Estimating Site-Specific Ingestion and Dermal Exposure Doses

To calculate exposure doses, ATSDR uses pathway-specific equations. For instance, the equation for soil/sediment ingestion is different than for dermal contact with surface water. For exposures via ingestion, ATSDR also breaks out doses by:

• Reasonable Maximum Exposure (RME), which refers to people who are at the high end of the exposure distribution (approximately the 95^th percentile). The RME scenario is intended to assess exposures that are higher than average, but still within a realistic exposure range.
• Central Tendency Exposure (CTE), which refers to individuals who have average or typical exposure to a contaminant.

When possible, use site-specific information to calculate doses. Depending on potential health concerns and site conditions, you can estimate doses for past, current, or future exposures. The ability to accurately estimate past and potential future exposure doses, however, might be limited. Information about past contaminant levels or exposures can be incomplete or unavailable. In some cases, exposures are characterized by using mathematical models, which can be used to estimate exposure concentrations.

At many sites, exposure to a single contaminant can occur through multiple routes of exposure, and exposure to multiple contaminants can also occur. Exposures to multiple contaminants from various pathways can be summed to derive a total exposure dose. More information about approaches to assessing doses to multiple contaminants and exposure pathways is presented in the In-depth Toxicological Effects Analysis section of this e-manual.

ATSDR calculates doses for standard age groups and custom groups, when appropriate. The health assessor should strive to estimate exposure doses by using site-specific or population-specific exposure information. The general dose equation is:

Exposure Dose (D)   =   (C * IR * EF) / BW

The variables in the equation represent the following exposure parameters (also called input parameters):

C         =    Media-specific contaminant concentration (the EPC)

IR        =    Media-specific intake rate

EF       =    Exposure factor

BW     =    Age-specific body weight

Selecting Exposure Parameters

This section defines each of the exposure parameters used in the general exposure dose equation and discusses key factors to consider when selecting appropriate variables. To ensure adequate protection of public health, begin by choosing conservative exposure parameters. As appropriate, refine the analysis by considering more realistic parameters consistent with what is known about site-specific exposures.

At some sites, conditions can result in exposures that differ from the standard default assumptions. Health assessors should use exposure parameters that represent appropriate exposure conditions and choose site-specific values whenever possible. You can obtain site-specific information by talking with community members, reading site reports, and other means. For example, you might learn that a site-related population does not rely exclusively on private wells for drinking water. Using default assumptions might therefore overestimate exposures. To ensure that the most reasonable, yet protective, exposure conditions are considered, select exposure parameters for the dose equation by carefully examining site-specific exposure conditions.

If site-specific values are not available, use default parameters. The main sources for default exposure parameters include PHAST, ATSDR’s Guidance Documents (available on the PHAGM Resources page), and EPA’s Exposure Factors Handbook (EFH). When selecting values from the EFH, choose those that best represent site conditions and would be adequately protective of the surrounding community. The most up-to-date default exposure parameters are found in PHAST.

You do not need to limit dose estimates to a single point estimate. Providing a range of realistic scenarios, including CTE and RME doses, as well as other types of doses when appropriate, shows a wider perspective of health implications and helps community members understand where their exposures might fit into the overall picture.

For example, an exposure to a contaminant in soil might be expected to result in long-term effects to workers regularly exposed to soils. However, for people who aren’t regularly contacting the soils, there might be no expected harmful health effects. PHAST will help you evaluate exposures for these and other types of scenarios. For instance, PHAST will use other types of doses (e.g., not CTE or RME) to evaluate exposures to contaminants for specific populations, such as exposures to soil for gardeners and trespassers.

In addition, EPA and others have developed tools for conducting probabilistic risk assessments that evaluate data distributions instead of point estimates (e.g., Monte Carlo analysis). Health assessors can use these tools in their evaluations to more adequately characterize variability and uncertainty. Be sure to work with experts in these analyses to determine their applicability, use, and interpretation at a particular site.

The exposure parameters included in the general exposure dose equation are defined below. Also refer to this table that includes additional pathway-specific parameters you may need to include depending on your site-specific scenarios.

• Contaminant concentration. The EPC is used to assess potential exposures from contaminants. Steps to calculate the EPC are outlined in ATSDR’s PHAT EPC Mini-Module [PDF – 2.2 MB].

• Exposure group/body weight. ATSDR calculates exposure doses for a standard set of exposure groups with associated default body weights for each exposure pathway-specific dose equation (see table). Health assessors can also evaluate custom exposure groups/body weights, if appropriate. When exposed to the same amount of a contaminant, people with lower body weights will receive a relatively higher dose of the contaminant than people with higher body weights.

