TABLE OF CONTENTS

5. SOIL INGESTION AND PICA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

5.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

5.2 SOIL INTAKE STUDIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

5.2.1 Key Studies of Primary Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

5.2.1.1 Davis et al., 1990 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

5.2.1.2 Calabrese et al., 1997a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5

5.2.1.3 Davis and Mirick, 2006 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7

5.2.2 Relevant Studies of Primary Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 5-8

5.2.2.1 Binder et al., 1986 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-8

5.2.2.2 Clausing et al., 1987 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10

5.2.2.3 Calabrese et al., 1989 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11

5.2.2.4 Van Wïjnen et al., 1990 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13

5.2.2.5 Calabrese et al., 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-14

5.2.2.6 Calabrese et al., 1999 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15

5.2.2.7 Stanek and Calabrese, 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15

5.2.2.8 Stanek et al., 2001b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15

5.2.3 Key Studies of Secondary Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-16

5.2.3.1 Stanek and Calabrese, 1995a . . . . . . . . . . . . . . . . . . . . . . . . . 5-16

5.2.3.2 Stanek and Calabrese, 1995b . . . . . . . . . . . . . . . . . . . . . . . . . 5-18

5.2.4 Relevant Studies of Secondary Analysis . . . . . . . . . . . . . . . . . . . . . . . 5-19

5.2.4.1 Thompson and Burmaster, 1991 . . . . . . . . . . . . . . . . . . . . . . 5-19

5.2.4.2 Calabrese and Stanek, 1992a . . . . . . . . . . . . . . . . . . . . . . . . . 5-20

5.2.4.3 Sedman and Mahmood , 1994 . . . . . . . . . . . . . . . . . . . . . . . . 5-21

5.2.4.4 Calabrese and Stanek, 1995 . . . . . . . . . . . . . . . . . . . . . . . . . . 5-22

5.2.4.5 Stanek et al., 2001a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-23

5.2.4.6 Zartarian et al., 2005 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-24

5.3 PICA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-25

5.3.1 Prevalence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-25

5.3.1.1 General Pica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-25

5.3.1.2 Soil Pica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-25

5.3.2 Soil Pica Among Children . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-26

5.3.2.1 Calabrese et al., 1991 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-26

5.3.2.2 Calabrese and Stanek, 1992b . . . . . . . . . . . . . . . . . . . . . . . . . 5-27

5.3.2.3 Calabrese and Stanek, 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . 5-27

5.3.2.4 Zartarian et al., 2005 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-29

5.4 RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-29

5.5 REFERENCES FOR CHAPTER 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-33
LIST OF TABLES

Table 5-1. Average Daily Soil Ingestion Values Based on Aluminum, Silicon, and Titanium as

Tracer Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-37

Table 5-2. Soil Ingestion Estimates for the Median of Best Four Trace Elements Based on

Food/Soil Ratios for 64 Anaconda Children (mg/day) Using Al, Si, Ti, Y, and Zr . . 5-37

Table 5-3. Dust Ingestion Estimates for the Median of Best Four Trace Elements Based on

Food/Dust Ratios for 64 Anaconda Children (mg/day) Using Al, Si, Ti, Y, and Zr . . 5-38

Table 5-4. Mean and Median Soil Ingestion (mg/day) by Family Member . . . . . . . . . . . . . . 5-38

Table 5-5. Estimated Daily Soil Ingestion Based on Aluminum, Silicon, and Titanium

Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-39

Table 5-6. Calculated Soil Ingestion by Nursery School Children . . . . . . . . . . . . . . . . . . . . 5-40

Table 5-7. Calculated Soil Ingestion by Hospitalized, Bedridden Children . . . . . . . . . . . . . . 5-41

Table 5-8. Mean and Standard Deviation Percentage Recovery of Eight Tracer Elements . . 5-41

Table 5-9. Soil and Dust Ingestion Estimates for Children Ages 1-4 Years . . . . . . . . . . . . . 5-42

Table 5-10. Geometric Mean (GM) and Standard Deviation (GSD) LTM Values for Children at

Daycare Centers and Campgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-43

Table 5-11. Estimated Geometric Mean Ltm Values of Children Attending Daycare Centers

According to Age, Weather Category, and Sampling Period . . . . . . . . . . . . . . . . . . . 5-44

Table 5-12. Distribution of Average (Mean) Daily Soil Ingestion Estimates per Child for 64

Children (mg/day) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-45

Table 5-13. Estimated Distribution of Individual Mean Daily Soil Ingestion Based on Data for

64 Subjects Projected over 365 Days

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-45

Table 5-14. Summary Statistics and Parameters for Distributions of Estimated Soil Ingestion by

Tracer Elementa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-46

Table 5-15. Positive/negative Error (Bias) in Soil Ingestion Estimates in the Calabrese et al.

(1989) Mass-balance Study: Effect on Mean Soil Ingestion Estimate (Mg/day)a . . . 5-47

Table 5-16. Daily Soil Ingestion Estimation in a Soil-Pica Child by Tracer and by Week

(mg/day) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-48
Table 5-17. Ratios of Soil, Dust, and Residual Fecal Samples in the Soil Pica Child . . . . . . 5-49

Table 5-18. Daily Variation of Soil Ingestion by Children Displaying Soil Pica in Wong (1988)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-50

Table 5-19 Key Studies Used to Derive Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . 5-51

Table 5- 20 . Summary of Estimates of Incidental Soil and Dust Ingestion by Children (1-7

years old) from Key Studies (mg/day) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-52

Table 5-21. Summary of Recommended Values for Soil Ingestion . . . . . . . . . . . . . . . . . . . . 5-53

Table 5-22. Confidence in Soil Intake Recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-54
5-1

1 5. SOIL INGESTION AND PICA

2

3 5.1 INTRODUCTION

4

5 The ingestion of soil is a potential source of human exposure to toxicants. Some people

6 are surprised to learn that soil ingestion occurs at all, and others are surprised to learn that some

7 people ingest soil intentionally, due to cravings or cultural practices. Children may ingest

8 significant quantities of soil, due to their tendency to play on floors and on the ground outdoors

9 and due to their mouthing behaviors.

10 At this point in time, knowledge of soil ingestion patterns within the United States is

11 somewhat limited. Only a few researchers in the U.S. have attempted to quantify soil ingestion

12 patterns in children, and these researchers have performed studies in only a few locales in the

13 northern parts of the United States. Based on the information that we do have, it appears that

14 children may ingest fairly substantial amounts of soil on a per-kilogram-body-weight basis, and

15 could receive a large proportion of their total exposure to certain toxicants via the soil ingestion

16 route. Thus, understanding soil ingestion patterns is an important part of understanding, and

17 estimating, children’s overall exposures to environmental toxicants.

18 The Centers for Disease Control and Prevention's Agency for Toxic Substances and

19 Disease Registry (ATSDR) held a workshop in June 2000 in which a panel of soil ingestion

20 experts developed definitions for soil ingestion, soil-pica, and geophagy, to distinguish aspects

21 of soil ingestion patterns that are important from a research perspective (ATSDR 2001):

22

23 Soil ingestion is the consumption of soil. This may be intentional or unintentional, resulting

24 from various behaviors including, but not limited to, mouthing, contacting dirty hands, eating

25 dropped food, or consuming soil directly.

26

27 Soil ingestion, as defined above, has been documented in U.S. children in several studies

28 that use a "tracer element" methodology. The tracer element methodology attempts to quantify

29 amounts of soil ingested by analyzing samples of soil from childrens' residences, and by

30 analyzing samples of the childrens' excreta (feces, and sometimes also urine). The soil, fecal,

31 and urine samples are analyzed for the presence and quantity of tracer elements - typically,
5-2

1 aluminum, silicon, titanium, and yttrium, and other elements. Because these metals/metalloids

2 are not metabolized into other substances in the body, their presence in feces and urine can be

3 used to estimate the quantity of soil ingested by mouth. None of the studies attempt to quantify

4 amounts excreted in perspiration, tears, glandular secretions, hair or nails. Early versions of this

5 methodology usually did not account for the contribution of tracer elements from non-soil

6 substances (food, medications, and non-food sources such as toothpaste) that children might

7 swallow. Later studies generally account for tracer element contributions from these non-soil

8 sources. 

9 Some study authors adjust their soil ingestion estimate results to account for the potential

10 contribution of tracer elements found in household dust as well as soil. Dust is the fine

11 particulate found indoors. It is composed of particles derived from outdoor sources such as soils,

12 smoke, pollen, etc. and indoor sources such as particles associated with construction activities,

13 wood burning, clothes drying, molds, etc. Dust ingestion can occur from inhalation, deposition

14 in the respiratory system and subsequent ingestion.

15

16 Soil-pica is a form of soil ingestion that is characterized by the recurrent ingestion of unusually

17 high amounts of soil (i.e., on the order of 1,000 - 5,000 milligrams per day). The soil ingestion

18 may be intentional or unintentional. 

19

20 Soil-pica, as defined above, has been documented in U.S. children with the same tracer

21 element methodology, but to a more limited extent (Calabrese et al., 1991), and documented in

22 Jamaican children (Wong 1988 as reported in Calabrese and Stanek 1993). The existing U.S.

23 studies on soil ingestion, which appear to include some study subjects who exhibited soil-pica

24 behavior, were of short duration, and had relatively small numbers of study participants. These

25 factors combined may obscure the true incidence of soil-pica in the population of U.S. children.

26 Groups at risk of soil-pica behavior include children aged 6 years and younger and individuals

27 who are developmentally delayed. It should be noted that pica behavior is not always associated

28 with developmentally impaired children and that pica behavior is observed in 50% of children

29 between 1 and 3 years of age (Sayetta, 1986).

30
5-3

1 Geophagy is a form of soil ingestion defined as the intentional ingestion of earths and is usually

2 associated with cultural practices. 

3

4 Geophagy is practiced in various places in the United States by members of various

5 cultural groups. It often involves the intentional ingestion of clay materials from

6 uncontaminated sources that are not surface soils (ATSDR 2001, Vermeer and Frate, 1979). 

7 However, because geophagy is defined above as the intentional ingestion of clay or earths, and

8 because determining whether a child's soil ingestion is intentional or not is difficult (ATSDR

9 2001), geophagy is not included as a separate concept in the rest of this chapter. 

10 The available studies on soil intake are summarized in the following sections. Some of

11 the later studies are re-analyses of data previously published. For this reason, the sections that

12 follow are organized into studies of primary analysis and studies of secondary analysis. Within

13 those two categories, there are studies considered "key" because their experimental design is

14 superior, or they are the only studies with a particular attribute needed for the recommendations. 

15 The studies not categorized as "key" are categorized as "relevant" either because they are based

16 on foreign data or because of limitations in the experimental design (e.g., not accounting for

17 tracers found in food and medicines). Recommended soil intake rates are based on the results of

18 key studies and are summarized in the last section, along with additional guidance to risk

19 assessors using soil intake estimates. 

