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2.0. ESTIMATING EXPOSURES AND RISKS
2.1. INTRODUCTION

In this chapter, the framework for assessing exposure and risk to 2,3,7,8-TCDD and related dioxin-like compounds will be described. Section 2.2 introduces the exposure equation and its key terms. Section 2.3 describes how risk is estimated given estimates of exposure. It also discusses the use of toxicity equivalency factors. Section 2.4 provides the overview of the procedures used in this document, and provides a roadmap for finding pertinent information in other chapters of the document. Section 2.5 describes the development of exposure scenarios for this assessment. Section 2.6 describes the exposure parameters chosen for the exposure pathways of this assessment.

The development of exposure assessment methods, scenarios and associated parameter values raises many issues which are generic to all chemicals. In order to keep the scope of this document reasonable, the decision was made to focus on issues specific to dioxin-like compounds and to avoid evaluating generic issues. Thus, priority is given to addressing issues such as fish bioconcentration, dermal absorption, degradation, and other chemical/physical properties of these compounds. The approach used to address generic issues such as soil ingestion rates, inhalation rates and other behavior parameters is based on previously published Agency documents, primarily the Exposure Factors Handbook (EPA, 1989a). Another generic issue which has been raised in connection with this document is the use of Monte Carlo procedures to define exposure scenarios. These procedures require distributions for the input parameters used in the assessment. Such distributions have not been established by the Agency. Decisions on the use and definition of such distributions affect assessments of all chemicals and cut across all Agency programs. Thus, it is not appropriate to establish such polices in this document. However, individuals outside of the Agency have published assessments which applied Monte Carlo procedures to problems involving dioxin-like compounds. In recognition of the high interest in this area, a general description of this technique and summaries of assessments which applied it to dioxin-like compounds are included in Chapter 7 of this Volume.

The Agency does have efforts underway to evaluate these generic issues. For example, the Office of Health and Environmental Assessment (OHEA) is in the process of revising the Exposure Factors Handbook and plans to hold public review meetings in 1993. In addition, OHEA is developing a guidance document on generating exposure scenarios which will be issued for review in 1993. Several offices have projects specific to Monte Carlo:

. Office of Health and Environmental Assessment - A Workshop on approaches to evaluating uncertainty (including the use of Monte Carlo) was held in 1992.

. Office of Policy, Planning and Evaluation - A workshop on using Monte Carlo methods is scheduled for 1993.

. Office of Pollution Prevention and Toxics - A handbook on the use of Monte Carlo is being developed and is scheduled for publication in 1993.

Readers interested in generic Monte Carlo procedures are best served in these forums.

2.2. EXPOSURE EQUATION

This document describes procedures for conducting exposure assessments to estimate either potential or internal dose. A potential dose is defined as a daily amount of contaminant inhaled, ingested, or otherwise coming in contact with outer surfaces of the body, averaged over an individual's body weight and lifetime. An internal dose is defined as the amount of the potential dose which is absorbed into the body (EPA, 1991). Section 2.3 below discusses the relevancy of this distinction for dioxin-like compounds.

The general equation used to estimate potential dose normalized over bodyweight and lifetime is as follows:

Equation V3 2-1

This procedure is used to estimate dose in the form needed to assess cancer risks. Each of the terms in this exposure equation is discussed briefly below:

. Exposure media concentrations: These include the concentrations in soil for dermal contact and soil ingestion exposure pathways, in vapor and particulate phase in air for inhalation exposure pathways, in water for a water ingestion pathway, and in food products such as fish, fruits and vegetables, and beef and milk, for food ingestion pathways. The concentrations used should represent a temporal average over the time of exposure. Chapter 4 provides procedures for estimating exposure media concentrations.

. Contact rate: These include the ingestion rates, inhalation rates, and soil contact rates for the exposure pathways. These quantities are generally the total amount of food ingested, air inhaled, etc. Only a portion of this material may be contaminated. The next term, the contact fraction, which is 1.0 or less, reduces the total contact rate to the rate specific to the contaminated media.

. Contact fraction: As noted, this term describes the distribution of total contact between contaminated and uncontaminated media. For example, a contact fraction of 0.8 for inhalation means that 80% of the air inhaled over the exposure period contains dioxin-like compounds in vapor form or sorbed to air-borne particulates. The contact fractions for the exposure pathways of air inhalation and water ingestion are related to the time individuals spend at home. Other pathways such as fish ingestion or ingestion of home grown foods are not related to time at home. Similarly, contact fractions for individuals exposed at work places relate largely to time spent at the work place.

EPA (1989a) discusses several time use studies which can be used to make assumptions about time spent at home (and outdoors at the home environment) versus time spent away from home. Generally, these time use studies asked participants to keep 24 hour diaries of all activities. Studies summarized were national in scope, involved large numbers of individuals, cross-sections of populations in terms of age and other factors, and up to 87 categories of activities. Results from different studies consistently indicate that the average adult spends between 68 to 73% of time at the home environment.

. Exposure duration: This is the overall time period of exposure. Values of 9 years and 20 years are used in the example scenarios described in Chapter 5. The value of 9 years corresponds to the average time spent at one residence (EPA, 1989a), and was used as an exposure duration for a non-farming family living in a rural setting. Twenty years was used as the exposure duration for farming families in a rural setting. Another exposure duration demonstrated in Chapter 5 is one associated with a childhood pattern of soil ingestion. The exposure duration in this case is 5 years.

. Body weight: For all the pathways, the human adult body weight of 70 kg is assumed. This value represents the United States population average. The body weight for child soil ingestion is 17 kg (EPA, 1989a).

. Lifetime: Following traditional assumptions, the average adult lifetime assumed throughout this document is 70 years. Even though actuarial data indicate that the United States average lifetime now exceeds 70 years, this convention is used to be consistent with other Agency assessments of exposure and risk.

