Appendix B

Human Anatomical and Intake Factors Used in the Exposure Assessment Calculations

In constructing dose models one needs to define the characteristics of individuals in various age/gender categories and the characteristics of the microenvironments in which they live or from which they obtain water and food. Appendix B defines the types of anatomical and activity data needed to carry out the exposure/dose assessment and explains how these data are obtained. For all factors used in this assessment, we define both an arithmetic mean value and a coefficient of variation (CV), which is the arithmetic standard deviation divided by the arithmetic mean.

B.1 Body Weight

We calculate the arithmetic-mean body weight and CV of body weight for three age groups—infant, child, and adult. The body weight for infants is needed for estimating exposures to contaminants in breast milk. Body weight for adults and children are needed to calculate lifetime-average contact rates per unit body weight and contact rates for an exposure duration of less than a full lifetime. The child age category applies to ages 0 to 15 years, but excludes breast-fed babies. General data on body-weight distributions by age and gender are available from the ICRP [1975], the U.S. EPA [1985, 1989a] and Najjar and Roland [1987]. Because it provides more details on age-specific variations, we used the Najjar and Roland [1987] data set to develop the mean value and CV of body weight for ages 0 to 1, 0 to 15, 15 to 70, and for lifetime. These values are listed in Table B-1.

Table B-1
Values of Human Anatomical and Intake Properties Used in the Exposure Calculations(a)

Parameter, symbol

Child(b)

Adult(b)

Combined(b)

Units

Body weight of infants
age 0 to 1 y, BW

7.2

(0.3)

--

 

--

--

kg

Body weight, BW

29

(0.24)

71

(0.2)

62

(0.2)

kg

Surface area, SAb

0.032

(0.09)

0.024

(0.06)

0.026

(0.07)

m2/kg

Working breathing rate, BRw

--

--

0.030

(0.3)

--

--

m3/kg-h

Active breathing rate, BRa

0.023

(0.3)

0.018

(0.3)

0.019

(0.3)

m3/kg-h

Resting breathing rate, BRr

0.008

(0.3)

0.006

(0.2)

0.0064

(0.2)

m3/kg-h

Fluid intake, Ifl

0.029

(0.2)

0.020

(0.2)

0.022

(0.2)

L/kg-d

Breast milk intake(c), Ibm

0.11

(1)

--

 

--

 

kg/kg-d

Water intake during recreation, Iflr

0.0007

(1)

0.0007

(1)

0.0007

(1)

L/kg-h

Ingestion of homegrown exposed produce, Iep

0.0016

(0.7)

0.00078

(0.7)

0.00096

(0.7)

kg/kg-d

Ingestion of homegrown unexposed produce, Iup

0.00095

(0.7)

0.00053

(0.7)

0.00062

(0.7)

kg/kg-d

Notes to Table B-1:

(a) Listed are the arithmetic-mean value and (in parentheses) the estimated coefficient of variation (CV), equal to the standard deviation divided by the mean. Body weights are from Najjar and Roland [1987], breathing rates are from ICRP [1975], tap water intakes are from Yang and Nelson [1986] and Ershow and Cantor [1989] and food intakes are from Yang and Nelson [1986].

(b) The child category covers ages 0 to 15, the adult category covers ages 16 to 70, the combined category is used to represent lifetime equivalent exposure and is obtained by multiplying the child category by 15/70, the adult category by 55/70, and then summing these products.

(c) Breast-milk intakes are from Butte et al. [1984] and Whitehead and Paul [1981].


B.2 Body Surface Area

Information on the relation between human body weight and surface area has been published by the ICRP [1975] and the U.S. EPA [1989b]. The EPA [1989b] reports that surface area (SA) in m2 can be estimated as 0.1 times body weight (BW) in kg raised to the 2/3 power. Using this formula, along with methods described in Bevington [1969], we estimate the mean value and standard deviation of surface area per unit body weight, SAb, in m2/kg for children and adults using the formula

 , (B-1)

where the second term is the standard deviation of the surface area-body weight ratio. The resulting surface-area values and CVs are given in Table B-1.

B.3 Breathing Rate

General data on breathing rates by age and gender are available from the EPA [1985, 1989a] and the ICRP [1975]. Values in Table B-1 are taken primarily from the ICRP [1975] with variances estimated by McKone and Daniels [1991]. The working breathing rate is for 8 hours of work and, when combined with 8 hours of breathing at the active rate and 8 hours at the resting rate, gives a daily equivalent intake of 30 m3 for an adult [EPA, 1989b]. Layton [1992] has derived breathing rates that are consistent with the quantities of oxygen needed to metabolize reported dietary intakes of fats, carbohydrates, and proteins. He has shown that the values in Table B-1 could be high by a factor of from 20% to 30%, but this is within the CV reported here.