20

21 5.2 SOIL INTAKE STUDIES

22 5.2.1 Key Studies of Primary Analysis 

23 5.2.1.1 Davis et al., 1990

24 Davis et al. (1990) used a mass-balance/tracer technique to estimate soil ingestion among

25 children. In this study, 104 children between the ages of 2 and 7 years were randomly selected

26 from a three-city area in southeastern Washington State. The study was conducted over a seven

27 day period, primarily during the summer. Daily soil ingestion was evaluated by analyzing soil

28 and house dust, feces, urine, and duplicate food samples for aluminum, silicon, and titanium. In

29 addition, information on dietary habits and demographics was collected in an attempt to identify

30 behavioral and demographic characteristics that influence soil intake rates among children. The

31 amount of soil ingested on a daily basis was estimated using the following equation:
5-4

(5-2)

1 where:

2 Si,e = soil ingested for child i based on tracer e (g);

3 DWf = feces dry weight (g);

4 DWp = feces dry weight on toilet paper (g);

5 Ef = tracer amount in feces (μg/g);

6 Eu = tracer amount in urine (μg/g);

7 DWfd = food dry weight (g);

8 Efd = tracer amount in food (μg/g); and

9 Esoil = tracer concentration in soil (μg/g).

10

11 The soil intake rates were corrected by adding the amount of tracer in vitamins and medications

12 to the amount of tracer in food, and adjusting the food quantities, feces dry weights, and tracer

13 concentrations in urine to account for missing samples.

14 Soil ingestion rates were highly variable, especially those based on titanium. Mean daily

15 soil ingestion estimates were 38.9 mg/day for aluminum, 82.4 mg/day for silicon and

16 245.5 mg/day for titanium (Table 5-1). Median values were 25 mg/day for aluminum, 59

17 mg/day for silicon, and 81 mg/day for titanium. The investigators also evaluated the extent to

18 which differences in tracer concentrations in house dust and yard soil impacted estimated soil

19 ingestion rates. The value used in the denominator of the mass balance equation was

20 recalculated to represent a weighted average of the tracer concentration in yard soil and house

21 dust based on the proportion of time the child spent indoors and outdoors. The adjusted mean

22 soil/dust intake rates were 64.5 mg/day for aluminum, 160.0 mg/day for silicon, and 268.4

23 mg/day for titanium. Adjusted median soil/dust intake rates were: 51.8 mg/day for aluminum,

24 112.4 mg/day for silicon, and 116.6 mg/day for titanium. The investigators also observed that

25 the following demographic characteristics were associated with high soil intake rates: male sex, 

26 racial groups other than white, low income, operator/laborer as the principal occupation of the

27 parent, and city of residence. However, none of these factors were predictive of soil intake rates

28 when tested using multiple linear regression.
5-5

1 The advantages of this study are that soil intake rates were corrected based on the tracer

2 content of foods and medicines and that a relatively large number of children were sampled. 

3 Also, demographic and behavioral information was collected for the survey group. However,

4 although a relatively large sample population was surveyed, these children were all from a single

5 area of the U.S. and may not be representative of the U.S. population as a whole. The study was

6 conducted over a one-week period during the summer and may not be representative of long7 term (i.e., annual) patterns of intake.

8

9 5.2.1.2 Calabrese et al. 1997a

10 Calabrese et al. (1997a) estimated soil ingestion rates for children residing on a

11 Superfund site using a mass-balance methodology in which eight tracer elements (i.e.,

12 aluminum, barium, manganese, silicon, titanium, vanadium, yttrium, and zirconium) were

13 analyzed. The methodology used in this study is similar to that employed in Calabrese et al.

14 (1989). As in Calabrese et al. (1989), 64 children ages 1-3 years and predominantly from

15 two-parent households were selected for this study. This stratified simple random sample of

16 children was selected from the Anaconda, MT area. Thirty-six of the 64 children were male, and

17 the children ranged in age from 1 to 3 years with approximately an equal number of children in

18 each age group. The study was conducted for seven consecutive days during a two week period

19 in the month of September. Duplicate samples of meals, beverages, and over- the-counter

20 medicines and vitamins were collected over the seven day period, along with fecal samples. In

21 addition, soil and dust samples were collected from the children’s home and play areas. 

22 Toothpaste containing nondetectable levels of the tracer elements, with the exception of silica,

23 was provided to all of the children. Infants were provided with baby cornstarch, diaper rash

24 cream, and soap which were found to contain low levels of tracer elements.

25 As in Calabrese et al. (1989), an additional study was conducted in which the identical 

26 mass-balance methodology used to estimate soil ingestion rates among children was used on

27 adults in order to validate that soil ingestion could be detected. Known amounts of soil were

28 administered to ten adults (5 males, 5 females) from Western Massachusetts over a period of 28

29 days. Each adult ingested for 7 consecutive days: 1) no soil during Week 1, 2) 20 mg of

30 sterilized soil during Week 2, 3) 100 mg of sterilized soil during Week 3, and 4) 500 mg of

31 sterilized soil during Week 4. Soil samples were previously characterized and were of sufficient
5-6

1 concentration to be detected in the analysis of fecal samples. Duplicate food and fecal samples

2 were collected every day during each study week and analyzed for the eight tracer elements (Al,

3 Si, Ti, Ce, La, Nd, Y, and Zr). The authors determined that a soil ingestion of 200 to 500 mg/day

4 could be detected in a reliable manner.

5 Calabrese et al. (1997a) estimated soil ingestion by each tracer element using the Best

6 Tracer Method (BTM), which allows for the selection of the most recoverable tracer for a

7 particular group of subjects (Stanek and Calabrese, 1995b). In this case Ba, Mn, and V were

8 dropped as they were found to be poor performing tracers. The median soil ingestion estimates

9 for the four best trace elements based on food/soil ratios for the 64 children using Al, Si, Ti, Y,

10 and Zr were presented (Table 5-2). The best estimate was calculated by taking the median of the

11 best four trace elements. Based on the soil ingestion estimate for the best tracer, the mean soil

12 ingestion rate was 66 mg/day and the median was 20 mg/day. The 95th percentile value was

13 283 mg/day. Using the median of the 4 best tracers, the mean was 7 mg/day and the 95th

14 percentile was 160 mg/day. These results are lower than the soil ingestion estimates obtained by

15 Stanek and Calabrese (1995a). Calabrese et al. (1997a) believe this may be due to the fact that

16 the families of the children who participated in this study were aware that they lived on an EPA

17 Superfund site and this knowledge might have resulted in reduced exposure. There was no

18 statistically significant difference found in soil ingestion estimates by gender or age. There was

19 also no significant difference in soil ingestion by housing or yard characteristics (i.e., porch,

20 deck, door mat, etc.), or between children with or without pets. 

21 The median dust ingestion estimates for the four best tracer elements using Al, Si, Ti, Y,

22 and Zr were also presented (Table 5-3). The estimate is based on food/dust ratios for the 64

23 Anaconda children. The mean dust ingestion rate based on the best tracer was 127 mg/day and

24 the 95th percentile rate was 614 mg/day.

25 The advantages of this study were the use of a longer 7 consecutive day study period

26 rather than two periods of 3 and 4 days (Stanek and Calabrese, 1995a), the use of the BTM, the

27 use of an expanded adult validation study which used 10 volunteers rather than 6 (Calabrese et

28 al., 1989), and the use of a dietary education program to reduce food tracer input and variability. 

29 However, the data presented in this study are from a single 7-day period during September which

30 may not reflect soil ingestion rates for other months or time-periods. In addition, the study

31 displayed a net residual negative error, which may have resulted in underestimated soil ingestion
5-7

1 rates. The authors believe that this error is not likely to affect the median by more than 40

2 mg/day.

3

4 5.2.1.3 Davis and Mirick, 2006

5 Davis and Mirick (2006) calculated soil ingestion for children and adults in the same

6 family using a mass balance approach. The families in this study were a subset of the 104

7 families who participated in the soil ingestion study by Davis et al. (1990), and the data were

8 collected in 1988, one year prior to the Davis et al. study. Nineteen families were selected for

9 the analyses in this study, and each consisted of a child participant between the age of 3 and 7, a

10 female, and a male parent or guardian living in the same house. Samples were collected for 11

11 consecutive days of all food items consumed, all feces excreted, twice-daily urine, and soil/house

12 dust. Tracer elements for this study included aluminum, silicon and titanium. In addition,

13 parents completed a daily diary of activities for themselves and the participant child for 4

14 consecutive days during the study period.

15 Soil ingestion rates are shown for all three family member participants in Table 5-4. The

16 mean and median estimates for children for all three tracers ranged from 36.7 to 206.9 mg/day

17 and 26.4 to 46.7 mg/day, respectively, and fall within the range of those reported by Davis et al.,

18 1990. Adult soil ingestion estimates ranged from 23.2 to 624.9 mg/day for mean values and

19 from 0 to 259.5 mg/day for median values, and were more variable than for the children in the

20 study regardless of the tracer. The authors believed that this higher variability may have

21 indicated an important occupational contribution of soil ingestion in some, but not all, of the

22 adults. Similar to previous studies, the soil ingestion estimates were the highest for titanium. 

23 Although toothpaste is a known source of titanium, the titanium content of the toothpaste used

24 by study participants was not determined.

25 Only three of a number of behaviors examined for their relationship to soil ingestion

26 were found to be associated with increased soil ingestion in this study: 

27

28 • reported eating of dirt (for children);

29 • occupational contact with soil (for adults); and

30 • hand washing before meals (for both children and adults).

31
5-8

T 

f F

i,e s

i,e i

i,e

= ×

(5-1)

1 Several typical childhood behaviors, however, including thumb-sucking, furniture licking, and

2 carrying around a blanket or toy were not associated with increased soil ingestion for the

3 participating children. Among both parents and children, neither nail-biting nor eating unwashed

4 fruits or vegetables was correlated with increased soil ingestion. When investigating correlations

5 within the same family, a child’s soil ingestion was not found to be associated with either

6 parent’s soil ingestion, nor did the mother and father’s soil ingestion appear to be correlated.

7 One advantage of this study is that it examines soil ingestion among family members,

8 both children and adults. However, the sample population was small. In addition, the families

9 were a subset of those in a previous study, chosen for their high compliance with the study

10 protocol, and as such may not be representative of the general population. 