Exposure Groups/Body Weights in Exposure Pathway Equations

• Intake rate. The intake rate is the amount of a contaminated medium to which a person is exposed during a specified period of time. Examples include the amount of water, soil, and food ingested on a daily basis; the amount of air inhaled; or the amount of water or soil that a person may contact through dermal exposures. Select intake rates that best characterize the exposed population.

ATSDR has standard, media-specific default CTE intake rates and RME intake rates that align with the standard set of exposure groups. Default CTE intake rates represent average or typical exposure to a contaminant, while the default RME intake rates represent people who have a higher than average exposure (approximately the 95^th percentile), but are still within a realistic range of exposure. Default intake rates are stored in PHAST and in Exposure Dose Guidance documents for individual exposure pathways. ATSDR has default intake rates for all exposure pathways in PHAST except for food ingestion.

Health assessors must evaluate whether the CTE and/or RME default intake rates are representative of site-specific conditions and potentially affected populations. If not, you will need to obtain more realistic parameters and provide justification for changing default values.

When selecting an appropriate intake rate, consider the unique behavior or exposure rates of site populations. Default assumptions are intended to be conservative, that is, lead to overestimates of doses for the population they describe. But in some instances, using default assumptions could underestimate exposures. For example, using intake rates for the general population to represent people drinking water who live in very warm climates could underestimate the actual intake. Make sure that consumption rates accurately reflect the habits and behaviors of the local population.

• Exposure factor (EF). The EF expresses how often (frequency) and how long (duration) a person could contact a contaminant in the environment over a certain amount of time (averaging time). The default EF value is usually 1, which indicates exposure occurs daily for acute, intermediate, and chronic durations, but health assessors can change the EF based on site-specific exposures. For instance, the EF can be used to adjust the exposure dose or concentration to account for intermittent or irregular exposure. ATSDR’s Exposure Dose Guidance for Life Expectancy and Exposure Factors provides the default values to use and discusses site-specific options for the EF equation.

Health assessors can use PHAST to calculate the EF. The EF is calculated using the following general equation (see the example EF calculation that follows):

                        Exposure Factor (EF)   =   (F * ED) / AT

Where:
F          =    Exposure frequency
ED       =    Exposure duration
AT       =    Averaging time

The exposure frequency is how frequently exposure occurs, often measured in days per week and weeks per year. ATSDR assumes daily exposures when developing CVs and health guidelines, but actual pathway- and media-specific exposures can occur less frequently. For example, the frequency of exposure could be less if the site has limited access.

The exposure duration is the length of time a population has been exposed to site contaminants. You can usually estimate the maximum duration by examining the site’s history. You can also base the exposure duration on the activities of the exposed population. For example, the exposure duration of people staying in a vacation rental home served by a contaminated water supply will be only as long as their stay at the home.

The averaging time is the time period over which exposure is averaged to arrive at a time-weighted exposure factor, used to express exposure in terms of an average daily dose or air concentration that can be compared to health guidelines and toxicity study results. For assessing cancer risks, the time is averaged over a lifetime of 78 years (based on the average life expectancy for men and women combined). The time exposure parameter for carcinogenic effects is therefore 28,470 days (365 days/year x 78 years). This approach for carcinogens assumes that exposure to a high dose or air concentration during a short period is the same as a corresponding low dose or air concentration during a lifetime. For assessing non-cancer effects, the time is averaged over the exposure duration (days, weeks, or years), which may not be a lifetime. For example, when estimating the dose for a child exposed to a contaminant in a playground for 3 years, the time exposure parameter would equal 1,095 days (365 days/year x 3 years). As with all assumptions, these might not be applicable in all situations.

• Bioavailability factor. The bioavailability factor is the amount of a contaminant that is absorbed into a person’s body. It is the percent of the total amount of a contaminant ingested, inhaled, or contacted that actually enters the bloodstream and is available to potentially harm a person. ATSDR assumes this factor to be 1 (i.e., all of a contaminant to which a person is exposed is assumed to be absorbed) for all exposure pathways and contaminants, with one exception: arsenic via soil ingestion. Thus, PHAST shows only the arsenic bioavailability factor for the soil ingestion equation, using a value of 0.60 (which can be altered by the health assessor if supported by site-specific information). Health assessors should not adjust the bioavailability factor of 1 for any contaminant other than arsenic without first consulting with a toxicologist and getting the approval of the ADS group.

