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Chapter 3: Exposure Scenario Selection Revised October 2008 

Chapter 3 

Exposure Scenario Selection 

What’s covered in Chapter 3: 

�	 Recommended Exposure Scenarios 

- Adult Resident 

- Child Resident 

�	 Exposure Scenario Locations 

�	 Quantifying Exposure 

This chapter summarizes the exposure scenarios selected for U.S. EPA risk assessment evaluation of wastes 

petitioned for delisting. The purpose of this chapter is to identify the types of human exposure to waste 

constituent emissions associated with the selected waste management scenarios (landfill and surface 

impoundment). The chapter addresses both identification of recommended exposure scenarios and selection 

of standardized exposure scenario locations. 

An exposure scenario is a combination of exposure pathways through which a single receptor may be exposed 

to a waste constituent. Receptors may come into contact with waste constituent emissions associated with 

the selected waste management scenarios via two primary exposure routes, either (1) directly via inhalation 

or ingestion of water or (2) indirectly via subsequent ingestion of soil and foodstuffs (such as fish) that 

become contaminated by waste constituents through the food chain. Receptors may also be exposed to waste 

constituents released from a waste management unit to surface media (for example, via volatilization to air 

or via windblown particulate matter) or to groundwater (for example, via ingestion of groundwater). 

Exposure pathways represent combinations of receptors and exposure routes. Each exposure pathway 

consists of four fundamental components: (1) a source and mechanism of waste constituent release (see 

Chapter 2); (2) a retention medium, or a transport mechanism and subsequent retention medium in cases 

involving media transfer of waste constituents (see Chapter 2); (3) an exposure route; and (4) a point of 

potential human contact with the contaminated medium, which is referred to as the POE and involves 

exposure of a specific receptor at a specific point. 

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The exposure scenarios recommended for evaluation in this chapter are generally conservative in nature and 

are not intended to be entirely representative of actual scenarios at all sites. Rather, they are intended to allow 

standardized and reproducible evaluation of risks across most sites and land use areas. Conservatism is 

incorporated to ensure protection of potential receptors not directly evaluated, such as special subpopulations, 

and land uses that are region-specific. The U.S. EPA Delisting Program believes that the recommended 

exposure scenarios and associated assumptions presented in this chapter are reasonable and conservative and 

that they represent a scientifically sound approach that allows protection of human health and the environment 

while recognizing the uncertainty associated with evaluating real-world exposure. Unless site-specific 

conditions warrant an exception approved by the delisting authority, the U.S. EPA Delisting Program 

recommends that these scenarios be used, at a minimum, in an initial site evaluation to identify primary risk 

concerns. Any exceptions, particularly deletion or modification of a recommended exposure scenario, should 

be well documented and approved by the delisting authority. 

The following sections discuss (1) the default exposure scenarios, (2) selection of the POEs to be evaluated 

in the risk assessment, and (3) quantification of exposure. 

3.1 DEFAULT EXPOSURE SCENARIOS 

Adult and child residents are the two receptors evaluated in this analysis. These receptors are discussed 

below along with special subpopulation characteristics. 

3.1.1 Adult Resident 

The adult resident exposure scenario is evaluated to account for the combination of exposure pathways to 

which an adult receptor may be exposed in an urban or rural (non-farm) setting. The adult resident is 

assumed to be exposed to waste constituents from an emission source through the following exposure 

pathways: 

C Surface Pathways 

- Direct inhalation of vapors and particles 

- Ingestion of fish 

- Ingestion of drinking water from surface water sources 

C Groundwater Pathways 

- Ingestion of drinking water from groundwater sources 

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- Dermal absorption from groundwater sources via bathing 

- Inhalation from groundwater sources via showering 

Further discussion of these exposure pathways, including numerical equations, parameter values, and waste 

constituent-specific inputs, can be found in Chapter 4. No other exposure pathways are evaluated for the 

adult resident exposure scenario. 