11

12 5.2.2. Relevant Studies of Primary Analysis

13 5.2.2.1 Binder et al., 1986

14 Binder et al. (1986) used a tracer technique modified from a method previously used to

15 measure soil ingestion among grazing animals to study the ingestion of soil among children 1 to

16 3 years of age who wore diapers. The children were studied during the summer of 1984 as part

17 of a larger study of residents living near a lead smelter in East Helena, Montana. Soiled diapers

18 were collected over a 3-day period from 65 children (42 males and 23 females), and composited

19 samples of soil were obtained from the children's yards. Both excreta and soil samples were

20 analyzed for aluminum, silicon, and titanium. These elements were found in soil but were

21 thought to be poorly absorbed in the gut and to have been present in the diet only in limited

22 quantities. This made them useful tracers for estimating soil intake. Excreta measurements were

23 obtained for 59 of the children. Soil ingestion by each child was estimated on the basis of each

24 of the three tracer elements using a standard assumed fecal dry weight of 15 g/day, and the

25 following equation:

26

27 where:

28 Ti,e = estimated soil ingestion for child i based on element e (g/day);
5-9

f 1 i,e = concentration of element e in fecal sample of child i (mg/g);

2 Fi = fecal dry weight (g/day); and

3 Si,e = concentration of element e in child i's yard soil (mg/g).

4

5 The analysis assumed that (1) the tracer elements were neither lost nor introduced during sample

6 processing; (2) the soil ingested by children originates primarily from their own yards; and (3)

7 that absorption of the tracer elements by children occurred in only small amounts. The study did

8 not distinguish between ingestion of soil and housedust, nor did it account for the presence of the

9 tracer elements in ingested foods or medicines.

10 The arithmetic mean quantity of soil ingested by the children in the Binder et al.

11 (1986) study was estimated to be 181 mg/day (range 25 to 1,324) based on the aluminum tracer;

12 184 mg/day (range 31 to 799) based on the silicon tracer; and 1,834 mg/day (range 4 to 17,076)

13 based on the titanium tracer (Table 5-5). The overall mean soil ingestion estimate, based on the

14 minimum of the three individual tracer estimates for each child, was 108 mg/day (range 4 to

15 708). The median values were 121 mg/day, 136 mg/day, and 618 mg/day for aluminum, silicon,

16 and titanium, respectively. The 95th percentile values for aluminum, silicon, and titanium were

17 584 mg/day, 578 mg/day, and 9,590 mg/day, respectively. The 95th percentile value based on

18 the minimum of the three individual tracer estimates for each child was 386 mg/day.

19 The authors were not able to explain the difference between the results for titanium and

20 for the other two elements, but they speculated that unrecognized sources of titanium in the diet

21 or in the laboratory processing of stool samples may have accounted for the increased levels. 

22 The frequency distribution graph of soil ingestion estimates based on titanium shows that a

23 group of 21 children had particularly high titanium values (i.e., >1,000 mg/day). The remainder

24 of the children showed titanium ingestion estimates at lower levels, with a distribution more

25 comparable to that of the other elements.

26 The advantages of this study are that a relatively large number of children were studied

27 and tracer elements were used to estimate soil ingestion. However, the children studied may not

28 be representative of the U.S. population, and the study did not account for tracers ingested via

29 foods or medicines. Also, the use of an assumed fecal weight instead of actual fecal weights

30 may have biased the results of this study. Finally, because of the short-term nature of the survey,

31 soil intake estimates may not be entirely representative of long-term behavior, especially at the

32 upper end of the distribution of intake.
5-10

1 5.2.2.2 Clausing et al., 1987

2 Clausing et al. (1987) conducted a soil ingestion study with Dutch children using a tracer

3 element methodology similar to that of Binder et al. (1986). Aluminum, titanium, and

4 acid-insoluble residue (AIR) contents were determined for fecal samples from children aged 2 to

5 4 years attending a nursery school and for samples of playground dirt at that school. Twenty6 seven daily fecal samples were obtained over a 5-day period for the 18 children examined. 

7 Using the average soil concentrations present at the school, and assuming a standard fecal dry

8 weight of 10 g/day, soil ingestion was estimated for each tracer. Eight daily fecal samples were

9 also collected from six hospitalized, bedridden children. These children served as a control

10 group, representing children who had very limited access to soil.

11 The average quantity of soil ingested by the school children in this study was as follows: 

12 230 mg/day (range 23 to 979 mg/day) for aluminum; 129 mg/day (range 48 to 362 mg/day) for

13 AIR; and 1,430 mg/day (range 64 to 11,620 mg/day) for titanium (Table 5-6). As in the Binder

14 et al. (1986) study, a fraction of the children (6/19) showed titanium values well above

15 1,000 mg/day, with most of the remaining children showing substantially lower values. Based

16 on the Limiting Tracer Method (LTM), mean soil intake was estimated to be 105 mg/day with a

17 population standard deviation of 67 mg/day (range 23 to 362 mg/day). Use of the LTM assumed

18 that "the maximum amount of soil ingested corresponded with the lowest estimate from the three

19 tracers" (Clausing et al., 1987). Geometric mean soil intake was estimated to be 90 mg/day on

20 the assumption that the maximum amount of soil ingested cannot be higher than the lowest

21 estimate for the individual tracers.

22 Mean (arithmetic) soil intake for the hospitalized children was estimated to be 56 mg/day

23 based on aluminum (Table 5-7). For titanium, three of the children had estimates well in excess

24 of 1,000 mg/day, with the remaining three children in the range of 28 to 58 mg/day. Using the

25 LTM method, the mean soil ingestion rate was estimated to be 49 mg/day with a population

26 standard deviation of 22 mg/day (range 26 to 84 mg/day). The geometric mean soil intake rate

27 was 45 mg/day. The data on hospitalized children suggest a major nonsoil source of titanium for

28 some children and may suggest a background nonsoil source of aluminum. However, conditions

29 specific to hospitalization (e.g., medications) were not considered. AIR measurements were not

30 reported for the hospitalized children. Assuming that the tracer-based soil ingestion rates

31 observed in hospitalized children actually represent background tracer intake from dietary and
5-11

1 other nonsoil sources, mean soil ingestion by nursery school children was estimated to be

2 56 mg/day, based on the LTM (i.e., 105 mg/day for nursery school children minus 49 mg/day for

3 hospitalized children).

4 The advantages of this study are that the investigators evaluated soil ingestion among two

5 populations of children that had differences in access to soil and corrected soil intake rates based

6 on background estimates derived from the hospitalized group. However, a smaller number of

7 children were used in this study than in the Binder et al. (1986) study and these children may not

8 be representative of the U.S. population. Tracer elements in foods or medicines were not

9 evaluated. Also, intake rates derived from this study may not be representative of soil intake

10 over the long-term because of the short-term nature of the study. In addition, one of the factors

11 that could affect soil intake rates is hygiene (e.g., hand washing frequency). Hygienic practices

12 can vary across countries and cultures and may be more stringently emphasized in a more

13 structured environment such as that found in child care centers in The Netherlands and other

14 European countries, compared to child care centers in the U.S.

15

16 5.2.2.3 Calabrese et al., 1989

17 Calabrese et al. (1989) studied soil ingestion among children using the basic tracer design

18 developed by Binder et al. (1986). However, in contrast to the Binder study, eight tracer

19 elements—aluminum, barium, manganese, silicon, titanium, vanadium, yttrium, and

20 zirconium— were analyzed instead of only three (aluminum, silicon, and titanium). Sixty-four

21 children between the ages of 1 and 4 years old were included in the study. These children were

22 all selected from the greater Amherst, MA area and were predominantly from two-parent

23 households where the parents were highly educated. The Calabrese et al. (1989) study was

24 conducted over a period of eight days to two weeks and included the use of a mass-balance

25 methodology in which duplicate samples of food, beverages, medicines, and vitamins were

26 collected and analyzed in addition to soil and dust samples collected from the child’s home and

27 play area. Fecal and urine samples were also collected and analyzed for tracer elements. 

28 In order to validate the mass-balance methodology used to estimate soil ingestion rates

29 among the children and to determine which tracer elements provided the most reliable data on

30 soil ingestion, known amounts of soil (300 mg over 3 days and 1,500 mg over 3 days) containing

31 eight tracers were administered to six adult volunteers (three males and three females). Soil,
5-12

1 feces, and samples of food were analyzed for tracer elements to calculate recovery rates of tracer

2 elements in soil. From this investigation the authors confirmed that the tracer methodology

3 could adequately detect tracer elements in feces at the levels expected for the study of soil intake

4 rates in children. Aluminum, silicon, and yttrium exhibited the lowest standard deviation of

5 recovery and were therefore identified as the most reliable of the eight tracer elements analyzed;

6 the percentage of recovery of these three tracers was closest to 100%. The recovery of these

7 three tracers ranged from 120 to 153 percent when 300 mg of soil had been ingested over a

8 three-day period and from 88 to 94 percent when 1,500 mg soil had been ingested over a three9 day period (Table 5-8).

10 Using the three most reliable tracer elements, the mean soil intake rate for children,

11 adjusted to account for the amount of tracer found in food and medicines, was estimated to be

12 153 mg/day based on aluminum, 154 mg/day based on silicon, and 85 mg/day based on yttrium

13 (Table 5-5). Median intake rates were somewhat lower (29 mg/day for aluminum, 40 mg/day for

silicon, and 9 mg/day for yttrium). Upper (95th 14 ) percentile values were 223 mg/day for

15 aluminum, 276 mg/day for silicon, and 106 mg/day for yttrium. Similar results were observed

16 when soil and dust ingestion was combined (Table 5-9). Intake of soil and dust was estimated

17 using a weighted ingestion for one child in the study ranged from approximately 10 to

18 14 grams/day during the second week of observation. Average soil ingestion for this child was

19 5 to 7 mg/day, based on the entire study period.

20 In a subsequent paper (Calabrese and Stanek, 1992a), the authors used statistical

21 modeling to revise these soil ingestion estimates downward, based on a more accurate

22 representation of the amount of outdoor soil in indoor dust (31.3%). These new analyses

23 indicate that the estimates of median outdoor soil ingestion presented in the previous study

24 should be reduced by 35%. These revised soil ingestion estimates are reduced from 29 to19

25 mg/d based on aluminum, 40 to 26 mg/d based on silicon, and 9 to 6 mg/d based on yttrium. 

26 However, this adjustment was not used in subsequent analyses by Stanek and Calabrese.

27 The advantage of this study is that intake rates were corrected for tracer concentrations

28 in foods and medicines . Also, intake was observed over a longer time period in this study than

29 in earlier studies and the number of tracers used was larger than for other studies. A relatively

30 large population was studied, but it may not be entirely representative of the U.S. population
5-13

1 because it was selected from a single location. The results presented in Calabrese et al. 1989

2 have been superseded by more refined analyses of the same data.