Calculating Exposure Pathway-Specific Doses

In addition to the general exposure dose equation described above, which ATSDR uses for drinking water and food ingestion, ATSDR has formulas for other exposure pathways that incorporate more components, such as for soil and water dermal exposure pathways. As with the general exposure dose equation, begin by choosing conservative exposure parameters and refining the analysis, as needed, by considering more realistic parameters. Refer to ATSDR’s Exposure Pathway-Specific EDGs and PHAST (see PHAST text box below) to locate the appropriate parameters. If you modify default intake parameters, document the basis of those modifications. Note that some pathway-specific equations use exposure parameters (described in this table) that are not in the general exposure dose equation.

The following sections present information on calculating exposure pathway-specific doses. The sections also include examples for each pathway using fictitious site and exposure information for training purposes.

Drinking Water

Ingestion of contaminated water can be a significant source of exposure to hazardous contaminants from a site. However, inhalation and dermal exposures can also significantly contribute to the total exposure dose when certain contaminants, such as volatile organic compounds (VOCs), are present in drinking water used for other household purposes. The magnitude of these exposures, which you can use the SHOWER Model to evaluate (see box), varies depending on the frequency of showering and bathing, time spent indoors, air exchange rates in the bathroom and house, and other factors.

The preferred way to estimate exposure from the ingestion of potable water is to analyze contaminant concentrations in indoor tap water samples from individual homes. You can also use data collected from exterior home spigots, private wells, or municipal wells (at the public water distribution point).

To calculate the most accurate exposure, consider site-specific information. More importantly, realize that the default intake rate represents fluid intake from all sources. As such, assuming the default intake rate suggests all fluids are from a single drinking water source.

See the equation below and an example for estimating doses via drinking water.

Soil/Sediment Ingestion

Soil ingestion can occur by inadvertently consuming soil on hands or food items, mouthing objects, ingesting unusually high amounts of soil (i.e., soil-pica), or intentionally ingesting earth as part of certain cultural practices (i.e., geophagy). With soil-pica, health assessors should assume this practice exists if you have any possible scenarios that could involve exposures for children. Health assessors should investigate geophagy only if they learn this practice occurs during the site visit or from talking with other agencies who might know about this practice locally.

Consider the use of and accessibility to the site and surrounding areas when evaluating a site’s soil exposure pathways. Sites with abandoned buildings, standing water, or streams can attract children, and exposures can occur at sites near playgrounds or school yards despite fencing and other efforts to restrict access. Both residential and recreational areas are likely to provide access for exposure. Family members and pets can also bring contaminated soil into homes on their feet and in their hair.

See the equation below and an example for estimating doses via soil ingestion.

Calculating Soil-Pica Exposures

Soil-pica is defined as the persistent eating of non-nutritive substances such as dirt or paint. Incidental ingestion of soil occurs in all age groups, particularly in children (who have more hand-to-mouth contact), but soil-pica is the intentional ingestion of larger amounts of soil. ATSDR evaluates soil-pica only under certain circumstances, such as when this behavior is known or suspected to have occurred. Health assessors should evaluate soil-pica exposures using the maximum detected concentration as the EPC. The guidance that applies for calculating EPCs for soil-pica exposures is discussed in the Soil/Sediment Ingestion section.

1. Run a default soil-pica intermediate and acute residential scenario in ATSDR’s Public Health Assessment Site Tool (PHAST) Soil/Sediment Exposure Calculator. You will use an EPC that represents the maximum concentration of surface soil that a child could access. The default scenario in PHAST assumes an exposure of 3/7 days a week and an intake rate of 5,000 milligrams per kilogram (mg/kg).

*If PHAST indicates the default scenario using the maximum concentration exceeds acute and/or intermediate health guidelines (as indicated by a health quotient > 1 in PHAST) go to step 2.

2. Enter site-specific parameters, if available, in PHAST to determine whether a single soil-pica event is a health concern. Note that site-specific data are often not available and are not required to evaluate this one-time dose. Typically, this one-time dose uses an exposure of 1 day and the default intake rate of 5,000 mg/kg.

Refer to ATSDR’s Soil and Sediment Ingestion EDG and ATSDR’s Discrete Sampling Guidance for more information.

Soil/Sediment Dermal Contact

During the PHA process, health assessors need to examine the potential for exposure by dermal absorption of contaminants via contact with contaminated soil. Dermal absorption of contaminants from soil or dust depends on the area of contact, duration of contact, chemical and physical attraction between the contaminant and the soil, and ability of the contaminant to penetrate the skin. Contaminant-specific factors, such as lipophilicity, polarity, volatility, molecular weight, and solubility also affect dermal absorption.