3.1.2 Child Resident 

For child receptors, the U.S. EPA Delisting Program evaluates two exposure pathways: (1) the dermal 

absorption while bathing with groundwater pathway and (2) the ingestion of soil contaminated with air 

particulate pathway. Child residents (1 to 6 years old) were not selected as receptors for the groundwater 

ingestion and inhalation pathways, the surface water pathways, or the direct air inhalation pathways because 

the adult resident receptor scenario has been found to be protective of children with regard to these pathways 

(U.S. EPA 1995a, 1995b). For most routes of exposure (e.g., drinking water ingestion, air inhalation, food 

ingestion), the intake rate remains relatively proportional to body weight throughout the lifetime. Child 

intake rates may be slightly higher (U.S. EPA 1990f, 1997b). However, the exposure duration for children 

would be considerably shorter than for adults. After consideration of the weighted average of exposure over 

a lifetime, the adult evaluation becomes the more sensitive scenario and no increased adverse effects would 

be seen by considering the child exposure pathway. The dermal absorption pathway and the soil ingestion 

pathway are two exceptions. 

The dermal absorption while bathing with groundwater exposure pathway is evaluated differently for child 

residents than it is for adult residents because of the following considerations: (1) the ratio of exposed skin 

surface area to body weight is slightly higher for children than for adults, resulting in a slightly larger average 

daily exposure for children than for adults; and (2) the exposure duration for such children is limited to 6 

years, thus lowering the lifetime average exposure to carcinogens. Typically, the adult scenario is more 

protective with regard to carcinogens (because of the longer exposure duration), and the child scenario is 

more protective with regard to noncarcinogens (because of the greater skin surface area to body weight ratio) 

(U.S. EPA 1995b). 

For soil ingestion, the receptor is a resident who ingests soil from childhood through adulthood. Young 

children (to 6 years) typically ingest a greater quantity of soil than adults while at the same time have a much 

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smaller average body weight than an adult. This results in a greater average daily exposure for children than 

adults. However, the exposure duration for the child is limited to the 6 years that a young individual would 

fall into the appropriate age range, thus lowering lifetime average exposure for carcinogens. Therefore, OSW 

policy for assessing exposure via soil ingestion to carcinogenic constituents is to consider exposure to a single 

individual through childhood and adulthood for soil ingestion exposures. This practice accounts for both the 

greater average daily dose of childhood and the longer exposure duration of adulthood. For noncarcinogens, 

the greater average daily dose has the stronger influence on the effects assessment and the child exposure 

pathway is considered. Further discussion of the exposure pathways, including algorithms, parameter values, 

and waste constituent-specific inputs, can be found in Chapter 4. 

3.1.3 Special Subpopulation Characteristics 

Special subpopulations are defined as human receptors or segments of the population that may be at higher 

risk because of receptor sensitivity to waste constituents (for example, the elderly, infants and children, and 

fetuses of pregnant women). The assumptions and methodology specified in the DTSD to complete the risk 

assessment (such as the conservative EPACMTP assumptions discussed in Section 2.2.2 and the use of 

reference doses [RfD] developed to account for toxicity to sensitive receptors) have been developed to 

protect human health, including that of special subpopulations. 

3.2 EXPOSURE SCENARIO LOCATIONS 

This section describes the exposure locations (POEs) for the groundwater, surface water, and air pathways 

(Sections 3.2.1, 3.2.2, and 3.2.3, respectively). A delisted waste may be disposed of by the petitioner in any 

authorized landfill or surface impoundment without regulatory control (see Section 1.2.1); therefore, the 

POEs selected for the risk assessment of a waste petitioned for delisting are individually standardized. 

3.2.1 Downgradient Groundwater-Drinking Water Well POE 

The EPACMTP, a probabilistic groundwater fate and transport model, is used to predict groundwater 

constituent concentrations at a hypothetical receptor well located downgradient from a waste management 

unit (as described in Section 2.2). This receptor well represents the POE at which potential exposure to the 

groundwater is measured; that is, the estimated constituent concentration at the POE is used to assess risk. 

The distance to the well is based on the results of a U.S. EPA OSW survey performed to determine the 

distance to the nearest drinking water well downgradient from municipal landfills (U.S. EPA 1988b). The 

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survey data are entered in the EPACMTP model as an empirical distribution: minimum = 0 m, median = 427 

m, and maximum = 1,610 m (approximately 1 mile) (U.S. EPA 1997e). At most of the facilities included in 

the survey, the direction of ambient groundwater flow is not precisely known; therefore, it cannot be 

ascertained whether the nearest receptor well is located along the plume center line or even within the plume. 