2.3. RISK EQUATION

Although estimation of risk is technically beyond the scope of an exposure assessment, the exposure assessor needs some background understanding in this area. The primary source of information on the health risks of the dioxin-related compounds is the Health Reassessment that EPA is publishing concurrently with this document (EPA, 1994). However some general considerations for using exposure estimates in support of cancer risk assessments are summarized here. The usual procedure used to calculate an upper-limit incremental cancer risk is as follows:

Equation V3 2-2

The derivation of this factor is described in EPA (1984a) and further background is provided in EPA (1981). The Agency is currently reevaluating this slope factor and the reader should consult the companion Health Reassessment (EPA, 1994) for the current policy. EPA derived the 1984 slope factor for 2,3,7,8-TCDD from animal feeding studies on the basis of potential (i.e., administered) dose. Thus, for purposes of consistency, when using this slope factor to estimate risk to humans, the exposure assessor should provide the dose estimate as a potential dose. This point raises issues specific to the various pathways.

The absorption which occurred during the animal experiments EPA used to derive the 1984 slope factor for 2,3,7,8-TCDD was estimated to be 55% (Farland, 1987). The review of literature on bioavailability in Appendix C of Volume 2 of this assessment indicates that the gut absorption of 2,3,7,8-TCDD in humans when the vehicle is soil is 20-40% of potential dose. Fries and Marrow (1975) found that 50-60% of the 2,3,7,8-TCDD was absorbed by rats from feed. Rose, et al. (1976) estimated that 86% of 2,3,7,8-TCDD in a mixture of acetone and corn oil fed by gavage to rats was absorbed. EPA (1984), using animal data and information on fate of particles in the respiratory system, estimated that the fraction of 2,3,7,8-TCDD absorbed into the body ranges from 0.25 to 0.29. What this discussion indicates is that the absorption for human ingestion and inhalation pathways might range from 20-80% of potential dose, which compares to 55% found in the laboratory experiments. If no adjustment were made to potential dose estimates, then human risk estimates might be overestimated (when absorption is in the 20% range) or underestimated (in the 80% range). This discrepancy is not felt to be large enough or certain enough to warrant an absorption correction factor.

The rate of absorption of vapor-phase 2,3,7,8-TCDD into the lungs has not been studied, but it seems reasonable to assume that the absorption in the vapor phase should exceed that of absorption from bound 2,3,7,8-TCDD on particulates, probably above 50%. There is also an unknown uncertainty introduced when assuming that the q1* developed from a feeding study can be used for an inhalation pathway. Thus, it is unclear what adjustment is needed to account for differences between the feeding study and a vapor-phase inhalation exposure. Accordingly, it is recommended that assessors not attempt any such adjustments at this point, but fully acknowledge the uncertainty.

Estimating risks associated with dermal exposure introduces several issues to consider. First, use of an oral dose-response function may not be applicable to the dermal route. Second, dermal absorption of dioxin-like compounds from soil appears to be much lower than that which occurred in the dose-response feeding studies. EPA (1992a) indicates that 0.1 - 3% of 2,3,7,8-TCDD may be dermally absorbed from soil, which is significantly less than the 55% absorption found in the laboratory feeding experiments. It is assumed for this assessment that an absorption fraction of 0.03 (3%) applies to 2,3,7,8-TCDD as well as the other dioxin-like compounds. Specifically, this assessment estimates the total amount of compounds applied to skin and then reduces it by 97% to estimate the absorbed dose. This is the only pathway in which an absorption fraction is used to adjust a dose. Because of this adjustment, an additional adjustment to the risk equation, Equation (2-2) above, is needed when estimating risk from dermal exposure in a manner consistent with other exposure pathways: the slope factor should be multiplied by (100%) / (55%), or about 2, to convert it to an absorbed basis. Finally, the assessor should acknowledge that considerable uncertainty is introduced by applying an oral based dose-response function to dermal exposure.

Another set of issues facing the exposure/risk assessor is how to estimate exposure to mixtures of dioxin-like compounds with differing slope factors. EPA (1989b) has proposed a procedure to address this issue, which is to adjust the risk estimate using a "toxicity equivalency factor", commonly referred to as TEF. The TEF for a congener of interest is the cancer potency of that congener divided by the cancer potency of 2,3,7,8-TCDD. As shown in Table 1-1 in Chapter 1, the TEF for 2,3,7,8-TCDD is 1 and all other dioxin-like compounds have TEFs less than 1. The combined risk resulting from exposure to a mixture of dioxin-like compounds can be computed using the TEFs and assuming that the risks are additive:

Equation V3 2-3

2.4. PROCEDURE FOR ESTIMATING EXPOSURE

Section 2.2 described the exposure equation as it applies to dioxin and dioxin-like compounds. Before making exposure estimates, the assessor needs to gain a more complete understanding of the exposure setting and be prepared to estimate exposure media concentrations. The purpose of this section is to provide guidance for the procedures followed in this assessment to define such settings and estimate exposure media concentrations. The approach used here is termed the exposure scenario approach. Brief descriptions of the steps and associated document chapters are presented below and summarized in Figure 2-1.

Step 1. Identify Source
Three principal sources are addressed in this document. The first, identified as "soil", is called a source in that the starting point of the assessment is soil contamination. Of course, the ultimate source for soil contamination is some unidentified cause for the soil to become contaminated. For exposure and risk assessment purposes, the cause for contamination is not relevant except to assume that the cause is not ongoing and that the impact of the "initial" levels is what is being evaluated. The soil source is further characterized as off-site or on-site. Off-site implies that the soil contamination is located some distance from the site of exposure. The site of exposure could be a residence or farm, and the site of contamination could be a landfill, for example. On-site implies that the soil contamination is on the site of exposure. The second principal source is called "stack emissions." Unlike the soil source, the contamination is assumed to be on-going. Stack emissions in particulate form are assumed to deposit onto the soils and vegetations of the site of exposure, and emissions in vapor form result in air-borne concentrations which transfer into vegetations at sites of exposure. ...
table Figure 2-1. Roadmap for assessing exposure and risk to dioxin and dioxin-like compounds.
... It is noted that individuals working at the site where stack emissions occur are also exposed. The procedures in this document only apply to residents who are not associated with the site where stack emissions occur.