Whether dermal exposure is a minor or major contributor to the overall exposure dose relative to contributors from ingestion and inhalation will depend on your site-specific exposure scenarios. Many organic contaminants bind to organic matter in soil and are therefore not readily available for absorption by the skin. In addition, if absorption occurs, it can only be with the fraction of the contaminant that is in direct contact with the skin. Therefore, the ability of a soil contaminant to be dermally absorbed depends on the diffusion of the contaminant through the soil matrix to the location where it can come in contact with someone’s skin.

There are many exposure parameters that are unique to the soil/sediment dermal dose equation (see table). One such factor is adherence, which is a soil-specific factor involved in dermal absorption. Adherence depends on soil properties, the part of the body exposed to the soil, and the type of activity being performed during soil contact. Where possible, consider site- and exposure-specific conditions. Refer to PHAST for default adherence values.

Another factor to consider is the exposure frequency and duration. Young children, older children, and adults are expected to have different exposure frequency and duration. Young children would typically have an increased exposure duration, because they tend to retain soil on their skin after coming indoors from not washing their hands as soon as adults would, and an increased exposure frequency because they tend to have more time to be exposed to outdoor soil. In general, adults would have a decreased exposure frequency because they tend to have less time to be exposed to outdoor soil.

See the equation below and an example for calculating an administered dermal dose (ADD) for soil contact.

Surface Water Ingestion

At some sites, you might need to evaluate exposures associated with swimming or recreational activities in surface water bodies. Most scenarios involving ingestion of surface water will be due to incidental ingestion while swimming.

Consider geographic factors (such as proximity or availability of surface water for recreation), seasonal factors, and age when determining the appropriate exposure factor. Activity pattern data from the EPA’s EFH can help you make some of the necessary determinations (e.g., swimming frequency/month).

Surface water scenarios often require adjusting the EF based on site-specific factors. For example, where warm-weather conditions prevail year-round (such as in southern Florida, Puerto Rico, or southern California) exposure duration can be chronic, intermediate, or acute, or a combination of these durations. But, where warm weather conditions exist only a few months of the year, only acute and intermediate duration exposures are likely. Another example is for scenarios where children may be visiting a location for the summer, such as camp, where only acute exposure is likely.

See the equation below and an example for calculating a dose for ingestion of surface water while swimming.

Surface Water Dermal Contact

Most scenarios involving skin contact with surface water involve swimming or wading. Dermal absorption of contaminants via surface water can be a significant exposure route depending on contaminant-specific characteristics. There are many exposure parameters that are unique to the surface water dermal dose equation (see table). One such parameter is skin permeability to a contaminant, which is influenced by the contaminant’s physicochemical properties, including its molecular weight (size and shape), electrostatic charge, hydrophobicity, and solubility in aqueous and lipid media. In general, contaminants that demonstrate high skin permeability are low in molecular weight, non-ionized, and lipid-soluble.

The chemical-specific permeability coefficient is another parameter considered in the estimation of dermal absorption of a chemical from water. Values for dermal permeability coefficients may vary over a large range, depending on the contaminant. When the permeability coefficient for a contaminant is known, the dermal absorption of a chemical from water can be estimated.

Another factor to consider when calculating exposure doses from dermal contact is the exposure frequency and duration. When calculating exposure doses, consider geographic factors, such as proximity or availability of surface waters for recreation, seasonal factors, and age.

See the equation below and an example for calculating an Administered Dermal Dose (ADD) of surface water through either swimming or wading.

Food Ingestion

To assess the human health risk from ingesting contaminated food, you need information on the quantities of contaminated foods consumed and the extent of contamination in these foods. Direct concentration measurements are the most reliable way to assess food contamination exposure. These measurements should be conducted on foods prepared for consumption or portions of contaminated plants and animals that are representative of the portions used as food.

If the food chain appears to be a significant pathway for human exposure, but you lack the appropriate information on contaminant levels, explicitly describe this gap in your document and recommend how to obtain the needed information. Make sure you understand the contaminants at the site and their tendencies to bioaccumulate in animals or plants so that you can recommend the most appropriate strategy.

To estimate the food chain exposure dose, you need to know the consumption rate of specific food items in the human diet. ATSDR does not have default intake rates for food, so health assessors should use the EPA’s EFH to obtain these intake rates. Be aware that consumption rates of the population (and subpopulations) in the vicinity of a hazardous waste site can differ considerably from national averages. For example, American Indians/Alaska Natives who subsist on fish from a primary source will likely have higher consumption rates. Use local consumption patterns if they are available and differ from national averages.

Commonly, as a health assessor, you will be concerned about exposures from consuming fish due to the potential for bioaccumulation of a contaminant(s) found in surface water or sediment. In addition, in the case of residential soil contamination, the consumption rate of homegrown foods and local wild plants is also of interest.

See the equation below and an example for calculating exposure doses for food ingestion.