3

4 5.2.2.4 Van Wïjnen et al., 1990

5 In a study by Van Wïjnen et al. (1990), soil ingestion among Dutch children ranging in

6 age from 1 to 5 years was evaluated using a tracer element methodology similar to that used by

7 Clausing et al. (1987). Van Wïjnen et al. (1990) measured three tracers (i.e., titanium,

8 aluminum, and acid insoluble residue (AIR)) in soil and feces and estimated soil ingestion based

9 on the LTM. An average daily feces dry weight of 15 g was assumed. A total of 292 children

10 attending daycare centers were sampled during the first of two sampling periods and 187

11 children were sampled in the second sampling period; 162 of these children were sampled during

12 both periods (i.e., at the beginning and near the end of the summer of 1986). A total of 78

13 children were sampled at campgrounds, and 15 hospitalized children were sampled. The mean

14 values for these groups were: 162 mg/day for children in daycare centers, 213 mg/day for

15 campers and 93 mg/day for hospitalized children. 

16 The authors also reported geometric mean LTM values because soil intake rates were

17 found to be skewed and the log transformed data were approximately normally distributed. 

18 Geometric mean LTM values were estimated to be 111 mg/day for children in daycare centers,

19 174 mg/day for children vacationing at campgrounds (Table 5-10) and 74 mg/day for

20 hospitalized children (70-120 mg/day based on the 95 percent confidence limits of the mean). 

21 AIR was the limiting tracer in about 80 percent of the samples. Among children attending

22 daycare centers, soil intake was also found to be higher when the weather was good (i.e., <2

23 days/week precipitation) than when the weather was bad (i.e., >4 days/week precipitation (Table

24 5-11). The authors suggest that the mean LTM value for hospitalized infants represents

25 background intake of tracers and should be used to correct the soil intake rates based on LTM

26 values for other sampling groups. Using mean values, corrected soil intake rates were 69 mg/day

27 (162 mg/day minus 93 mg/day) for daycare children and 120 mg/day (213 mg/day minus 93

28 mg/day) for campers. Corrected geometric mean soil intake was estimated to range from 0 to 90

29 mg/day with a 90th percentile value of 190 mg/day for the various age categories within the

30 daycare group and 30 to 200 mg/day with a 90th percentile value of 300 mg/day for the various

31 age categories within the camping group.
5-14

1 The advantage of this study is that soil intake was estimated for three different

2 populations of children; one expected to have high intake, one expected to have "typical" intake,

3 and one expected to have low or background-level intake. Van Wïjnen et al. (1990) used the

4 background tracer measurements to correct soil intake rates for the other two populations. The

5 major limitation of this study is that tracer concentrations in food and medicine were not

6 evaluated. Also, the population of children studied was relatively large, but may not be

7 representative of the U.S. population. This study was conducted over a relatively short time

8 period. Thus, estimated intake rates may not reflect long-term patterns, especially at the high9 end of the distribution. Another limitation of this study is that values were not reported element10 by-element, which would be the preferred way of reporting. In addition, one of the factors that

11 could affect soil intake rates is hygiene (e.g., hand washing frequency). Hygienic practices can

12 vary across countries and cultures. 

13

14 5.2.2.5 Calabrese et al. 1996

15 Calabrese et al., 1996 examined the hypothesis that one cause of the variation between

16 tracers seen in soil ingestion studies could be related to differences in soil tracer concentrations

17 by particle size. In this study, the soil that was used by Calabrese et al. 1997a from Anaconda,

18 Montana was reanalyzed for the tracer concentration after it had been sieved to a particle size of

19 <250 μm in diameter (<2 mm soil particle size in the original study). The smaller particle size

20 was examined based on the assumption that children and adults principally ingest soil of small

21 particle size adhering to fingertips and under fingernails. For five of the tracers used in the

22 original study (Al, Si, Ti, Y, and Zr), soil concentration was not changed by particle size. 

23 However, the soil concentrations of three tracers (La, Ce, and Nd) were increased two- to

24 fourfold at the smaller soil particle size. Soil ingestion estimates for these three tracers were

25 decreased by approximately 60% at the 95th percentile.

26 The importance of this study is that it provides further insights regarding the selection of

27 tracers for soil ingestion studies.

28

29 5.2.2.6 Calabrese et al. 1999

30 Calabrese et al. 1999 extends the findings from Calabrese et al. 1996 by quantifying trace

31 element concentrations in soil based on sieving to particle sizes of 100 to 250 μm and to particle
5-15

1 sizes of 53 to < 100 μm. This study used the data from soil concentrations from the Anaconda,

2 Montana site reported by Calabrese et al. 1997. Results of the study indicated that soil

3 concentrations of Al, Si, and Ti do not increase at the two finer particle size ranges measured. 

4 However, soil concentrations of Ce, La, and Nd increased by a factor of 2.5 to 4.0 in the 100-250

5 μm particle size range when compared with the 0 to 2 μm particle size range. There was not a

6 significant increase in concentration in the 53 to 100 μm particle size range. 

7 The importance of this study is that it provides further insights regarding the selection of

8 tracers for soil ingestion studies.

9

10 5.2.2.7 Stanek and Calabrese 2000

11 In Stanek and Calabrese, 2000, the authors reanalyzed the soil ingestion data from the

12 Anaconda study. The authors assumed a lognormal distribution for the soil ingestion estimates

13 in the Anaconda study to predict average soil ingestion for children over a longer time period. 

Using best linear unbiased predictors, the authors predicted 95th 14 percentile soil ingestion values

over time periods of 7 days, 30 days, 90 days, and 365 days. The 95th 15 percentile soil ingestion

16 values were predicted to be 133 mg/day over 7 days, 112 mg/day over 30 days, 108 mg/day over

17 90 days, and 106 mg/day over 365 days. Based on this analysis, estimates of the distribution of

longer term average soil ingestion are expected to be narrower, with the 95th 18 percentile estimates

19 being as much as 25% lower (Stanek and Calabrese, 2000). 

20

21 5.2.2.8 Stanek et al. 2001b

22 Stanek et al. (2001b) developed best linear unbiased predictors to reduce the biasing

23 effect of short-term soil ingestion estimates. This study estimated long-term average soil

24 ingestion distribution using daily soil ingestion estimates from children who participated in the

25 Anaconda, MT study. In this long-term (annual) distribution, the soil ingestion estimates were:

mean 31, median 24, 75th percentile 42, 90th percentile 75, and 95th 26 percentile 91 mg/day. A

27 limitation of this analysis is that the distribution of long-term soil ingestion uses the median soil

28 ingestion estimate on a day, rather than the average of the tracer element estimate.

29

30 5.2.3 Key Studies of Secondary Analysis

31 5.2.3.1 Stanek and Calabrese, 1995a
5-16

1 Stanek and Calabrese (1995a) presented a methodology which links the physical passage

2 of food and fecal samples to construct daily soil ingestion estimates from daily food and fecal

3 trace-element concentrations. Soil ingestion data for children obtained from the Amherst study

4 (Calabrese et al., 1989) were reanalyzed by Stanek and Calabrese (1995a). In the Amherst

5 study, soil ingestion measurements were made over a period of 2 weeks for a non-random

6 sample of sixty-four children (ages of 1-4 years old) living adjacent to a university in western

7 Massachusetts. During each week, duplicate food samples were collected for 3 consecutive days

8 and fecal samples were collected for 4 consecutive days for each subject. The total amount of

9 each of eight trace elements present in the food and fecal samples was measured. The eight trace

10 elements are aluminum, barium, manganese, silicon, titanium, vanadium, yttrium, and zirconium. 

11 The authors expressed the amount of trace element in food input or fecal output as a "soil

12 equivalent," which was defined as the amount of the element in average daily food intake (or

13 average daily fecal output) divided by the concentration of the element in soil. A lag period of

14 28 hours between food intake and fecal output was assumed for all respondents. Day 1 for the

15 food sample corresponded to the 24 hour period from midnight on Sunday to midnight on

16 Monday of a study week; day 1 of the fecal sample corresponded to the 24 hour period from

17 noon on Monday to noon on Tuesday. Based on these definitions, the food soil equivalent was

18 subtracted from the fecal soil equivalent to obtain an estimate of soil ingestion for a trace

19 element. A daily overall ingestion estimate was constructed for each child as the median of trace

20 element values remaining after tracers falling outside of a defined range around the overall

21 median were excluded. Additionally, estimates of the distribution of soil ingestion projected

22 over a period of 365 days were derived by fitting log-normal distributions to the overall daily

23 soil ingestion estimates.

24 Table 5-12 presents the estimates of mean daily soil ingestion intake per child (mg/day)

25 for the 64 study participants. (The authors also presented estimates of the median values of daily

26 intake for each child. For most risk assessment purposes the child mean values, which are

27 proportional to the cumulative soil intake by the child, are needed instead of the median values.) 

28 The approach adopted in this paper led to changes in ingestion estimates from those presented in

29 Calabrese et al. (1989). 

30 Specifically, among elements that may be more useful for estimation of ingestion, the

31 mean estimates decreased for Al (153 mg/d to 122 mg/d) and Si ( 154 mg/d to 139 mg/d), but
5-17

1 increased for Ti (218 mg/d to 271 mg/d) and Y (85 mg/d to 165 mg/d). The overall mean

2 estimate from this reanalysis was 179 mg/d. Table 5-13 presents the empirical distribution of the 

3 the “overall” mean daily soil ingestion estimates for the 8-day study period (not based on

4 lognormal modeling). The estimated intake based on the overall estimates is 45 mg/day or less

5 for 50 percent of the children and 208 mg/day or less for 95 percent of the children. The upper

6 percentile values for most of the individual trace elements are somewhat higher. Next, estimates

7 of the respondents soil intake averaged over a period of 365 days were presented based upon the

8 lognormal models fit to the daily ingestion estimates (Table 5-13). The estimated median value

9 of the 64 respondents' daily soil ingestion averaged over a year is 75 mg/day, while the

10 95th percentile is 1,751 mg/day.

11 A strength of this study is that it attempts to make full use of the collected data through

12 estimation of daily ingestion rates for children. The screening of data to remove less consistent

13 tracer estimates, and the aggregation of the remaining values, may introduce error to the

14 analysis. Individual daily estimates of ingestion will be subject to larger errors than are weekly

15 average values, particularly since the assumption of a constant lag time between food intake and

16 fecal output may be not be correct for many subject days. The aggregation approach used to

17 arrive at the overall ingestion estimates rests on the assumption that the mean ingestion estimates

18 across acceptable tracers provides the most reliable ingestion estimates. The validity of this

19 assumption depends on the particular set of tracers used in the study, and is not fully assessed.

20 In developing the 365-day soil ingestion estimates, data that were obtained over a short

21 period of time (as is the case with all available soil ingestion studies) were extrapolated over a

22 year. The 2-week study period may not reflect variability in tracer element ingestion over a year.

23 While Stanek and Calabrese (1995a) attempt to address this through lognormal modeling of the

24 long term intake, new uncertainties are introduced through the parametric modeling of the

25 limited subject day data. Also, the sample population size of the original study was small and

26 site limited, and, therefore, is not representative of the U.S. population. Study mean estimates of

27 soil ingestion, such as the study mean estimates presented in Table 5-12, are substantially more

28 reliable than any available distributional estimates. 