To reflect uncertainties and variations in the location of the receptor well in relation to the direction of 

ambient groundwater flow, the modeled well is located anywhere on the downgradient side of the waste 

management unit within a 1-mile radius (U.S. EPA 1997e). In contrast to the 1990 TC Rule (U.S. EPA 

1990d), there is no requirement that the well lie within the leachate plume. 

For carcinogenic waste constituents, the exposure concentration is defined as the maximum 30-year average 

receptor well concentration; for noncarcinogens, the exposure concentration is taken to be the highest receptor 

well concentration during the modeled 10,000-year period (U.S. EPA 1996b). A 10,000-year limit was 

imposed on the exposure period; that is, the calculated exposure concentration is the peak or highest 30-year 

average concentration occurring within 10,000 years following the initial release from the waste management 

unit (U.S. EPA 1996b). The Monte Carlo fate and transport simulation provides a probability distribution 

of receptor well concentrations as a function of expected leachate concentration. Regulatory leachate 

concentration limits were determined using a back-calculation procedure corresponding to a 90th percentile 

protection level. These leachate concentration limits are the maximum leachate concentrations at which 

groundwater concentrations at 90 percent of the hypothetical downgradient receptor wells are not expected 

to exceed the given toxicity benchmark (usually either an MCL or HBN value) (U.S. EPA 1996b). 

3.2.2 Downgradient Surface Water Body POE 

Human exposure routes for surface water include ingestion of surface water used as drinking water and 

ingestion of fish from nearby surface water bodies. For the surface water ingestion exposure route, the 

surface water POE modeled is a fifth-order stream 100 m from the waste management unit (U.S. EPA 1993c). 

Fifth-order streams were chosen for analysis because U.S. EPA assumes that a fifth-order stream is the 

smallest stream capable of serving as a community water supply (U.S. EPA 1993c). The assumption of a 

100-m distance to the nearest surface water body is a conservative assumption based on available data. A 

U.S. EPA survey of municipal landfill facilities showed that 3.6 percent of the surveyed facilities are located 

within 1 mile of a river or stream and that the average distance from these facilities to the closest river or 

stream is 586 m (U.S. EPA 1988b). 

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For the fish ingestion exposure route, a second-order stream was chosen for analysis. This stream segment 

was determined to be the smallest stream capable of supporting fisheries (U.S. EPA 1993c). The surface 

water body POE for collection of fish is assumed to be 100 m downgradient from the disposal facility (U.S. 

EPA 1993c). 

3.2.3 Downwind POE 

Human exposure to emissions of windblown particulate from landfills and to emissions of volatiles from 

landfills and surface impoundments includes inhalation of these particulate and volatiles. These are the 

exposure routes evaluated for the U.S. EPA Delisting Program. For the air pathway, the POE is 305 m (1,000 

feet) downwind of the facility (U.S. EPA 1993c, 1994a). 

3.3 QUANTIFICATION OF EXPOSURE 

This section describes the factors to be evaluated in quantifying the exposure received under each of the 

exposure scenarios outlined above. The calculation of constituent-specific exposure rates for each exposure 

pathway evaluated is based on (1) the estimated concentration in a given medium as calculated in Chapter 

2, (2) the contact rate, (3) receptor body weight, and (4) the frequency and duration of exposure. This 

calculation is repeated for each constituent and for each exposure pathway included in an exposure scenario. 

Exposure pathway-specific equations are presented in the appropriate sections of Chapter 4. Sections 3.3.1 

through 3.3.8 describe a generic exposure rate calculation and the exposure pathway-specific variables that 

may affect this calculation. The default exposure parameters (e.g., body weight, skin surface area) used in 

RCRA delisting risk assessment are reported in the US EPA Exposure Factors Handbook (EFH) (U.S. EPA 

1990f, 1997b). 

3.3.1 Generic Exposure Rate Equation 

Exposure to hazardous constituents is assumed to occur over a period of time. To calculate an average 

exposure per unit of time, the total exposure can be divided by the time period. In this analysis, the exposures 

are intended to represent reasonable maximum exposure (RME) estimates for each applicable exposure route. 