The third principal source is called "effluent discharges". Such discharges represent point source inputs to surface water bodies.

Like the stack emission source, impacts to surface water bodies are assumed to be ongoing during the period of exposure. Unlike either of the above two sources, only the impacts to water and fish are considered for this source category.
expand table Figure V3 2-1

Step 2. Estimate Release Rates
Estimating the release of contaminants from the initial source is the first step towards estimating the concentration in the exposure media. Releases from soil contamination include volatilization, and wind and soil erosion. Chapter 4 on estimating exposure media concentrations describes fate and transport modeling procedures for estimating soil releases. Stack emissions and effluent discharges are point source releases into the environment. Background on stack emissions including details on modeling from the stack to a site of exposure are provided in Chapter 3.

Step 3. Estimate Exposure Point Concentrations
Contaminants released from soils, emitted from stacks, or discharged into surface waters move through the environment to points where human exposure may occur. Contaminated soil that is near but not at the site of exposure is assumed to slowly erode and contaminate the exposure site soil, but to a level lower than the level at the contaminated site. The only time when the source concentrations equal the exposure concentrations is for the soil pathways, soil ingestion and dermal contact, when the soil contamination is on-site. Chapter 3 describes the use of the COMPDEP Model used to estimate dispersion of stack plumes to arrive at air-borne concentrations at the site of exposure as well as deposition rates of stack emitted particulates. Chapter 4 describes how soil and vegetation concentrations are estimated given particulate deposition rates, and also how release rates from soil initially contaminated translate to exposure point concentrations. Chapter 4 also describes a simple dilution model which translates effluent discharges into surface water and fish tissue concentrations.

Step 4. Characterize Exposed Individuals and Exposure Patterns
The patterns of exposure are described in Sections 2.5 and 2.6 of this Chapter. Exposed individuals in the scenarios of this assessment are individuals who are exposed in their home environments. They are adult residents who also recreationally fish, have a home garden, farm, and are children ages 2-6 for the soil ingestion pathway. Each of these pathways are evaluated separately. Since it is unlikely that single individuals would experience all of these pathways, the exposures across pathways are not added. Exposure pathways evaluated, which have generally been alluded to in discussions above, include inhalation, ingestion, and soil dermal contact. Each pathway has the set of parameters including contact rates, contact fractions, body weights, and lifetime. These parameters were defined earlier in Section 2.2.

Step 5. Put It Together in Terms of Exposure Scenarios
A common framework for assessing exposure is with the use of "settings" and "scenarios." Settings are the physical aspects of an exposure area and the scenario characterizes the behavior of the population in the setting and determines the severity of the exposure. A wide range of exposures are possible depending on behavior pattern assumptions. An exposure scenario framework offers the opportunity to vary any number of assumptions and parameters to demonstrate the impact of changes to exposure and risk estimates. Exposure estimates for six example scenarios are demonstrated in Chapter 5.

Step 6. Estimate Exposure and Risk
Section 2.2 described the basic equation that estimates exposure for every assumed pathway in an exposure scenario. Chapter 5 demonstrates the methodology on six example scenarios, which includes the generation of exposure estimates for ten different exposure pathways and three different dioxin-like compounds.

Step 7. Assess Uncertainty
Chapter 7 provides a discussion on the validity of exposure media concentration estimation, and possible sources of uncertainty associated with this methodology. These uncertainties should be considered when applying this procedures to a particular site. Chapter 6 on User Considerations includes discussions on other pertinent topics such as sensitivity of model results to parameter selection, and judgements on use of the parameters selected for the demonstration scenarios for other applications.

2.5. STRATEGY FOR DEVISING EXPOSURE SCENARIOS

EPA (1992b) states, "In exposure scenario evaluation, the assessor attempts to determine the concentrations of chemicals in a medium or location and link this information with the time that individuals or populations contact the chemical. The set of assumptions about how this contact takes place is an exposure scenario." These assumptions can be made many different ways producing a wide variety of scenarios and associated exposure levels. The number of people exposed at different levels form a distribution of exposures. Ideally assessors would develop this entire distribution to fully describe the exposed population. Such distributions could be defined using Monte Carlo techniques if sufficient input data are available. However, as discussed in Section 2.1 above, generic issues surrounding use of Monte Carlo are not evaluated here.

Discussions of how other assessors have applied Monte Carlo to problems involving dioxin-like compounds are presented in Chapter 7. Since the necessary information for developing a population distribution is rarely available, EPA (1992b) recommends developing a central and high end scenario to provide some idea of the possible range of exposure levels. This section will illustrate this procedure as applied to the dioxin-like compounds. In addition, this section identifies the exposure pathways which are relevant to these compounds, and provides background and justification for the exposure parameters which were selected for the demonstrations in Chapter 5.

For any physical setting, a wide variety of exposure scenarios are possible. The range of exposure levels results from a number of different factors including individual behavior patterns, proximity of individuals to the source of contamination, the fate characteristics of the contaminant, and others. In order to illustrate the possible range, the assessor should try to characterize a central and high end scenario. The general strategy recommended here for defining these scenarios is to first identify and quantify the source of contamination. Next, the assessor should determine the geographic area that is impacted by this source. The contaminant levels are likely to vary widely over this area. Select locations of interest within this area such as the location of the nearest exposed individual or most heavily populated area. For each of these locations, identify behavior patterns which characterize central and high end exposure patterns. Central scenarios correspond to average or median levels and high end scenarios are defined as levels above the 90th percentile but within the actual range of exposure levels (EPA, 1992b). Statistical data are rarely available to precisely define such scenarios. Instead judgement is usually required to identify behavior patterns meeting these criteria.