29

30 5.2.3.2 Stanek and Calabrese, 1995b
5-18

1 Stanek and Calabrese (1995b) recalculated ingestion rates that were estimated in three

2 previous mass-balance studies (Calabrese et al., 1989 and Davis et al., 1990 for children's soil

3 ingestion, and Calabrese et al., 1990 for adult soil ingestion) using the Best Tracer Method

4 (BTM). This method allows for the selection of the most recoverable tracer for a particular

5 subject or group of subjects. The selection process involves ordering trace elements for each

6 subject based on food/soil (F/S) ratios. These ratios are estimated by dividing the total amount

7 of the tracer in food by the tracer concentration in soil. The F/S ratio is small when the tracer

8 concentration in food is almost zero when compared to the tracer concentration in soil. A small

9 F/S ratio is desirable because it lessens the impact of transit time error (the error that occurs

10 when fecal output does not reflect food ingestion, due to fluctuation in gastrointestinal transit

11 time) in the soil ingestion calculation. Because the recoverability of tracers can vary within any

12 group of individuals, the BTM uses a ranking scheme of F/S ratios to determine the best tracers

13 for use in the ingestion rate calculation. To reduce biases that may occur as a result of sources of

14 fecal tracers other than food or soil, the median of soil ingestion estimates based on the four

15 lowest F/S ratios was used to represent soil ingestion among individuals.

16 For children, the authors used data on 8 tracers from Calabrese et al., 1989 and data on 3

17 tracers from Davis et al. (1990) to estimate soil ingestion rates. The median of the soil ingestion

18 estimates from the lowest four F/S ratios from the Calabrese et al. (1989) study most often

19 included Al, Si, Ti, Y, and Zr. Based on the median of soil ingestion estimates from the best

20 four tracers, the mean soil ingestion rate was 132 mg/day and the median was 33 mg/day. The

21 95th percentile value was 154 mg/day. These estimates are based on data for 128 subject weeks

22 for the 64 children in the Calabrese et al. (1989) study. For the 101 children in the Davis et al.

23 (1990) study, the mean soil ingestion rate was 69 mg/day and the median soil ingestion rate was

24 44 mg/day. The 95th percentile estimate was 246 mg/day. These data are based on the three

25 tracers (i.e., Al, Si, and Ti) from the Davis et al. (1990) study. When the Calabrese et al. (1989)

26 and Davis et al. (1990) studies were combined, soil ingestion for children was estimated to be

27 104 mg/day (mean); 37 mg/day (median); and 217 mg/day (95th percentile), using the BTM.

28 When the adult data from the Calabrese et al. (1990) study were reevaluated, soil

29 ingestion rates were estimated to be 64 mg/day (mean); 87 mg/day (median); and 142 mg/day

30 (95th percentile), using the BTM.
5-19

1 This study provides a reevaluation of previous studies. Its advantages are that it

2 combines data from two studies for children, one from Washington and one from Massachusetts,

3 which increases the number of observations. It also corrects for biases associated with the

4 differences in tracer metabolism. The limitations associated with the data used in this study are

5 the same as the limitations described in the summaries of the Calabrese et al. (1989), Davis et al.

6 (1990) and Calabrese et al. (1990) studies.

7

8 5.2.4. Relevant Studies of Secondary Analysis

9 5.2.4.1 Thompson and Burmaster, 1991

10 Thompson and Burmaster (1991) developed parameterized distributions of soil ingestion

11 rates for children based on a reanalysis of the key study data collected by Binder et al. (1986). 

12 In the original Binder et al. (1986) study, an assumed fecal weight of 15 g/day was used. 

13 Thompson and Burmaster reestimated the soil ingestion rates from the Binder et al. (1986) study

14 using the actual stool weights of the study participants instead of the assumed stool weights. 

15 Because the actual stool weights averaged only 7.5 g/day, the soil ingestion estimates presented

16 by Thompson and Burmaster (1991) are approximately one-half of those reported by Binder et

17 al. (1986). Table 5-11 presents the distribution of estimated soil ingestion rates calculated by

18 Thompson and Burmaster (1991) based on the three tracers elements (i.e., aluminum, silicon,

19 and titanium), and on the arithmetic average of soil ingestion based on aluminum and silicon. 

20 The mean soil intake rates were 97 mg/day for aluminum, 85 mg/day for silicon, and 1,004

21 mg/day for titanium. The 90th percentile estimates were 197 mg/day for aluminum, 166 mg/day

22 for silicon, and 2,105 mg/day for titanium. Based on the arithmetic average of aluminum and

23 silicon for each child, mean soil intake was estimated to be 91 mg/day and 90th percentile intake

24 was estimated to be 143 mg/day.

25 Thompson and Burmaster (1991) tested the hypothesis that soil ingestion rates based on

26 the adjusted Binder et al. (1986) data for aluminum, silicon and the average of these two tracers

27 were lognormally distributed. The distribution of soil intake based on titanium was not tested for

28 lognormality because titanium may be present in food in high concentrations and the Binder et

29 al. (1986) study did not correct for food sources of titanium. Although visual inspection of the

30 distributions for aluminum, silicon, and the average of these tracers all indicated that they may

31 be lognormally distributed, statistical tests indicated that only silicon and the average of the
5-20

1 silicon and aluminum tracers were lognormally distributed. Soil intake rates based on aluminum

2 were not lognormally distributed. Table 5-14 also presents the lognormal distribution

3 parameters and underlying normal distribution parameters (i.e., the natural logarithms of the

4 data) for aluminum, silicon, and the average of these two tracers. According to the authors, “the

5 parameters estimated from the underlying normal distribution are much more reliable and

6 robust” (Thompson and Burmaster, 1991).

7 The advantages of this study are that it provides percentile data and defines the shape of

8 soil intake distributions. However, the number of data points used to fit the distribution was

9 limited. In addition, the study did not generate “new” data. Instead, it provided a reanalysis of

10 previously-reported data using actual fecal weights. This analysis is based on a study that did

11 not correct for tracer intake from food or medicine and the results may not be representative of

12 long-term intake rates because the data were derived from a short-term study.

13

14 5.2.4.2 Calabrese and Stanek 1992a

15 Calabrese and Stanek 1992a estimated the amount of outdoor soil in indoor dust using

16 statistical modeling. The model used data from 60 homes who participated in the Calabrese et

17 al. 1989 study. Scatter plots of each tracer concentration in soil versus dust for the subject

18 population were developed. Correlation analysis of the scatter plots was performed and an

19 estimate of the proportion of outdoor soil in indoor dust was developed using a model based on

20 the soil and dust data from Calabrese et al. 1989. The scatter plots show little evidence of a

21 consistent relationship between outdoor soil and indoor dust concentrations. The model uses

22 several simplifying assumptions. First, it assumes that the amount of dust produced every day

23 from both indoor and outdoor sources in a house is constant for all houses. Second, the model

24 assumes that the proportion of indoor dust due to outdoor soil is constant for all houses. Third, it

25 assumes that the concentration of the tracer element in dust produced from indoor sources is

26 constant for all houses. Using these assumptions, the model predicts that 31.3% of indoor dust

27 comes from outdoor soil. This model was then used to adjust the soil ingestion estimates from

28 Calabrese et al. 1989. In 1989, Calabrese assumed that all the excess fecal tracers were of soil

29 origin. Stanek and Calabrese 1992a reported that 50% of the excess fecal tracers were from

30 indoor origin. Taking that 50% multiplied by 31.3% results in 15%; which added to the 50%
5-21

1 indicates that approximately 65% of the total residual excess fecal tracer were of soil origin

2 (Calabrese and Stanek 1992a).

3 This study provides a refinement to the calculations from Calabrese et al. 1989. 

4 However, several assumptions were made to estimate the total residual excess fecal tracer that

5 comes from soil. The validity of these assumptions cannot be evaluated. Subsequent papers by

6 Stanek and Calabrese did not make use of this adjustment. 

7

8 5.2.4.3 Sedman and Mahmood , 1994

9 Sedman and Mahmood (1994) used the results of two previous children’s tracer studies

10 (Calabrese et al. 1989; Davis et al. 1990) to determine estimates of average daily soil ingestion

11 in young children and for over a lifetime. In the two studies, the intake and excretion of a

12 variety of tracers were monitored, and concentrations of tracers in soil adjacent to the children’s

13 dwellings were determined. The authors determined soil ingestion in these children using a mass

14 balance approach, dividing the excess tracer intake (i.e., quantity of tracer recovered in the feces

15 in excess of the measured intake) by the average concentration of tracer in soil samples from

16 each child's dwelling. They adjusted the mean estimates of soil ingestion in children for each

17 tracer (Y) from both studies to reflect that of a 2-year old child using the following equation:

18

19 Y i = − × xe 0 112 . yr (5-3)

20 where:

Yi 21 = adjusted mean soil ingestion (mg/day)

22 x = a constant

23 yr = average age (2 years)

24

25 The average ages of children in the two previous studies were 2.4 years in Calabrese

26 et al. (1989) and 4.7 years in Davis et al. (1990). The mean of the adjusted levels of soil

27 ingestion for a two year old child was 220 mg/kg for the Calabrese et al. (1989) study and 170

28 mg/kg for the Davis et al. (1990) study. From the adjusted soil ingestion estimates, based on a

29 normal distribution of means, the mean estimate for a 2-year old child was 195 mg/day and the

30 overall mean of soil ingestion and the standard error of the mean was 53 mg/day. Based on

31 uncertainties associated with the method employed, Sedman and Mahmood (1994)
5-22

1 recommended a conservative estimate of soil ingestion in young children of 250 mg/day. Based

2 on the 250 mg/day ingestion rate in a 2-year old child, an average daily soil ingestion over a

3 lifetime was estimated to be 70 mg/day. The lifetime estimates were derived using the equation

4 presented above that describes changes in soil ingestion with age.

5

6 5.2.4.4 Calabrese and Stanek, 1995

7 Calabrese and Stanek (1995) explored sources and magnitude of positive and negative

8 errors in soil ingestion estimates for children on a subject-week and trace element basis. 

9 Calabrese and Stanek (1995) identified possible sources of positive errors to be:

10 C Ingestion of high levels of tracers before the start of the study and low ingestion

11 during the study period may result in over estimation of soil ingestion; and

12 C Ingestion of element tracers from a non-food or non-soil source during the study

13 period.

14 Possible sources of negative bias identified by Calabrese and Stanek (1995) are the following:

15 C Ingestion of tracers in food, but the tracers are not captured in the fecal sample either

16 due to slow lag time or not having a fecal sample available on the final study day; and

17 C Sample measurement errors which result in diminished detection of fecal tracers, but

18 not in soil tracer levels.

