anthropogenic sources of soil contamination might include the rupture of underground storage tanks, application of fertilizers and pesticides, oil and fuel dumping, leaching of wastes from landfills, and direct discharge of industrial wastes to the soil.
- Natural sources that could impact soil systems include volcanic activity, hurricanes, forest fires, or earthquakes. These natural disasters can cause dramatic changes to the landscape and have devastating impacts to agriculture and can leave behind significant amounts of contaminated dust and debris.
Fate and Transport
Fate and transport processes “link” the release of contaminants at a source with the resultant environmental concentrations to which receptors can be exposed. When a contaminant is released from a source, it is subject to transportMovement within a medium or between media. and transformationChange in a chemical or physical state. in the environment. Compounds can also transfer from an environmental medium to biota, a process referred to as bioconcentration or bioaccumulation.
| Migration Process | Examples Relevant to Soil and Dust |
|---|---|
| Transport |
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| Transformation |
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| Transfer – Environment to Biota |
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For additional information on the uptake of contaminants from soil to plants and animals used as a source of food, see the Food Module in the Media Tool Set of EPA ExpoBox.
Information on soil properties may be useful for fate and transport modeling of these soil contaminants. Contaminants can be physically or chemically attached to soil particles or can be trapped in the pore spaces between soil particles.
Soil characteristics such as texture (percent sand, silt, and clay), structure (arrangement of the soil particles), pore space, and percent organic matter play a large role in the transport processes of contaminants through soil. For example, chemicals can move quickly through sandy soils because compared with silt and clay they have larger particle sizes, larger pore spaces, and lower organic matter; therefore, contaminants do not attach easily to sandy soils and they drain quickly.
Chemicals can attach more easily to soils with a higher content of clay and organic matter. In addition, sandy soils generally do not contain a large amount of soil organisms compared with other soil types. Soils with more aggregation and smaller pore spaces will slow the movement of water and generally have a higher diversity and population of soil organisms that can metabolize the contaminant.
Soil adherence to skin is also a function of soil type and particle size. Other properties of soil that could affect chemical fate and transport include soil pH, ionic strength, and presence of other pollutants.
Certain physicochemical properties of the pollutant are also important (see box). Physicochemical properties data will help determine whether a chemical is likely to remain in the soil, partition to other media, or transform physically, chemically, or biologically after release. Climatic conditions such as precipitation can also determine how contaminants are physically transported through media (e.g., by contributing to runoff, drainage, leaching).
Tools for assessing exposure to contaminated soil and indoor (settled) dust (via ingestion or dermal contact) are provided in this module.
Indoor settled dust includes particles in building interiors that have settled onto objects, surfaces, floors, and carpets. These particles may include soil particles that have migrated or been tracked in from the outdoors. Settled indoor dust and airborne indoor dust that has been inhaled and subsequently swallowed represent potential ingestion exposures (U.S. EPA, 2011).
Outdoor settled dust includes particles that have deposited (by wet or dry deposition) onto outdoor objects and surfaces. It is not possible to distinguish between soil and outdoor settled dust; outdoor settled dust would generally be present on the soil surface. Therefore, when talking about soil and the soil ingestion pathway, we are including both soil and outdoor settled dust (U.S. EPA, 2011).
Tools for evaluating inhalation exposures to indoor or outdoor dust that is airborne are provided in the Air Module in the Media Tool Set of EPA ExpoBox. It is important to recognize the cycle of deposition and resuspension for some contaminants, in which the inhalation, dermal, and ingestion exposure pathways might all be relevant.
There are a number of sources that provide information that is useful in predicting fate and transport of contaminants in soil and dust.
Special Considerations for House Dust
Pollutants can accumulate in house dust over time and become “trapped” in carpets, curtains, and upholstery. House dust is not as exposed to moisture, sunlight, and the frequent temperature/climate changes that typically aid in the breakdown of chemicals in the outdoor environment. In addition, pollutants may be less mobile in indoor environments where dispersion is limited (Paustenbach et al., 1997).