The RME approach is intended to combine upper-bound and mid-range exposure factors so that the result 

represents an exposure scenario that is both protective and reasonable, not the worst possible case (U.S. EPA 

1991a). All exposures quantified in this analysis are (1) normalized for time and body weight, (2) presented 

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in units of milligrams per kilogram of body weight per day, and (3) termed “intakes.” Equation 3-1 is a 

generic equation used to calculate constituent dose (U.S. EPA 1997b). 

(3-1) 

where: 

I =	 intake — the amount of constituent at the exchange boundary (mg/kg/day); for evaluating 

exposure to noncarcinogenic constituents, this intake is referred to as the average daily 

dose (ADD); for evaluating exposure to carcinogenic constituents, this intake is referred 

to as the lifetime average daily dose (LADD) 

Cgen = generic constituent concentration — the media average concentration contacted over the 

exposure period (for example, mg/kg for soil and mg/L for water) 

CR gen = generic contact rate — the amount of contaminated medium contacted per unit of time or 

per event (for example, kg/day for soil and L/day for water) (upper-bound value) 

EF = exposure frequency (days/yr) (upper-bound value) 

ED = exposure duration (yrs) (upper-bound value) 

BW = body weight of the receptor over the exposure period (kg) (average value) 

AT = averaging time — the period over which the exposure is averaged (days); for carcinogens, 

the averaging time is 27,375 days, based on a lifetime exposure of 75 yrs; for 

noncarcinogens, averaging time equals exposure duration (in yr) multiplied by 365 days/yr 

3.3.2 Contact Rate 

The contact rate is the amount of contaminated medium contacted per unit of time or event. Contact rates 

for subsistence food types (for example, fish for the fish ingestion pathway), are assumed to be 100 percent 

from the hypothetical assessment area (for example, water body). The following sections describe exposure 

pathway-specific considerations regarding contact rate. 

3.3.2.1 Groundwater and Surface Water Exposure Pathway Parameters 

This section describes the following variables specific to the groundwater and surface water exposure 

pathways: drinking water intake, other contact with groundwater, individual inhalation rate, and individual 

skin surface area. 

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Drinking Water Intake 

For groundwater and surface water ingestion, the intake rate is assumed to be 2.0 L/day, the average amount 

of water that an adult ingests (U.S. EPA 1991a). This value, which is currently used to set drinking water 

standards, is close to the current 90th percentile value for adult drinking water ingestion (2.3 L/day) reported 

in the US EPA Exposure Factors Handbook (EFH) (U.S. EPA 1997b). Also, this value is comparable to the 

8 glasses of water per day historically recommended by health authorities (U.S. EPA 1991a). 

Other Contact with Groundwater 

For the groundwater pathway of dermal exposure during bathing, the contact with water is assumed to occur 

in the shower. In this analysis, it is assumed that the average adult resident is in contact with groundwater 

during bathing for 0.25 hour per event and that the average child resident is in contact with groundwater 

during bathing for 0.33 hour per event, with one event per day (U.S. EPA 1991a, 1997b). 

For the groundwater pathway of inhalation exposure during showering, the contact with water is assumed to 

occur principally in the shower and in the bathroom, although an additional longer exposure to much lower 

concentrations in the house following shower and bathroom use is also included. In this analysis, it is 

assumed that the average adult resident spends 11.4 minutes per day in the shower, an additional 48.6 minutes 

per day in the bathroom, and another 900 minutes (15 hours) in the house after shower and bathroom use 

(U.S. EPA 1991a, 1997b). 

Individual Inhalation Rate 

Daily inhalation rates vary depending on activity, gender, age, and so on (U.S. EPA 1997b). Citing a need 

for additional research, the latest EFH does not recommend a reasonable upper-bound inhalation rate value 

3 (U.S. EPA 1997b). The EFH-recommended value for the average inhalation rate is 15.2 m /day for males

3 3 and 11.3 m /day for females (U.S. EPA 1997b). The inhalation rate of 20 m /day has been commonly used 

in past U.S. EPA risk assessments (U.S. EPA 1997b). The upper-bound value for an individual’s inhalation 

3 3 rate has been established as 0.83 m /hr (20 m /day) (U.S. EPA 1991a). This value was derived by combining 

inhalation rates for indoor and outdoor activities in a residential setting (U.S. EPA 1991a). A conservative 

3 inhalation rate of 20 m /day has been selected for use in this analysis.