For example, most rural areas probably include both farming and nonfarming residents. Farmers who grow or raise much of their own food could be selected to represent the high end scenario and those living in typical residential areas could represent the central scenario. Alternatively, if more detail is desired, central and high end scenarios could be defined for both segments of the population, i.e., farmers and residents. For each scenario, determine relevant exposure pathways and assign values for exposure parameters such as contact rate, exposure duration, and so on, which represent a central and/or high end pattern for the type of receptor. Finally, compute the associated exposure level. The resulting range of exposure levels for each location can be used to illustrate the possible range of exposures.

Reference has been made in this chapter to the example scenarios found in Chapter 5, Demonstration of Methodology. Four "source categories", categories of contamination sources described in Chapter 4, are demonstrated in Chapter 5. The on-site soil and stack emission sources are assumed to expose a relatively large population in a rural area containing residences and farms. For these sources, both central and high end scenarios are defined in the manner outlined above. Specifically, a central scenario is based on typical behavior at a residence and the high end is based on a farm family that raises a portion of its own food. For the other two sources, off-site soil and effluent discharges, only one scenario each will be defined and demonstrated.

The off-site soil source category will be demonstrated with a high end scenario - a farm is located near the site of contamination. Soil on the farm becomes impacted through the process of soil erosion. Other individuals within a community can also be impacted by a site of high soil contamination. Such individuals would include those visiting or trespassing on the site, volatilized residues can reach their residences, they may obtain water and fish from a nearby impacted water body, and so on. As such, alternate scenarios demonstrating the impact of a site of soil contamination could be developed. For the sake of brevity, and also considering that those residing nearest the contaminated are most impacted, only a high end scenario is developed for the off-site soil source category. The effluent discharge source category is unique in that only the pathways of water ingestion and fish ingestion are considered. For this category, fish and water ingestion patterns will be those adopted for the central scenarios. Again, other patterns of fish and water ingestion could be evaluated for this source category. As a matter of brevity again, only central patterns of behavior with regard to fish and water ingestion are demonstrated.

The methodologies used to estimate exposure media concentrations are described in Chapter 4 as screening level in their technical sophistication, but site specific in their application. Defining populations that are typical of central and/or high end exposures is clearly a site specific exercises. Assessors need to make the kinds of assumptions discussed here for their own source and populations of concern. Many acceptable ways could be used to define central and high end scenarios. The approach used here was done for demonstration purposes only. On the other hand, the example scenarios in Chapter 5 were carefully crafted to be plausible and meaningful, considering key factors such as source strength, fate and transport parameterization, exposure parameters, and selection of exposure pathways. For example, a beef and milk exposure pathway is demonstrated only for the high end scenarios. Farmers raise cattle for beef and dairy products and are assumed to obtain a portion of their intake of these products from their own farm, whereas non-farming residents are assumed not to be exposed to contaminated farm products.

Key source strength terms were carefully developed and defined. Key source strength terms include soil concentrations, effluent discharge rates, and stack emission rates. For the demonstration of methodologies of this assessment, concentrations of dioxin-like compounds in contaminated soils for the off-site soil source category were set at 1 ppb, which was a typical concentration of 2,3,7,8-TCDD found in Superfund-like sites studied in the National Dioxin Study (EPA, 1987). Concentrations in soil used for the on-site source category were characterized as typical of background levels and assigned a value of 1 ppt, three orders of magnitude lower than the 1 ppb for off-site soil contamination. Researchers investigating concentrations of 2,3,7,8-TCDD in "background" or "rural" settings have typically found it in the ppt range or not detected it (with a detection limit generally less than 1 ppt). Introductory sections of Chapter 5 provide a more complete description of the example scenarios.

2.6. EXPOSURE PATHWAYS AND PARAMETERS

The dioxin-like compounds have been found primarily in air, soil, sediment and biota and to a lesser extent in water. Thus, the most likely exposure pathways are:

. Ingestion of soil, water, beef, dairy products, fish, fruit, and vegetables. Dermal contact with soil

. Inhalation of particulates and vapors.

The following sections describe the selection of central and high end exposure parameters for these pathways. Table 2-1 summarizes all the exposure parameters selected to represent the central and high end demonstration scenarios of Chapter 5.

2.6.1. Soil Ingestion

Soil ingestion occurs commonly among children during activities such as mouthing of toys and other objects, nonsanitary eating habits, and inadvertent hand-to-mouth transfers. In addition to normal soil ingestion activities, some individuals exhibit behavior known as pica which involves intentional soil ingestion. Soil ingestion rates associated with pica are probably much higher. No measured values for pica patterns have been reported in the literature, though EPA (1989a) reports that other assessments have assumed values such as 5 and 10 g/day. This document considers only normal soil ingestion among children.

To a lesser extent, soil ingestion also occurs among adults from activities such as hand-to-mouth transfer when eating sandwiches or smoking. However, the data to estimate the adult rate of soil ingestion is essentially unavailable, so adult soil ingestion is not demonstrated in this assessment. Paustenbach (1987) and Sheenan et al. (1991) have suggested calculating exposures for this pathway (as well as dermal contact and inhalation) separately over three to four age periods to reflect major changes in body weight, surface area and inhalation rates. In general, exposure assessments can be refined by estimating exposures separately over each year of age that is of interest and summing to get the total. Age specific data for body weight, surface area and inhalation rate are presented in EPA (1989a and 1992b). These procedures are not presented here, but readers interested in refining exposure estimates are encouraged to check the above references for further guidance.

table Table 2-1. Summary of exposure pathway parameters selected for the demonstration scenarios of Chapter 5.
Based on the review of literature, particularly the studies of Binder et al. (1986) and Clausing et al. (1986), the following values for soil ingestion were suggested in EPA (1989a): average soil ingestion in the population of young normal children (under the age of 7) is estimated at approximately 0.1 to 0.2 g/d.