Data
There are a number of sources that provide data that are useful in predicting environmental fate and transport of contaminants in soil and dust.
Environmental fate and transport information is sometimes obtained from field studies. These studies can help us understand what happens to parent compounds and their breakdown products after they are released to the environment. Data generated can provide quantitative environmental fate characterization to assist in the estimation of exposure to a chemical.
Models
Fate and transport models can be used to estimate concentrations of contaminants at the point-of-contact for a receptor population. Models might also be used to describe the multimedia transport and fate of pollutants from soil to other media. A variety of mathematical methods or models - each with specific data needs - are available or are under development to characterize soil contamination and describe the multimedia transport and fate of pollutants in the environment. There are also several resources that contain information on input parameters necessary for fate and transport models.
Concentrations
Characterizing contaminant concentrations for an exposure scenario is typically accomplished using some combination of the following approaches:
- Sampling soil and dust and measuring contaminant concentrations
- Modeling the concentrations based on source strength, media transport, and chemical transformation processes
- Using existing, available measured concentration data collected for related analysis or compiled in databases
EPA ExpoBox provides information on measuring or modeling soil/dust concentrations and on available monitoring data. Information on sampling techniques and analytical methods is available to support the measurement of contaminants in soil and dust. In the absence of monitoring data, a variety of models can be used to estimate contaminant concentrations in soil and dust. Resources that provide information on the bioavailability of contaminants in soil are also included below.
Measuring Concentrations
A number of sampling techniques or protocols have been established for monitoring and characterizing soil and dust contamination. Analytical methods for measuring concentrations of contaminants in soil or dust may include methods required for specific chemicals or groups of chemicals that are regulated by EPA or methods for unregulated chemicals or groups of chemicals of interest.
Modeling Concentrations
In the absence of monitoring data, models may be used to estimate the concentrations of contaminants in soil and dust, such as the following.
Available Data
There are a number of information sources that provide monitoring data on contaminant concentrations in soil or dust, including the following.
Bioavailability
Bioavailability is a measure of how much of a contaminant is absorbed when people are exposed to that contaminant through inhalation, skin contact or food intake. Resources are available that provide information related to the bioavailability of contaminants in soil.
Exposure Scenarios
Exposure to contaminants in soil or indoor settled dust can be estimated by first defining the exposure scenarioA set of facts, assumptions, and inferences about how exposure takes place that aids the exposure assessor in evaluating or quantifying exposure. of interest. Exposure scenarios typically include information on the sources and pathways of exposure, contaminants of concern, and receptor populations. They might also describe a receptor population’s activities that may affect exposure and the timeframe over which exposure occurs.
Exposure to soil and dust can occur by intentional ingestion (e.g., pica or geophagy), incidental ingestion (e.g., from hand-to-mouth contact), or via dermal contact (see the Air Module in the Media Tool Set for information on evaluating inhalation exposures to indoor or outdoor dust that is airborne).
Dermal contact with soil could occur during outdoor recreational, gardening, or construction-related activities. Receptors could also come into contact with contaminants in soil and dust that have settled on carpets, floors, clothing, counter tops, or other surfaces.
Ingestion of soil and dust are potential routes of exposure for children and adults. However, children are particularly susceptible to these soil and dust exposures because certain behaviors (e.g. tendency to mouth objects or hands) and activities (e.g., playing on the ground outdoors. Crawling or playing on the floor indoors) may increase their contact with soils and dust.
For soil/dust ingestion and dermal contact scenarios, concentrations of the contaminants in soil/dust are needed to estimate exposure or dose (see the module on Lifestages in the Lifestages and Populations Tool Set of EPA ExpoBox for information on assessing exposure to children).
After characterizing the exposed population and identifying exposure concentrations, it is important to define all appropriate exposure factor inputs used to estimate potential exposures and risks. These inputs (intake rates, dermal contact rates, and other relevant patterns of behavior) can be obtained from the Exposure Factors Handbook: 2011 Edition (see Exposure Factors tab in the Indirect Estimation Module of Approaches Tool Set).