19 The authors developed an approach that attempted to reduce the magnitude of error in the

20 individual trace element ingestion estimates. Results from a previous study conducted by

21 Calabrese et al. (1989) were used to quantify these errors based on the following criteria: (1) a

22 lag period of 28 hours was assumed for the passage of tracers ingested in food to the feces (this

23 value was applied to all subject-day estimates); (2) a daily soil ingestion rate was estimated for

24 each tracer for each 24-hr day a fecal sample was obtained; (3) the median tracer-based soil

25 ingestion rate for each subject-day was determined; and (4) negative errors due to missing fecal

26 samples at the end of the study period were also determined. Also, upper- and lower-bound

27 estimates were determined based on criteria formed using an assumption of the magnitude of the

28 relative standard deviation(RSD) presented in another study conducted by Stanek and Calabrese

29 (1995a). Daily soil ingestion rates for tracers that fell beyond the upper and lower ranges were

30 excluded from subsequent calculations, and the median soil ingestion rates of the remaining

31 tracer elements were considered the best estimate for that particular day. The magnitude of
5-23

1 positive or negative error for a specific tracer per day was derived by determining the difference

2 between the value for the tracer and the median value.

3 Table 5-16 presents the estimated magnitude of positive and negative error for six tracer

4 elements in the children's study (conducted by Calabrese et al., 1989). The original mean soil

5 ingestion rates ranged from a low of 21 mg/day based on zirconium to a high of 459 mg/day

6 based on titanium (Table 5-15). The adjusted mean soil ingestion rate after correcting for

7 negative and positive errors ranged from 97 mg/day based on yttrium to 208 mg/day based on

8 titanium (Table 5-16). Calabrese and Stanek (1995) concluded that correcting for errors at the

9 individual level for each tracer element provides more reliable estimates of soil ingestion.

10 This study is valuable in providing additional understanding of the nature of potential

11 errors in trace element specific estimates of soil ingestion. However, the operational definition

12 used for estimating the error in a trace element estimate was the observed difference of that

13 tracer from a median tracer value. The authors did not specifically identify sources of error or

14 seek direct evidence that individual tracers were indeed in error. Corrections to individual tracer

15 means were made according to how different values for that tracer were from the median values. 

16 This approach is based on the hypothesis that the median tracer value is the most accurate

17 estimate of soil ingestion, and the validity of this assumption depends on the specific set of

18 tracers used in the study and need not be correct. The approach used for the estimation of daily

19 tracer intake is the same as in Stanek and Calabrese (1995a), and some limitations of that

20 approach are mentioned in the review of that study.

21

22 5.2.4.5 Stanek et al., 2001a

23 In order to identify and evaluate biasing factors for soil ingestion estimates, the authors

24 developed a simulation model based on data from previous soil ingestion studies. The soil

25 ingestion data used in this model were taken from Calabrese et al., 1989 (the Amherst study);

26 Davis et al., 1990; Calabrese et al., 1997a (the Anaconda study) and Calabrese et al., 1997b, and

27 relied only on the aluminum and silicon trace element estimates provided in these studies. 

28 Of the biasing factors explored, the impact of study duration was the most striking, with a

positive bias of more than 100% for 95th 29 percentile estimates in a 4-day mass balance study. A

30 smaller bias was observed for the impact of absorption of trace elements from food. Although

31 the trace elements selected for use in mass balance studies are believed to have low absorption,
5-24

1 whatever amount is not accounted for will result in an underestimation of the soil ingestion

2 distribution. In these simulations, the absorption of trace elements from food of up to 30% was

3 shown to negatively bias the estimated soil ingestion distribution by less than 20 mg/day. No

4 biasing effect was found for misidentifying play areas for soil sampling (i.e., ingested soil from a

5 yard other than the subject’s yard). 

6

7 5.2.4.6 Zartarian et al., 2005

8 Zartarian et al. (2005) conducted an analysis of soil ingestion rates from several studies

9 in the literature to be used as input for the Stochastic Human Exposure and Dose Simulation

10 (SHEDS) model which was used in the EPA report entitled A Probabilistic Exposure Assessment

11 for Children Who Contact CCA-Treated Playsets and Decks (U.S. EPA 2005).

12 Soil ingestion rate estimates were derived for the SHEDS-Wood model (this refers to the

13 application of the SHEDS model to wood preservative exposure scenarios) using data from

14 Calabrese’s Amherst and Anaconda studies. Data statistics from both of these studies were used

15 to fit distributions of soil/dust ingestion rates. The statistical distributions generated for

16 variability and uncertainty distributions relied upon two tracers only, Al and Si, in estimating the

17 parameters of the lognormal variability and uncertainty distributions. Using Monte-Carlo

18 sampling, values from the fitted distribution were sampled. The sampled values were separated

19 into those values under 500 mg/day and values that exceeded 500 mg/day. The model assumes

20 that soil ingestion values that exceed 500 mg/day are representative of pica behavior. The soil

21 ingestion rate distribution for non-pica behavior children developed for the SHEDS model has a

mean of 61, standard deviation 81, median 30, 25

th 

percentile 12, 75

th 

percentile 73, 95

th 22

percentile 236, and 99

th 

23 percentile 402 (mg/day). This distribution was simulated using only the

24 results generated below 500 mg/day. For children exhibiting pica behavior the summary

statistics are: mean 962 mg/day, standard deviation 758, median 735, 25

th 

percentile 590, 75

th 25

percentile 1046, 95

th 

percentile 2130, 99

th 

26 percentile 3852 mg/day. 

27 The strength of this analysis is that it provides variability and uncertainty distributions. It

28 also provides estimates for pica behavior. A limitation of this analysis is that pica children and

29 incidental ingestion were simulated separately. The distribution for incidental soil ingestion

30 does not take into account that for long-term behavior, children are expected to have days where

31 they may ingest unusually high levels of soil. 
5-25

1

2 5.3 PICA

3 5.3.1. Prevalence

4 5.3.1.1 General Pica

5 Feldman (1986) defines pica as “the repeated eating of non-nutritive substances” . 

6 Numerous articles have been published that report on the incidence of pica among various

7 populations. However, most of these papers describe pica for substances other than soil

8 including sand, clay, paint, plaster, hair, string, cloth, glass, matches, paper, feces, and various

9 other items. These papers indicate that pica behavior occurs in approximately half of all

10 children between the ages of 1 and 3 years (Sayetta, 1986). The incidence of behavior

11 ingesting non-nutritive substances in children has been shown to differ for different

12 subpopulations. The incidence rate appears to be higher for black children than for white

13 children. Approximately 30 percent of black children aged 1 to 6 years are reported to have 

14 pica behavior, compared with 10 to 18 percent of white children in the same age group (Danford,

15 1982). There do not appear to be any sex differences in the incidence rates for males or females

16 (Kaplan and Sadock, 1985). Lourie et al. (1963) states that the incidence of pica is higher

17 among children in lower socioeconomic groups (i.e., 50 to 60 percent) than in higher income

18 families (i.e., about 30 percent). Pica behavior appears to be more common in rural areas

19 (Vermeer and Frate, 1979). A higher rate of pica has also been reported for pregnant women and

20 individuals with poor nutritional status (Danford, 1982). In general, pica behavior is more

21 frequent and more severe in mentally retarded children than in children in the general population

22 (Behrman and Vaughan 1983, Danford 1982, Forfar and Arneil 1984, Illingworth 1983, Sayetta

23 1986).

24

25 5.3.1.2 Soil Pica

26 It should be noted that the pica statistics cited above apply to the incidence of general

27 pica and not soil pica. A soil pica workshop conducted by ATSDR defined soil pica as the

28 recurrent ingestion of unusually high amounts of soil (i.e., 1,000 - 5,000 mg/day)(ATSDR,

29 2001). Information on the incidence of soil pica is limited, but it appears that soil pica is less

30 common than general pica. In addition, parental observations regarding children who are likely

31 to be high soil ingesters have been found to be inaccurate (Calabrese et al., 1997b). A study by
5-26

1 Vermeer and Frate (1979) showed that the incidence of geophagia (i.e., intentional earth-eating)

2 was about 16 percent among children from a rural black community in Mississippi. However,

3 geophagy was described as a cultural practice among the community surveyed and may not be

4 representative of the general population. Average daily consumption of soil was estimated to be

5 50 g/day. Bruhn and Pangborn (1971) reported the incidence of pica for “dirt” to be 19 percent

6 in children, 14 percent in pregnant women, and 3 percent in nonpregnant women. However,

7 “dirt” was not clearly defined. The Bruhn and Pangborn (1971) study was conducted among 91

8 non-black, low income families of migrant agricultural workers in California. Based on the data

9 from the five key tracer studies (Binder et al., 1986; Clausing et al., 1987; Van Wïjnen et al.,

10 1990; Davis et al., 1990; and Calabrese et al., 1989) only one child out of the more than 600

11 children involved in all of these studies ingested an amount of soil significantly greater than the

12 range for other children. Although these studies did not include data for all populations and

13 were representative of short-term ingestions only, it can be assumed that the incidence rate of the

14 recurrent ingestion of unusually high amounts of soil in the general population is low. 

15 However, it is incumbent upon the user to use the appropriate value for their specific study

16 population.

17

18 5.3.2. Soil Pica Among Children

19 Information on the amount of soil ingested by children with pica behavior is limited.

20 However, some evidence suggests that a rate on the order of 10 g/day may not be unreasonable.

21

22 5.3.2.1. Calabrese et al., 1991

23 Calabrese et al. (1991) estimated that upper range soil ingestion values may range from

24 approximately 5 to 7 g/day. This estimate was based on observation of one pica child among the

25 64 children who participated in the study. In the study, a 3.5-year-old female exhibited extremely

26 high soil ingestion behavior during one of the two weeks of observation. Intake ranged from 74

27 to 2200 mg/day during the first week of observation and from 10,100 to 13,600 mg/day during

28 the second week of observation (Table 5-16). These results are based on mass-balance analyses

29 for seven tracer elements (aluminum, barium, manganese, silicon, titanium, vanadium, and

30 yttrium) of the eight used. Intake rates based on zirconium were significantly lower. In a
5-27

1 subsequent paper, the authors indicated that this may have resulted from sample loss due to GI

2 absorption (Stanek and Calabrese, 1994).

3

4 5.3.2.2. Calabrese and Stanek, 1992b

5 Using a methodology that compared differential element ratios, Calabrese and Stanek

6 (1992b) quantitatively distinguished outdoor soil ingestion from indoor dust ingestion in a soil

7 pica child. This study was based on a previous mass-balance study (Calabrese et al., 1991) in

8 which a 3.5-year-old child ingested 10 to 13 grams of soil per day during the second week of a 2-

9 week soil ingestion study. 