B.4 Water Ingestion

Tap-water intake includes all household tap water that is consumed as a beverage or used to prepare foods and beverages. Yang and Nelson [1986] have published tap-water and total-fluid intakes in mL/d for the U.S. population by age, sex, and region of the country. Ershow and Cantor [1989] have published population-based estimates of sex-, region-, and season-averaged tap-water intakes per unit body weight by the U.S. population in mL/kg-d. From these two papers, we have derived intakes of tap water in L/kg-d for children, adults, and lifetime equivalent. We also need to determine the amount of incidental ingestion that occurs during water recreation. Based on EPA [1989b] data, we use 0.0007 L/kg-h (CV equal to 1) as the ingestion rate of any surface water during recreational use. The mean values and CVs of tap-water intake and surface water intake are listed in Table B-1.

B.5 Breast-Milk Ingestion

Data on ingestion of breast milk by infants are available in Butte et al. [1984] and Whitehead and Paul [1981]. We calculate the breast-milk ingestion per unit body weight for infants ages 0 to 12 months as 0.11 kg/kg-d with a CV of 0.2. This CV is based on other ingestion factors.

B.6 Ingestion of Homegrown Fruits, Vegetables, and Grains

Ingestion of homegrown foods obtained from gardens in the vicinity of the NTLF are included in the risk assessment. For the food intakes included in the exposure assessment, we calculate here the arithmetic mean and standard deviation of homegrown food intakes per unit body weight for children, adults, and lifetime equivalent exposure, all on body-weight basis. We consider homegrown foods to be those produced on the land associated with a household and consumed within that household. The following food groups are considered in the exposure assessment:

• Leafy vegetables, which include exposed produce such as cabbage, cauliflower, broccoli, celery, lettuce, and spinach;
• Other exposed produce, such as apples, pears, berries, cucumber, squash, grapes, peaches, tomatoes, string beans, etc.;
• Protected produce or root crops, such as carrots, beets, turnips, potatoes, legumes, melons, citrus fruits, etc.; and
• Grains such as wheat, corn, rice, barley, millet, etc.

Total intake of foods in each of these groups is obtained from data compiled by the Nationwide Food Consumption Survey (NFCS) [USDA, 1983]. In this survey, the U.S. Department of Agriculture (USDA) used a stratified probability sample of households in the 48 conterminous states and the District of Columbia in each of four seasons from April 1977 through March 1978. The samples were designed to be representative of the United States and were classified according to geographic regions of the country, geographic divisions within each of the regions, and central city, suburban, and non-metropolitan populations. We calculate total annual average food intakes using results compiled by Yang and Nelson [1986], who analyzed the data from the USDA survey. The variance of intake-per-body-weight ratio used to determine the CV is calculated under the assumption that food intake correlates with body weight to the two-thirds power. The mean and variance of body weight used in this estimate are taken from Table B-1. Listed first in Table B-1 is our estimated total annual average population intake of fruits and vegetables and of grains expressed on a body-weight and age-specific basis.

Limited data are available on the amount of home-grown food that is produced and consumed in California or in the U.S. The U.S. EPA [1989a] has compiled for U.S. households data on the fractions of consumed produce that come from home gardens. For all categories of fruits and vegetables reported in this study, the fraction that is homegrown is in the range 0.04 to 0.75, and for the one grain listed (corn), the average fraction that is homegrown is 0.45 for the 25% of the individuals surveyed who consumed homegrown corn. From these data we estimated that the fraction of consumed fruits and vegetables that are homegrown is 0.24 with a CV of 0.7, and that the fraction of consumed grains (mostly corn) that are homegrown is 0.11 also with a CV of 0.7. These values represent households with home gardens and not necessarily the average of total homegrown consumption in either the U.S. or California.

According to Yang and Nelson (1986), 47% of all consumed produce (fruits and vegetables) consists of leafy vegetables and other exposed produce, which intercept contaminants from the atmosphere. The remaining 53% of fruits and vegetables consists of protected produce or root crops, in which contaminant transfer to the edible portion is primarily by root uptake. All grain crops are assumed to be exposed primarily to air contaminants.

Based on the information provided in the previous paragraphs, we estimated the mean and CV of the ingestion of exposed (above-ground) and unexposed homegrown produce in households near the NTLF. Table B-1 lists our estimates of the mean average annual ingestion of both exposed and unexposed homegrown foods on a body-weight basis for children, for adults, and for a lifetime of exposure. As is the case for other contact rates, both the mean value and the CV are provided.

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