An upper-range ingestion estimate among children with a higher tendency to ingest soil materials, although not a pica pattern, could be as high as 1 g/d. However, a value of 0.8 g/day is recommended for high end exposure estimates. The values of 0.2 g/d and 0.8 g/d were the values adopted for the central and high end exposure scenarios in Chapter 5.
expand table Table V3 2-1

Exposure Duration:
A duration of 9 years was assumed for the "residence" setting based on mobility data showing the average time in one residence was 9 years (EPA, 1989a). A duration of 20 years was evaluated as the 90th percentile of time in one residence, and was selected for the "farm" settings, which assumes that farming families live in a given residence longer than a non-farming family. These values apply to all pathways with no variation, except for soil ingestion, which was demonstrated only for children and was at 5 years.

Body Weight/Lifetime:
The standard assumptions of a 70 kg adult and 70 years lifetime were assumed for all pathways, except that of soil ingestion. In that case, a body weight of 17 kg was used.

A number of investigators have suggested that lower values are more appropriate for soil ingestion rates. These suggestions are being evaluated by the Office of Health and Environmental Assessment in connection with the revisions to the Exposure Factors Handbook (EPA, 1989a). To date, a final position has not yet been reached. As discussed in Section 2.1, it was decided to not independently evaluate such generic issues in this document. Thus, the soil ingestion rates adopted here reflect those previously accepted by the Agency, but should be updated if and when the Agency adopts new values.

Note that the general need to update values for exposure factors as new information becomes available applies to all factors. It has been emphasized in this discussion on soil ingestion just because it appears that changes are most imminent here. For the soil ingestion pathway, contact fraction refers to the portion of ingestion soil which is contaminated. For the residential setting, the assumption is made here that all soil ingestion by children occurs in and around the home, and that all the soil at the home is contaminated.

Thus, a value of 1 has been adopted in the example scenarios presented in Chapter 5. If the soil contact occurs primarily outdoors, climatic factors such as snow cover, frozen soil, rain, etc. can substantially limit contact and ingestion of soil. In situations where the contaminated area is located remote from where children live, and children have some access to these areas (if the areas are parks or playgrounds, e.g.), lower fractions would be appropriate.

2.6.2. Soil Dermal Contact

The total annual dermal contact, expressed in mg/yr, is the product of three terms: the contact rate per soil contact event, the surface area of contact, and the number of dermal contact events per year. EPA (1992a) recommends the following ranges for these terms:

. Contact rate: 0.2 to 1.0 mg/cm2-event

. Adult surface area: 5000 to 5800 cm2

. Event frequency: 40 to 350 events/year.

An event frequency and contact rate near the upper end of these ranges may be appropriate for an high end exposure activity pattern such as farming, where the individual more often comes in contact with soil and may be exposed to fugitive dust emissions while they work. An event frequency of 40 and 350, and a contact rate of 0.2 and 1.0 mg/cm2-event, are assumed for the central and high end exposure scenarios in Chapter 5. However, the exposed surface area of 5000 cm2 may be reduced for farmers. This area corresponds to 25% of the total body area and apparel such as short sleeves, shoes, socks and short pants. Farmers working in the field are likely to wear long pants at least, if not also long sleeves. Although EPA (1992a) indicates that clothing is not always effective in preventing dermal contact, it seems reasonable that a value of 1000 cm2 (5% of total body area) representing hands, neck, and face might be more appropriate in a farming scenario. Values of 5000 and 1000 cm2 were selected for the central and high end scenarios in Chapter 5.

The considerations for contact fraction are similar to those for soil ingestion; i.e., that all contact occurs with contaminated soil at the residence or farm site. Accordingly a value of 1 was selected for the example scenarios presented in Chapter 5.

One further adjustment was made for this exposure pathway. The contact as estimated above is the amount of soil which contacts the body. EPA (1992a) indicates that only a small percent of strongly hydrophobic organic compounds such as 2,3,7,8-TCDD are absorbed into the body from soil dermal contact. The "dermal absorption fraction" recommended for 2,3,7,8-TCDD in EPA (1992a) was 0.001 (0.1%) to 0.03 (3%). EPA (1992a) recommends using the upper end of this range for application to other dioxin-like compounds as a conservative assumption until these compounds have been tested. An absorption fraction of 0.03 was used for the three compounds demonstrated in Chapter 5. The dermal contact exposure pathway was the only one in which such an absorption fraction was used.

2.6.3. Vapor and Dust Inhalation

EPA (1989a) describes derivation of the commonly used ventilation rates of 20 and 23 m3/day. As noted in that reference, these values assume 16 hours of light activity and 8 hours of resting. Other recommendations in that reference are a rate of 30 m3/day for high end exposures, and to derive specific ventilation rates (EPA (1989a) gives information to do so) for specific activity patterns. The example scenarios of this assessment all use 20 m3/day.

An additional assumption needs to be made for the vapor and dust inhalation pathways. This pertains to an assumption concerning the differences in air quality between indoor and outdoor conditions. Algorithms for both particulate and vapor-phase air-borne concentrations of contaminants are specific to outdoor air. Hawley (1985) assumed, based on several other studies in which measurements were made, that the concentration of suspended particulate matter in indoor air is equal to 75% of that outside. Also, his report stated that most household dust is outdoor dust that is transported into the house, and that only a small percentage is developed from sources within. He then concluded that 80% of the indoor dust is identical in contaminant content to outdoor soil. Refinements to the concentration of contaminants on indoor versus outdoor dust should have a minor effect on exposure estimates. A similar trend is assumed for air-borne vapor phase concentrations. For this reason, differences between indoor and outdoor concentrations are not specifically considered, or equivalently, no distinctions are made for outdoor and indoor air quality.