10 Table 5-17 presents tracer ratios of soil, dust, and residual fecal samples in the soil pica

11 child. The authors reported that there was a maximum total of 28 pairs of tracer ratios based on

12 eight tracers. However, only 19 pairs of tracer ratios were available for quantitative evaluation,

13 as shown in Table 5-17. Of these 19 pairs, nine fecal tracer ratios fell between the limits for soil

14 and dust (Table 5-17). For these nine tracer soils, an interpolation was performed to estimate the

15 relative contribution of soil and dust to the residual fecal tracer ratio; this analysis indicates that

16 from 71 to 99% of the tracer originated from soil. All of the other 10 fecal tracer ratios that fell

17 outside the soil and dust limits were indicative of 100% soil origin. Therefore, the authors

18 conclude that the predominant proportion of the fecal tracers originated from outdoor soil and

19 not indoor dust.

20

21 5.3.2.3. Calabrese and Stanek, 1993

22 Calabrese and Stanek (1993) reviewed a study by Wong (1988) that attempted to estimate

23 the amount of soil ingested by two groups of children. Wong studied a total of 52 children in two

24 government institutions in Jamaica. The younger group (from the Glenhope Place of Safety)

25 contained 24 children with an average age of 3.1 years (range of 0.3 to 7.6 years). The older

26 group (from the Reddies Place of Safety) contained 28 children with an average age of 7.2 years

27 (range of 1.8 to 14 years). Fecal samples were obtained from the subject children and the

28 amount of silicon in dry feces was measured to estimate soil ingestion.

29 An unspecified number of daily fecal samples were collected from a hospital control

30 group of 30 children with an average age of 4.8 years (range of 0.3 to 12 years). Dry feces were

31 observed to contain 1.45% silicon, or 14.5 mg Si per gram of dry feces. This quantity was used
5-28

1 as a baseline representing the background level of silicon ingestion from dietary sources.

2 Observed quantities of silicon greater than 1.45% were interpreted as originating from soil

3 ingestion.

4 The amount of soil ingested was calculated using the formula of Binder et al. (1986). One

5 fecal sample was collected each month from each subject over the four-month study period.

6 For the 28 children in the older group, soil ingestion was estimated to be 58 mg/day,

7 based on the mean minus one outlier, and 1520 mg/day, based on the mean of all the children.

8 The outlier was a child with an estimated average soil ingestion rate of 41 g/day over the 4

9 months. Of the 28 children in the group, 7 had an average soil ingestion greater than 100 mg/day,

10 4 had an average soil ingestion greater than 200 mg/day, and 1 had an average soil ingestion

11 greater than 300 mg/day; 8 children showed no indication of soil ingestion.

12 Estimates of soil ingestion were higher in the younger group of children. The mean soil

13 ingestion of all the children was 470 ± 370 mg/day. Due to some sample losses, of the 24

14 children studied, only 15 had samples for each of the 4 months of the study. Of the 24 children in

15 the group, 14 had an average soil ingestion less than 100 mg/day, 10 had an average soil

16 ingestion greater than 100 mg/day, 5 had an average soil ingestion greater than 600 mg/day, and

17 4 had an average soil ingestion greater than 1000 mg/day; 5 children showed no indication of

18 soil ingestion. 

19 Over the entire 4-month study period, 9 of 84 samples (or 10.5%) yielded soil ingestion

20 estimates in excess of 1 g/day, indicating pica behavior. Of the 52 children studied, 6 displayed

21 soil pica behavior. The estimated soil ingestion for each of these subjects is shown in Table 5-18.

22 For the younger group of children, 5 of 24 (or 20.8%) displayed pica behavior on at least one

23 occasion. A high degree of daily variability in soil ingestion was observed among the six pica

24 children; three (#11, 12, and 22) showed pica behavior on only 1 of 4 days. The other three (#14,

25 18, and 27) showed pica behavior on 2, 3, and 4 days, respectively. Subject #27 consumed the

26 most soil (3.7 to 60.6 g/day); however, it was indicated that this child was mentally retarded,

27 whereas the other pica children were considered to have normal mental capabilities.

28 Sources of uncertainty or error in this study include differences between the hospital

29 study group (the background control) and the two study groups, lack of information on the intake

30 of silicon for the children in the study, use of a single fecal sample, and loss of fecal samples.

31 The use of a single soil tracer may also introduce error because there may be other sources of the
5-29

1 tracer in the children’s environment. For example, some toothpastes have extremely high silica

2 concentrations, and children may ingest significant quantities of toothpaste. Tracers may also be

3 found in indoor dust that children could ingest. However, despite these uncertainties, the results

4 are important in that they indicate that soil pica is not a rare occurrence in younger children.

5

6 5.3.2.4. Zartarian et al. 2005

7 Zartarian et al. (2005) conducted an analysis of soil ingestion rates from several studies

8 in the literature to be used as input for the Stochastic Human Exposure and Dose Simulation

9 (SHEDS-Wood) model which was used in the EPA report entitled A Probabilistic Exposure

10 Assessment for Children Who Contact CCA-Treated Playsets and Decks (U.S. EPA 2005).

11 Soil ingestion rate estimates were derived for the SHEDS-Wood model using data from

12 Calabrese’s Amherst and Anaconda studies. Data statistics from both of these studies were used

13 to fit distributions of soil/dust ingestion rates. Zartarian et al. (2005) derived a soil pica

14 distribution by sampling from the fitted lognormal distribution and retaining values above 500

mg/day. The mean and 95th 15 percentile values for this population was estimated to be 963 mg/day

16 and 2170 mg/day, respectively. The distribution is presented in Table 5-20. 

17

18 5.4 RECOMMENDATIONS

19 The key studies described in this section were used to recommend values for soil intake

20 among children. The list of these studies is provided in Table 5-19. Estimates of the amount of

21 soil ingested by children based on the key studies are summarized in Table 5-20 and the

22 recommended values are presented in Table 5-21. The mean values ranged from 38 mg/day to

23 193 mg/day with a weighted average of 90 mg/day for soil ingestion and 106 mg/day when it

24 was considered that a portion of the soil ingested comes from dust. These estimates are based

25 on weighted averages using aluminum and silicon as tracers, except for Calabrese et al. (1997),

26 which uses the best tracer methodology. These tracer elements were considered the most

27 reliable based on a review of the current literature. Results obtained using titanium as a tracer

28 were not considered in the derivation of the recommendations because titanium exhibits greater

29 variability compared to other tracers. Results using titanium are consistently higher than other

30 tracers. This may indicate that there are other non-food and non-soil sources of titanium being

31 ingested. For example, titanium is used in paper coatings and as paper fillers and in paints,
5-30

1 lacquers, and enamels (IPCS, 1982). Ingestion of these non-food items has not been

2 investigated.

3 Most of the studies used in this chapter did not categorize soil intake by the age groups

4 recommended in EPA 2005. Therefore, the recommended values in Table 5-21 apply to

5 children from age 1 to 7 years. At this time, the raw data from these studies are not available.

6 Data are particularly lacking for children < 1 year of age. Van Wijnen et al. (1990) (Table 5-10)

7 derived soil ingestion estimates for various age categories. Although Van Wijnen et al. (1990)

8 was not considered a key study because the presence of tracers in food and medicines was not

9 taken into consideration and the study may not representative of U.S. children, it showed that

10 children 4 to < 5 years of age had median soil ingestion rates up to 1.8 times higher than children

11 <1 year of age. However, one needs to consider that infants may spend most of their time indoors

12 and dust concentration may be more appropriate for calculating ingestion rates for this group. 

13 Using dust concentrations for estimating intake rates generally results in higher ingestion rates. 

14 Dust samples were not collected by Van Wijnen et al. (1990).

15 There are a number of limitations with the data presented in Table 5-20. A number of

16 studies have indicated that aluminum and silicon can be absorbed in small amounts from the

17 digestive tract in adults (Davis and Mirick 2006). Therefore, these soil ingestion values may be

18 biased low. It is also worth noting that even though there are five key studies presented in Table

19 5-20, they represent only four populations (i.e, Amherst, Anaconda, Tri-city area in southeastern

20 Washington, and a subset of the Tri-city area study) adding up to 241 children. Other studies are

21 reanalyses of these populations. Therefore, in some instances the same population is counted

22 more than once in the weighted averages presented in Table 5-20. In addition, since the children

23 were studied for short periods of time and the prevalence of pica behavior is not known,

24 excluding children with pica behavior from the calculations may underestimate long-term soil

25 intake rates. It is plausible that many children may exhibit some pica behavior if studied for

26 longer periods of time. Since young children may spend a significant number of hours indoors,

27 it may not be appropriate to assume that all the soil ingested came from outdoor exposure.

28 Therefore, the recommended soil ingestion values are based on soil and dust estimates. 

29 Rounding up to the nearest hundred, 100 mg/day is the best estimate of the mean soil

30 ingestion for children under 7 years of age. Over the period of study, 95th percentile values

31 ranged from 217 mg/day to 449mg/day with an average of 236 mg/day for soil ingestion and 449
5-31

1 mg/day when both soil and dust are considered. Rounding to the nearest hundred, the

2 recommended 95th percentile soil ingestion rate for children is 400 mg/day based on soil

3 and dust ingestion. A distribution of soil ingestion values is presented in Table 5-20. However,

4 since the children were studied for a short period of time and usually during the summer months,

5 these values are not estimates of usual intake.

6 Data on soil ingestion rates for children who exhibit pica behavior (i.e., ingest unusually

7 high amounts of soil on a recurrent basis) are also limited. In conducting a risk assessment for

8 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), U.S. EPA (1984) used 5 g/day to represent the soil

9 intake rate for pica children. The Centers for Disease Control (CDC) also investigated the

10 potential for exposure to 2,3,7,8-TCDD via soil ingestion. CDC used a value of 10 g/day to

11 represent the amount of soil that a child with pica behavior might ingest (Kimbrough et al.,

12 1984). These values are consistent with those observed by Calabrese et al. (1991). An ingestion

13 rate of 10 g/day is a reasonable value for use in acute exposure assessments, based on the

14 available information. This value is based on only one pica child observed in the Calabrese et al.

15 (1989) study where the intake ranged from 10-14 grams/day during the second week of

16 observation. In addition, a statistical designation is not assigned to this value.

17 These recommendations are based on studies that used different survey designs and

18 populations. For example, in some studies soil ingestion estimates were adjusted to account for

19 the contribution of house dust to this estimate. Other studies used best tracer methodology while

20 others relied on estimates from specific tracers. Despite these differences, the mean and upper21 percentile estimates reported for these studies are relatively consistent. The confidence rating

22 for soil intake recommendations is presented in Table 5-22. It is important to understand,

23 however, the various uncertainties associated with these values. First, individuals were not

24 studied for sufficient periods of time to get a good estimate of the usual intake. Therefore, the

25 values presented in this section may not be representative of long-term exposures. Second, the

26 experimental error in measuring soil ingestion values for individual children is also a source of

27 uncertainty. For example, incomplete sample collection of both input (i.e., food and non food

28 sources) and output (i.e., urine and feces) is a limitation for some of the studies conducted. In

29 addition, an individual's soil ingestion value may be artificially high or low depending on the

30 extent to which a mismatch between input and output occurs due to individual variation in the

31 gastrointestinal transit time. Third, the degree to which the tracer elements used in these studies
5-32

1 are absorbed in the human body is uncertain. Accuracy of the soil ingestion estimates depends

2 on how good this assumption is. Fourth, there is uncertainty with regard to the homogeneity of

3 soil samples and the accuracy of parents’ knowledge about their child’s playing areas. Fifth, all

4 the soil ingestion studies presented in this section with the exception of Calabrese et al. (1989)

5 were conducted during the summer when soil contact is more likely.