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Individual Skin Surface Area 

For dermal bathing exposure to contaminated groundwater, the selected receptors are an adult and a young 

child (1 to 6 years old). During bathing, generally all of the skin surface is exposed to water. The total adult 

2 2 body surface area can vary from about 17,000 to 23,000 cm . The EFH reports a value of 20,000 cm as the 

2 median value for adult skin surface area (U.S. EPA 1997b). A value of 6,900 cm has been commonly used

for a child receptor in U.S. EPA risk assessments; this value is approximately the average of the median 

values for male children aged 2 to 6 years (U.S. EPA 1992b). The EHF presents a range of recommended 

values for estimates of the skin surface area of children by age (U.S. EPA 1997b). The mean skin surface 

2 2 area at the median for boys and girls 5 to 6 years of age is 0.79 m or 7,900 cm (U.S. EPA 1992b and 1997b). 

Given that the age for children is defined as 0 to 6 years (see Section 3.3.4), a skin surface area value for ages 

5 to 6 years would be a conservative estimate of skin surface area for children. For calculation of dermal 

exposure to waste constituents, a value of 7,900 cm2

 is assumed for the skin surface area of children, and a 

value of 20,000 cm2

 is assumed for the skin surface area of adults. 

The major factors that must be considered when estimating the intake associated with dermal exposure to 

groundwater (that is, the absorbed dose per event or DA ) are the constituent concentration in contact with event 

the skin, the surface area of the skin exposed, the duration of exposure, the absorption of the constituent 

through the skin, and the amount of constituent that can be delivered to a target organ. A detailed discussion 

of these factors can be found in “Guidelines for Exposure Assessment” (U.S. EPA 1992a) and “Dermal 

Exposure Assessment: Principles and Applications” (U.S. EPA 1992b). Detailed information concerning 

the algorithms and parameter values used to calculate DA is provided in Chapter 4. event 

3.3.2.2 Air Exposure Pathway Parameters 

For air exposure pathways, receptors are assumed to be directly exposed to constituents as a result of normal 

respiration. Inhalation of vapors and particulate will be influenced by the relative amount of time that a 

receptor spends indoors. Although vapors entering buildings as a result of air exchange are likely to remain 

airborne and therefore may be inhaled, particulate entering these buildings are more likely to settle out and 

not be inhaled. However, for the purposes of this analysis, it is assumed that vapor and particulate may be 

inhaled throughout the day, both indoors and outdoors. Again, it is assumed that the upper-bound estimate 

3 3 respiration rate for an individual is 0.83 m /hr (20 m /day) (U.S. EPA 1991a, 1997d). 

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The U.S. EPA Delisting Program also considers child ingestion of soil contaminated with air-deposited 

particulate windblown from a nearby landfill. Two different receptors are used for carcinogens and 

noncarcinogens. For carcinogens, exposure in both childhood and adulthood is considered for the same 

individual. For noncarcinogens, only exposure in childhood is considered. For carcinogens, exposure is 

averaged over a lifetime (therefore, the averaging time is 75 years, the average lifetime recommended in the 

latest EFH). Due to the long-term cumulative effect of carcinogens, exposure to adults is usually assumed. 

For most routes of exposure (e.g., drinking water ingestion, air inhalation, food ingestion), the intake rate 

remains relatively proportional to body wight throughout the lifetime. Child intake rates may be slightly 

higher. After consideration of the weighted average of exposure over a lifetime, little or no effect would be 

seen by considering child exposure. However, the intake rate of soil is inversely proportional to body weight, 

because children to 6 years of age are considered more likely to ingest greater quantities of soil than adults 

and their body weight is a fraction of an adult. Exposure to contaminants via soil ingestion is calculated as 

a weighted average of exposure for 6 years as a child and as an adult. The total exposure for carcinogenic 

risk is still averaged over a lifetime. For noncarcinogens, the daily intake is back-calculated by direct 

comparison to the reference dose (RfD). The RfD being the dose to which a person may be exposed daily 

over a lifetime or significant portion of a lifetime without adverse effect. Because the RfD is a exposure dose, 

and because the daily exposure dose from soil ingestion is likely to be considerably higher for children than 

adults, the daily intake (and therefore the delisting level) is calculated for children. The mean soil ingestion 

values for children range from 39 to 271 mg/day, with an average of 146 mg/day for soil ingestion and 191 

mg/day for soil and dust ingestion (U.S. EPA 1989c, 1997b). 