The contact fraction for this pathway is equal to the fraction of total inhaled air which is contaminated. Thus it relates largely to percent of time spent in the contaminated area. For the example exposure scenarios presented in Chapter 5, the contact fraction corresponds to percent time at home. EPA (1989a) suggests a range of 0.75 to 1.0; the lower value will be adopted as the central value used in the residence setting. The value selected for high end scenarios will be 0.90 instead of 1.00, recognizing that 1.00 is more likely a bounding rather than a high end estimate.

2.6.4. Water Ingestion

The water ingestion rate of 2 L/day is traditionally assumed for exposure through drinking water. However, EPA (1989a), after review of several literature sources, concludes that 2.0 L/day may be more appropriately described as a 90% value, or a value for high end exposure estimates. For this reason, a water ingestion rate of 2 L/day is assumed only for the high end exposure estimates. Since the high end scenario includes a farm and the farming family, it is also argued that farm labor requirements justify the higher rate of water ingestion. EPA (19899a) recommends a rate of 1.4 L/day as representative of average adult drinking water consumption. This is the rate used for central scenarios in Chapter 5. The difference in central and high end tendencies is also modeled using the contact fraction. Again, this fraction is based on the time spent at home. The value of 0.75 is used to model the central estimate, for the residence setting, and the value of 0.90 is used to model the high end estimate, for the farm setting.

2.6.5. Beef and Dairy Product Ingestion

If contaminated beef or dairy products from one source are marketed along with uncontaminated products from many sources, only a small percent of the product consumed by an individual may be contaminated. The potential effects of such "market dilution" of beef and dairy products on human exposure are discussed briefly in EPA (1984a), at more length by Fries (1986), and at much greater length in EPA (1985) for the particular case of cattle production in Missouri. Aspects of the beef industry in this region specifically noted in EPA (1985) as important to estimating exposure were type of activity within the industry (e.g. cow-calf production, "backgrounding" - preparing calves for feedlots, feeding for slaughter), replacement rates as a function of activity, fractions of cattle fed to maturity outside contaminated areas before slaughter, and slaughter categories and rates relative to national figures.

EPA (1984a and 1985) concluded that dilution will vary widely between different marketing areas. EPA (1984a and 1985) and Fries (1986) noted that the subpopulations most likely to receive high exposures are beef producers, dairy farmers, and their direct consumers. The residents in the central exposure scenario in Chapter 5 are not assumed to be producers or direct consumers of farm products, and for this reason, the central estimate scenarios do not include a beef and milk pathway. The high end farming scenarios do have these pathways, meaning that the farmers home slaughter for beef and also obtain a portion of their milk from their lactating cattle.

Average consumption rates and fat content data for beef and dairy products are described in EPA (1989a). Summary information presented in that reference comes principally from a U.S. Department of Agriculture Nationwide Food Consumption Survey (NFCS) conducted in 1977-1978 (described in EPA, 1984b), with additional information added by Fries (1986). The NFCS covered intake of 3,735 possible food items by 30,770 individuals characterized by age, sex, geographic location, and season of the year. The average beef fat consumption rates listed in EPA (1989a) ranges from 14.9 to 26.0 g per 70-kg person/day, with a single high consumption estimate of 30.6 g per 70-kg person/day. Based on this information, EPA (1989a) recommends using a beef fat consumption rate of 22 g/day (based on an arithmetic mean from studies summarized in EPA (1989a) of 100 g/day whole beef and 22% fat content).

This may underestimate the amounts eaten by households who home slaughter; i.e., the availability of beef by farming families raising beef for slaughter might lead to consumption habits for beef that exceed those of the general population. Milk fat consumption from all dairy products ranges from 18.8 to 43 g per 70-kg person/day. Considering fresh milk only, the milk fat consumption is reported as 8.9 to 10.7 g per 70-kg person/day in various studies summarized in EPA (1989a), with a single high consumption estimate of 35 g per 70-kg person/day. An arithmetic mean milk fat consumption rate of 10.5 g/day is derived in EPA (1989a) (this assumes 300 g/day whole milk and 3.5% fat). This may also underestimate the consumption rate of farming families who consume milk supplied by their own cattle. The rate of beef and milk fat consumption assumed for the high end farming scenarios in Chapter 5 are 22 and 10.5 g/day, respectively.

Consumption rates of beef and milk are expressed in terms of fat ingested per day because dioxin-like compounds tend to partition strongly toward lipids. It is assumed that virtually all of such compounds will be found in the fat portion of milk or beef. Further, the algorithms to estimate concentrations in these food products estimated fat concentrations and not whole product concentrations.

EPA (1989a) also reports on another survey of 900 rural farm households (USDA, 1966), including some where the farm's beef and dairy cattle supply a portion of the household's beef and milk. In these situations, the average percent of homegrown beef and milk (dairy products) is 44% and 40%, respectively. Contact fractions of 0.44 and 0.40 were used in this assessment for the high end farming scenarios. Lacking better information, EPA (1989a) recommends a contact fraction for beef and dairy of 75% if the intent is to estimate high end estimates for a farmer who uses a portion of his farm's products.

2.6.6. Fish Ingestion

EPA (1989a) concludes that consumption rate data from two studies, that of Puffer (1981) and Pierce, et al. (1981) are most appropriate for estimating consumption rates for recreational fishing from large water bodies. The recommended 50th percentile consumption rate, or typical rate, for this subpopulation is 30 g/day, and the 90th percentile rate is 140 g/day. Table 2-2 contains ingestion rates for freshwater and estuarine fish and shellfish. These are based on an analysis of the results of the USDA 1977-78 National Food Consumption Survey. If using these data, the assessor should consider the following points:

1) The survey was conducted over a three day period. Thus, it does not represent long term behavior patterns which is the interest of exposure assessments used to support analysis of chronic health effects. This problem introduces uncertainty into the estimates of medians (50th percentile) and other percentiles. It can provide appropriate estimates of the average.