The table below provides some additional examples of scenarios involving contaminants in soil and dust. The list of examples is not meant to be exhaustive. There are numerous other ingestion and dermal scenarios that may be constructed based on the specific needs of the assessment.
There are also numerous variations of the examples provided in the table. Additional information on exposure scenarios involving contaminated air may be found in the Indirect Estimation Module in the Approaches Tool Set of EPA ExpoBox.
| Medium/Route | Receptor Population | Relevant Activity Pattern | Intake Rate/Skin Contact | Exposure Period |
|---|---|---|---|---|
| Soil and dust ingestion | Residential children | Residence time (mobility) [Table 16-4] |
Child soil and dust ingestion rate (mg/day) [Table 5-1] |
Acute or subchronic |
| Indoor Dust ingestion | School children | Time spent at school [Table 16-17] |
Child soil and dust ingestion rate (mg/day) [Table 5-1] |
Subchronic |
| Soil and dust ingestion | Adults gardeners | Residence time (mobility) [Table 16-4] Time spent working with soil in garden [Table 16-44] |
Adult soil and dust ingestion rate (mg/day) [Table 5-1] |
Chronic |
| Soil and dust dermal contact | Adults gardeners | Residence time [Table 16-4] Time spent working with soil in garden [Table 16-44] |
Adult skin surface area [Table 7-2] and soil adherence factor [Table 7-4] |
Chronic |
Several resources are available that illustrate soil and dust exposure scenarios.
Exposure Factors
To estimate human exposure to contaminants in soil and dust, exposure factor information is needed. Exposure factors are human behaviors and characteristics that help determine an individual's exposure to an agent. They may include ingestion rates, and the factors that affect dermal exposure such as body surface area, dermal adherence of solids to skin, and dermal transfer factors.
Data on soil and dust ingestion rates are available in Chapter 5 of EPA’s Exposure Factors Handbook: 2011 Edition. Ingestion rates are reported in units of mg/day. Ingestion rates should be selected to represent the appropriate medium (soil, dust, or soil + dust combined); age/lifestage; and intake category (general population, soil pica, or geophagy).
As described in the Handbook, the ‘soil + dust’ medium includes:
- outdoor soil
- containerized indoor soil
- dust that is a combination of outdoor settled dust, indoor settled dust, and airborne particulate matter that is inhaled and subsequently swallowed; and
- outdoor soil that has been tracked indoors
Body surface area, activity-specific soil adherence factors, and skin transfer efficiencies needed for assessing dermal contact with soil and dust are available in Chapter 7 of the Handbook. Chemical-specific factors related to dermal absorption and internal dose, however, are not provided in Chapter 7.
Activity-specific factors that may be relevant for assessing exposures involving dermal contact with soil or dust are available in Chapter 7 and Chapter 16 of the Handbook. These might include time spent indoors, time spent outdoors, or time spent doing specific activities (e.g., playing on dirt or grass, gardening, participating in outdoor recreation).
Other exposure factors that might be needed for assessing ingestion exposures include:
- Body weight (Chapter 8)
- Life expectancy values, specifically when evaluating cancer risk (Chapter 18)
Exposure factor data may be accessed from the Exposure Factors Tab of the Indirect Estimation Module.
Calculation Tools
A variety of tools are available for quantifying ingestion exposures (dose) and risks associated with contaminants in soil and dust. These calculation tools have typically been developed for specific situations or program offices but may be tailored to meet the needs of the user.
Guidance
A number of guidance documents are available to support various components of EPA programs involving soil and dust ingestion.
References
- Paustenbach, DJ; Finley, BL; Long, TF. (1997). The Critical Role of House Dust in Understanding the Hazards Posed by Contaminated Soils . Int J Toxicol 16: 339-362.
- U.S. EPA. (2011). Exposure Factors Handbook 2011 Edition (Final). (EPA/600/R-09/052F). Washington, DC.