This group of mean values is consistent with the 200-mg/day value that U.S. EPA programs have used as a 

conservative mean estimate. However, considering pica behavior (deliberate ingestion of soil by children), 

the upper percentile recommended value for soil ingestion by children is 400 mg/day (U.S. EPA 1997b). 

Because the prevalence of pica behavior is not known, and because the EFH states that 200 mg/day may be 

used as a conservative estimate of the mean, a delisting risk assessment uses 200 mg/day as the soil ingestion 

rate for children. 

3.3.3 Exposure Frequency 

An exposure frequency of 350 days per year is applied to all exposure scenarios (U.S. EPA 1991a, 1997d). 

Until better data become available, the common assumption that residents take 2 weeks of vacation per year 

is used to support a value of 15 days per year spent away from home (leaving 350 days per year spent at 

home) (U.S. EPA 1991a). 

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3.3.4 Exposure Duration 

The exposure duration reflects the length of time that an exposed individual may be expected to reside near 

the constituent source. For the adult resident, this value is taken to be 30 years, and for the child resident, 

this value is taken to be 6 years (U.S. EPA 1997d). The adult resident is assumed to live in one house for 

30 years, the approximate average of the 90th percentile residence times from two key population mobility 

studies (U.S. EPA 1997b). For the child resident, the exposure duration is assumed to be 6 years, the 

maximum age of the young child receptor (U.S. EPA 1997d). For carcinogens, exposures are combined for 

children (6 years) and adults (24 years). 

3.3.5 Averaging Time 

For noncarcinogenic constituents, the averaging time (AT) equals the exposure duration in years multiplied 

by 365 days per year. For an adult receptor, the exposure duration is 30 years, and for a child receptor, the 

exposure duration is 6 years (U.S. EPA 1991a). For carcinogenic constituents, the AT has typically been 

25,550 days, based on a lifetime exposure of 70 years at 365 days per year (U.S. EPA 1991a). The life 

expectancy value in the 1997 EFH is 75 years (U.S. EPA 1997b). Given this life expectancy value, the AT 

for a delisting risk assessment is 27,375 days, based on a lifetime exposure of 75 years at 365 days per year. 

3.3.6 Body Weight 

The choice of body weight for use in the risk characterization equations presented in Chapter 5 depends on 

the definition of the receptor at risk, which in turn depends on exposure and susceptibility to adverse effects. 

“Risk Assessment Guidance for Superfund (RAGS)” (U.S. EPA 1989) defines the body weight of the receptor 

as either adult weight (70 kg) or child weight (1 to 6 years, 15 kg) on the basis of data presented in 

“Evaluation and Selection of Models for Estimating Air Emissions from Hazardous Waste Treatment, 

Storage, and Disposal Facilities” (U.S. EPA 1984). However, most U.S. EPA guidance defines the child (1 

to 6 years) weight as 15 kg (U.S. EPA 1991a). The EFH-recommended value of 71.8 kg for an adult differs 

from the 70-kg value commonly used in U.S. EPA risk assessments (U.S. EPA 1997b). The Integrated Risk 

Information System (IRIS) uses the 70-kg body weight assumption in deriving cancer slope factors and unit 

risks. In keeping with the latest EFH recommendation, DRAS uses a 72-kg adult weight and a 15-kg child 

weight in delisting risk assessments. 