2) Because most of the persons surveyed did not eat fish or shellfish during the survey period, the 50th percentile values are zero. The mean values are more appropriate to use as central tendency estimates of fish and shellfish consumption over a lifetime. However, these averages are on a per capita basis, ie. averaged across all survey participants (including fish eaters and nonfish eaters). The average fish consumed by fish eaters is probably a more relevant estimate of central exposures. This value would be higher than the per capita average.
table Table 2-2 Fish consumption estimates from the USDA 1977-78 National Food Consumption Survey (consumptions were recorded for three day periods; N = 36249; units are grams/day/person; SF = shellfish).
3) These data represent total ingestion rates of store-bought fish. Obviously, what is of interest for a site specific survey is the amount of fish consumed from waters within the study area. Assuming local surveys are not available EPA (1989b) recommends approaching this problem by using judgement to estimate the number of fish meals (100 to 200 g) per year that a person may reasonably consume from the water body of concern. By comparing these judgement based values to the national survey data the assessor can make some evaluation of their reasonableness. If evidence exists that subsistence fishing occurs in the area of interest, then even higher levels than those given in Table 2-1 may be warranted. Wolfe and Walker (1987) found subsistence fish ingestion rates up to 300 g/d in a study conducted in Alaska.
expand table Table V3 2-2

For smaller water bodies, EPA (1989a) recommends that site-specific information be obtained via surveys of local fisherman to obtain the most appropriate fish consumption information for site-specific assessments. Alternately, EPA (1989a) recommends using judgement regarding how many fish meals per year an individual could obtain from the contaminated waters and assuming meal sizes of 100 to 200 g. Consumption of commercial fish (at restaurants or from markets) raises market dilution issues analogous to those described earlier for beef and milk. For this reason, exposed individuals in both the central and high end scenarios in Chapter 5 are assumed to obtain their contaminated fish intake from a nearby contaminated stream or pond; other fish they may consume is not considered in this assessment.

The examples used in this assessment assume that the contaminated waters are small lakes or streams which are occasionally fished on a recreational basis. Further it is assumed that an individual could eat 3 to 10 meals per year from the contaminated waters. Assuming an average meal size of 150 g, this translates to 450 to 1500 g/year or an average of 1.2 to 4.1 g/day. The central estimate for the example scenarios in Chapter 5 will therefore be 1.2 g/day, and the high end will be 4.1 g/day. Since these fish ingestion rates are rates of ingestion of contaminated fish, the contact fraction would be 1

2.6.7. Fruits and Vegetables

EPA (1989a) estimated ingestion rates for individuals who have home gardens and hence grow a portion of their fruit and vegetable intake. Their approach was to review the literature and derive average intake rates for all individuals, whether or not they have a home garden, and considering a variety of different fruits and vegetables. A typical and high end exposed individual had the same total ingestion rates. Their exposure was distinguished by the contact fractions; high end exposed individuals grew a larger proportion of their intake in their home gardens.

The average amounts of fruit and vegetable consumption are 200 and 140 g/day, respectively. These total ingestion rates are further refined considering two factors pertinent to estimation of concentration of dioxin-like compounds: whether the vegetation is grown below (carrots, e.g.) or above ground (tomatoes), and whether the edible portion is protected (citrus) or unprotected (apples). Chapter 4 discusses distinct procedures for estimating vegetative concentrations for below and above ground vegetation. Also, both algorithms assume that inner portions of vegetation are largely unimpacted, whereas outer portions of both above and below ground vegetation are impacted (see Chapter 4 for further detail on these algorithms and assumptions). Therefore, for fruits or vegetables which are protected, it can be assumed that there will be no exposure since the outer portions are not eaten. Results from a food consumption survey, such as that from Pao, et al. (1982), can be used to determine percent of total fruit/vegetable intake which is below/above ground and which is protected/unprotected.

Such an exercise was undertaken using data from Pao, et al. (1982) summarized in EPA (1989a) to arrive at the following percentages:

Diagram V3 2-1

As seen, it was found that there are no fruit grown underground, and there was a fairly similar proportion of protected and unprotected fruit. Fruits considered protected for this exercise included oranges, grapefruits, and cantaloupe; unprotected fruits included apples, peaches, pears, and strawberries. It is noted that this is clearly not a complete inventory, but only those fruits from the survey of Pao as summarized in EPA (1989a). Similarly, these percentages are not being recommended as general values for other site-specific assessments. For this assessment, it will be assumed that a total ingestion rate of unprotected above ground fruit is 88 g/day (0.44*200 g/day), and that there is no ingestion of unprotected under ground fruit. Vegetables above ground and unprotected include: cabbage, cucumbers (including cucumbers as pickles), lettuce, tomatoes, broccoli, spinach, string beans, and squash. Above ground protected vegetables include: corn, lima beans, and peas (several kinds). Below ground unprotected vegetables included potatoes and carrots; mature onions were considered below ground and protected. Assumed for this assessment are ingestion rates of unprotected above ground vegetables of 76 g/day (0.54*140 g/day) and unprotected below ground vegetables of 28 g/day (0.20*140 g/day).

These ingestion rates are defined as total ingestion rates of unprotected above/below ground fruits/vegetables. Only a portion of these are homegrown. Data summarized in EPA (1989a) shows that the fraction of vegetables consumed that are homegrown ranges from 0.04 to 0.75, depending on type. The overall average of the data is 0.25, which is recommended as a contact fraction for the average home gardener. The recommendation for the high end exposure was 0.40. These contact fractions were adopted as the central and high fractions for the example scenarios in Chapter 5. Similar data for fruits show a homegrown range of 0.09 to 0.33, with an average of 0.20, which is the central estimate used in Chapter 5. EPA (1989a) recommends a high end value of 0.30, which is the value used for the high end exposure scenarios.