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The daily intake for an exposure pathway is expressed as the dose rate per body weight. Because children 

have lower body weights, typical ingestion exposures per body weight, such as those for soil and foods, are 

higher for children. Therefore, average daily exposure normalized to body weight would be greater for 

children than adults. However, when factoring in the exposure duration for adults over children, the overall 

lifetime exposure would be less and the actual risk/hazard would be greater for adults. Use of two body 

weights may not account for significant differences between the weights of infants and toddlers or the weights 

of teenagers and adults. However, average body weight, not actual chronological age, defines a child in this 

context; obviously, the weight of a child changes significantly over the first 6 years of life. The average 

weight used is assumed to be a realistic average estimate for all 6 years that overestimates the weight of the 

child for the early years and then underestimates it for the later years (U.S. EPA 1997b). 

3.3.7 Fish Consumption 

Factors that affect human exposure via ingestion of fish from a surface water body include (1) sediment and 

water waste constituent concentrations and (2) the percentage of dietary fish caught in the surface water body 

affected by the waste management unit. The amount of fish consumed also affects exposure because people 

who eat large amounts of fish will tend to have higher exposures. Fish consumption rates vary greatly, 

depending on geographic region and social or cultural factors. Because 100 percent of a receptor’s dietary 

fish may not originate from the surface water body near the waste management unit, the percentage of locally 

caught dietary fish is also a variable that must be considered in estimating exposure. 

The EFH-recommended value for fish consumption for all fish is 0.28 g fish/kg body weight/day for an 

average adult (72 kg) (U.S. EPA 1997b). This value equates with a fish consumption rate of 20.1 g/day for 

all fish. For delisting risk assessments, U.S. EPA Region 6 estimates that an exposed individual eats 20 g of 

fish per day, representing one 8-ounce serving of fish approximately once every 11 days. 

3.3.8 Age-adjusted Contact Rates 

Because contact rates may be different for children and adults, carcinogenic risks during the first 30 years of 

life were calculated using age-adjusted factors (adj). Use of age-adjusted factors is especially important for 

soil ingestion exposures, which are higher during childhood and decrease with age. However, for the 

purposes of combining exposures across pathways, additional age-adjusted factors are used for inhalation and 

dermal exposures. These factors approximate the integrated exposure from birth until age 30 and combine 

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contact rates, body weights, and exposure durations for two age groups: small children and adults. Ageadjusted factors are either obtained from RAGS Part B or developed by analogy. 

For soils, noncarcinogenic waste constituents are evaluated for children separately from adults; no ageadjusted factor is used in this case. The focus on children is considered to be protective because of the higher 

daily rate of soil intake by children and their lower body weight. 

Equation 3-2, 3-3, and 3-4 are used to develop age-adjustment factors for soil ingestion, inhalation, and water 

ingestion, respectively. 

Soil ingestion ([mgAyr]/[kgAd]): 

(3-2) 

Inhalation ([m 3Ayr]/[kgAdays]): 

(3-3) 

Water ingestion ([LAyr]/[kgAd]): 

(3-4) 

where: 

Default 

IFSadj = Ingestion factor, soils ([mgAyr]/[kgAdays]) 113 (U.S. EPA 1991a) 

ED c = Exposure duration, child (yrs) 6 (U.S. EPA 1991a) 

IRS c = Soil ingestion rate, child (mg/day) 200 (U.S. EPA 1997b) 

BWc = Body weight, child (kg) 15 E (U.S. EPA 1997b) 

ED r = Exposure duration, residential (yrs) 30 (U.S. EPA 1991a) 

IRS a = Soil ingestion rate, adult (mg/day) 100 (U.S. EPA 1997b) 

BWa = Body weight, adult (kg) 72 (U.S. EPA 1997b) 

IFAadj = Inhalation factor ([m 3

Ayr]/[kgAdays]) 10.7 (U.S. EPA 1991a) 3 IRA c = Inhalation rate, child (m /day) 10 (U.S. EPA 1989a) 

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3 IRA a = Inhalation rate, adult (m /day) 20 (U.S. EPA 1991a) 

IFWadj = Ingestion factor, water ([LAyr]/[kgAdays]) 1.07 (U.S. EPA 1991a) 

IRW a = Drinking water ingestion rate, adult (L/day) 2 (U.S. EPA 1989a) 

IRW c = Drinking water ingestion rate, child (L/day) 1 (U.S. EPA, 1991a) 

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