REFERENCES FOR CHAPTER 2

  • Binder, S.; Sokal, D.; Maughn, D. (1986) The use of tracer elements in estimating the amount of soil ingested by young children. Arch. Environ. Health 41: 341-345.
  • Clausing, P.; Brunekreff, B.; Van Wijen, J.H. (1987) A method for estimating soil ingestion by children. Int. Arch. Occupational Environ. Health 59: 73-82.
  • Farland, W.H. (1987) Memorandum titled, "Absorption fraction when calculating upper-limit risks due to dioxin exposure", dated September 2, 1987, to Michael Callahan, Exposure Assessment Group, Washington, DC. from William Farland, U.S. Environmental Protection Agency, Office of Health and Environmental Assessment, Washington, D.C.
  • Fries, G.F. (1986) Assessment of potential residues in foods derived from animals exposed to TCDD-contaminated soil. Presented at 6th international symposium on chlorinated dioxins and related compounds; September; Fukuoka, Japan.
  • Fries, G.F.; Marrow, G.S. (1975) Retention and excretion of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) by rats. J. Agric. Food Chem. 23: 265-269.
  • Hawley, J.K. (1985) Assessment of health risk from exposure to contaminated soil. Risk Analysis 5(4): 289-302.
  • Pao, E.M., K.H. Fleming, P.M. Guenther, et al. (1982) Foods commonly eaten by individuals: amount per day and per eating occasion. U.S. Department of Agriculture. Home Economics Report No. 44.
  • Paustenbach, D.J. (1987) Assessing the potential human health hazards of dioxin contaminated soils. Commun. Toxicol. 1:185-220.
  • Pierce, R.S.; Noviello, D.T.; Rogers, S.H. (1981) Commencement Bay seafood consumption report. Preliminary report. Tacoma, WA: Tacoma-Pierce County Health Department.
  • Puffer, H. (1981) Consumption rates of potentially hazardous marine fish caught in the metropolitan Los Angeles area. EPA Grant #R807 120010.
  • Rose, J.Q.; Ramsey, J.C.; Wentzler, T.H. (1976) The fate of 2,3,7,8-tertrachloridbenzo-p-dioxin following single and repeated oral doses to the rat. Toxicol. Appl. Pharmacol. 36: 209-226.
  • Sheenan, P.J.; Meyer, D.M.; Sauer, M.M.; Paustenbach, D.J. (1991) Assessment of the human health risks posed by exposure to chromium-contaminated soils. J. Toxicol. Environ. Health 34: 161-201.
  • U.S. Department of Agriculture. (1983) Food Consumption: Households in the U.S., Seasons and Year 197-1978. Consumer Nutrition Division, Hyattsville, MD. NFCS 1977-78. Report No. H-6. USDA Nationwide Food Consumption Survey.
  • U.S. Department of Agriculture. (1966) Household food consumption survey 1965-1966. Report 12. Food Consumption of households in the U.S., Seasons and years 1965-1966. United States Department of Agriculture, Washington, D.C. U.S. Government Printing Office.
  • U.S. Environmental Protection Agency. (1981) Risk assessment on (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T), (2,4,5-trichlorophenoxy)propionic acid (silvex), and the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Office of Health and Environmental Assessment. EPA-600/6-81-003. NTIS PB81-234825.
  • U.S. Environmental Protection Agency. (1984a) Risk Analysis of TCDD Contaminated Soil. Office of Health and Environmental Assessment. EPA-600/8-84-031.
  • U.S. Environmental Protection Agency. (1984b) Stochastic processes applied to risk analysis of TCDD contaminated: a case study. Internal report dated May 31, 1984; Exposure Assessment Group, Office of Health and Environmental Assessment, Office of Research and Development, U.S. Environmental Protection Agency.
  • U.S. Environmental Protection Agency. (1985) Dioxin Transport From Contaminated Sites to Exposure Locations: A Methodology for Calculating Conversion Factors. Office of Health and Environmental Assessment, EPA-600/8-85-012. NTIS PB85-214310.
  • U.S. Environmental Protection Agency. (1987) National Dioxin Study. Office of Solid Waste and Emergency Response. EPA/530-SW-87-025. August, 1987.
  • U.S. Environmental Protection Agency. (1989a) Exposure Factors Handbook. Office of Health and Environmental Assessment, EPA/600/8-89/043. July, 1989.
  • U.S. Environmental Protection Agency. (1989b) Interim procedures for estimating risks associated with exposures to mixtures of chlorinated dibenzo-p-dioxins and -dibenzofurans (CDDs and CDFs) and 1989 update. Risk Assessment Forum, Washington, DC; EPA/625/3-89/016.
  • U.S. Environmental Protection Agency. (1992a) Dermal Exposure Assessment: Principals and Applications. Office of Health and Environmental Assessment EPA/600/8-91/011B.
  • U.S. Environmental Protection Agency. (1992b) Guidelines for exposure assessment. Office of Health and Environmental Assessment. EPA/600-Z-92/001. published in Federal Register, May 29, 1992, p. 22888-22938.
  • U.S. Environmental Protection Agency. (1992c) Environmental Equity. Reducing Risk for all Communities. Vol II. Office of Policy Analysis. EPA 230-R-92-008A.
  • U.S. Environmental Protection Agency. (1994) Health Assessment for 2,3,7,8-TCDD and Related Compounds. External Review Draft. EPA/600/BP-92/001a-c.
  • Wolfe, R.J.; Walker, R.J. (1987) Subsistence economica in Alaska: productivity, geography and developmental impacts. Arctic Anthropology 24(2):56-81.