6 Although the recommendations presented in this section are derived from studies which

7 were mostly conducted in the summer, exposure during the winter months, when the ground is

8 frozen or snow covered in many regions of the United States, should not be considered as zero. 

9 Exposure during these months, although lower than in the summer months, would not be zero

10 because some portion of house dust comes from outdoor soil.

11

12

13
5-33

1 5.5 REFERENCES FOR CHAPTER 5

2

3 ATSDR (2001) Summary report for the ATSDR soil-pica workshop. ATSDR, Atlanta, GA. March 20, 2001. 

4 Available on line at: http://www.atsdr.cdc.gov/NEWS/soilpica.html.

5

6 Binder, S.; Sokal, D.; Maughan, D. (1986) Estimating soil ingestion: the use of tracer elements in estimating the

7 amount of soil ingested by young children. Arch. Environ. Health. 41(6):341-345.

8

9 Behrman, L.E.; Vaughan, V.C., III. (1983) Textbook of Pediatrics. Philadelphia, PA: W.B. Saunders Company.

10

11 Bruhn, C.M.; Pangborn, R.M. (1971) Reported incidence of pica among migrant families. J. of the Am. Diet.

12 Assoc. 58:417-420.

13

14 Calabrese, E.J.; Pastides, H.; Barnes, R.; Edwards, C.; Kostecki, P.T.; et al. (1989) How much soil do young

15 children ingest: an epidemiologic study. In: Petroleum Contaminated Soils, Lewis Publishers, Chelsea, MI. 

16 pp. 363-397.

17

18 Calabrese, E.J.; Stanek, E.J.; Gilbert, C.E.; and Barnes, R. (1990) Preliminary adult soil ingestion estimates: Results

19 of a pilot study. Regul. Toxicol. Pharmacol. 12:88-95.

20

21 Calabrese, E.J.; Stanek, E.J.; Gilbert, C.E. (1991) Evidence of soil-pica behavior and quantification of soil ingested. 

22 Hum. Exp. Toxicol. 10:245-249.

23

24 Calabrese, E.J.; Stanek, E.J. (1992) What proportion of household dust is derived from outdoor soil? J. Soil Contam. 

25 1(3): 253-263.

26

27 Calabrese, E.J.; Stanek, E.J. (1992) Distinguishing outdoor soil ingestion from indoor dust ingestion in a soil pica

28 child. Regul. Toxicol. Pharmacol. 15:83-85.

29

30 Calabrese, E.J.; Stanek, E.J. (1993) Soil pica: not a rare event. J. Environ. Sci. Health. A28(2):373-384.

31

32 Calabrese, E.J.; Stanek, E.J. (1995) Resolving intertracer inconsistencies in soil ingestion estimation. Environ.

33 Health Perspect. 103(5):454-456.

34

35 Calabrese, E.J.; Stanek, E.J.; Barnes, R.M. (1996) Methodology to estimate the amount and particle size of soil

36 ingested by children: Implications for exposure assessment at waste sites. Regul. Toxicol. Pharmacol. 24:

37 264-268.
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1 Calabrese, E.J.; Stanek, E.J.; Pekow, P.; Barnes, R.M. (1997a) Soil ingestion estimates for children residing on a

2 Superfund site. Ecotoxicology and Environmental Safety. 36:258-268.

3

4 Calabrese, E.J.; Stanek, E.J.; Barnes, R.M. (1997b) Soil ingestion rates in children identified by parental

5 observation as likely high soil ingesters. J. Soil Contam. 6(3): 271-279.

6

7 Clausing, P.; Brunekreef, B.; Van Wijnen, J.H. (1987) A method for estimating soil ingestion by children. Int.

8 Arch. Occup. Environ. Health (W. Germany) 59(1):73-82.

9

10 Danford, D.C. (1982) Pica and nutrition. Annual Review of Nutrition. 2:303-322.

11

12 Davis, S.; Waller, P.; Buschbon, R.; Ballou, J.; White, P. (1990) Quantitative estimates of soil ingestion in normal

13 children between the ages of 2 and 7 years: population based estimates using aluminum, silicon, and titanium

14 as soil tracer elements. Arch. Environ. Hlth. 45:112-122.

15

16 Davis, S. and Mirick, D. (2006) Soil ingestion in children and adults in the same family. Journal of Exposure

17 Analysis and Environmental Epidemiology. 16:63-75.

18

19 Feldman, M.D. (1986) Pica: current perspectives. Psychosomatics (USA) 27(7):519-523. 

20

21 Forfar, J.O.; Arneil, G.C., eds. (1984) Textbook of Paediatrics. 3rd ed. London: Churchill Livingstone.

22

23 Illingworth, R.S. (1983) The normal child. New York: Churchill Livingstone.

24

25 IPCS (1982). Environmental health criteria 24: Titanium. International Programme on Chemical Safety. Published

26 under the joint sponsorship of the United Nations Environment Programme, the International Labour

27 Organisation, and the World Health Organization. Available on-line at

28 http://www.inchem.org/documents/ehc/ehc/ehc24.htm#SectionNumber:3.1 

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30 Kaplan, H.I.; Sadock, B.J. (1985) Comprehensive textbook of psychiatry/IV. Baltimore, MD: Williams and

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32

33 Kimbrough, R.; Falk, H.; Stemr, P.; Fries, G. (1984) Health implications of 2,3,7,8-tetrachlorodibenzo-p-dioxin

34 (TCDD) contamination of residential soil. J. Toxicol. Environ. Health 14:47-93.

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36 Lourie, R.S.; Layman, E.M.; Millican, F.K. (1963) Why children eat things that are not food. Children 10:143-146.

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38 Sayetta, R.B. (1986) Pica: An overview. American Family Physician 33(5):181-185.
5-35

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2 Sedman, R.; Mahmood, R.S. (1994) Soil ingestion by children and adults reconsidered using the results of recent

3 tracer studies. Air and Waste, 44:141-144.

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5 Stanek, E.J.; Calabrese, E.J. (1994) Bias and the detection limit model for soil ingestion. J. Soil Contam. 3(2): 183-

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7

8 Stanek, E.J.; Calabrese, E.J. (1995a) Daily estimates of soil ingestion in children. Environ. Health Perspect.

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10

11 Stanek, E.J.; Calabrese, E.J. (1995b) Soil ingestion estimates for use in site evaluations based on the best tracer

12 method. Human and Ecological Risk Assessment. 1:133-156.

13

14 Stanek, E.J.; Calabrese, E.J.; Barnes, R. (1999) Soil ingestion estimates for children in Anaconda using trace element

15 concentrations in different particle size fractions. Human Ecol. Risk Assessment 5(3): 547-558.

16

17 Stanek, E.J.; Calabrese, E.J. (2000) Daily soil ingestion estimates for children at a Superfund site. Risk Analysis

18 20(5): 627-635.

19

20 Stanek, E.J.; Calabrese, E.J.; Zorn, M. (2001a) Biasing factors for simple soil ingestion estimates in mass balance

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22

23 Stanek, E.J.; Calabrese, E.J.; Zorn, M. (2001b) Soil ingestion distributions for Monte Carlo risk assessment in

24 children. Human and Ecological Risk Assessment. 7(2): 357-368.

25

26 Thompson, K.M.; Burmaster, D.E. (1991) Parametric distributions for soil ingestion by children. Risk Analysis. 

27 11:339-342.

28

29 U.S. EPA. (1984) Risk analysis of TCDD contaminated soil. Washington, DC: U.S. Environmental Protection

30 Agency, Office of Health and Environmental Assessment. EPA 600/8-84-031.

31

32 U.S. EPA. (2005) Guidance on selecting age groups for monitoring and assessing childhood exposures to

33 environmental contaminants. Risk Assessment Forum, Office of Research and Development, Washington,

34 DC. EPA/630/P-03/003F.

35

36 Van Wijnen, J.H.; Clausing, P.; Brunekreff, B. (1990) Estimated soil ingestion by children. Environ. Res. 51:147-

37 162.

38
5-36

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2 nutritional implications. Am. J. Clin. Nutr. 32:2129-2135. 

3

4 Wong, M.S. (1988) The Role of Environmental and Host Behavioural Factors in Determining Exposure to Infection

5 with Ascaris lumbricoldes and Trichuris trichlura. Ph.D. Thesis, Faculty of Natural Sciences, University of

6 the West Indies. 1988.

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8 Zartarian, V.G., J. Xue, H. A. Ozkaynak, W. Dang, G. Glen, L. Smith, and C. Stallings. (2005). A probabilistic

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11 Environmental Protection Agency, Washington, DC, EPA/600/X-05/009. 

12
5-37

Table 5-1. Average Daily Soil Ingestion Values Based on Aluminum, Silicon, and Titanium as Tracer Elementsa

Element Mean

(mg/d)

Median

(mg/d)

Standard Error of

the Mean

(mg/d)

Range

(mg/d)b

Aluminum 38.9 25.3 14.4 279.0 to 904.5

Silicon 82.4 59.4 12.2 -404.0 to 534.6

Titanium 245.5 81.3 119.7 -5,820.8 to 6,182.2

Minimum 38.9 25.3 12.2 -5,820.8

Maximum 245.5 81.3 119.7 6,182.2

a

Excludes three children who did not provide any samples (N=101).

b

Negative values occurred as a result of correction for nonsoil sources of the tracer elements.

Source: Adapted from Davis et al. (1990).

Table 5-2. Soil Ingestion Estimates for the Median of Best Four Trace Elements Based on Food/Soil Ratios for 64

Anaconda Children (mg/day) Using Al, Si, Ti, Y, and Zr

Category

Soil Ingestion (mg/day)a

Min P5 P10 P25 P50 P75 P90 P95 Max Mea

n

SD

Median of

best 4

-101.3 -91.0 -53.8 -38.0 -2.4 26.8 73.1 159.8 380.2 6.8 74.5

Best tracer -53.4 -24.4 -14.4 2.2 20.1 68.9 223.6 282.4 609.9 65.5 120.3

2nd best -115.9 -62.1 -48.6 -26.6 1.5 38.4 119.5 262.3 928.5 33.2 144.8

3rd best -170.5 -88.9 -67.0 -52.0 -18.8 25.6 154.7 376.1 1293.5 31.2 199.6

4th best -298.3 -171.0 -131.9 -74.7 -29.3 0.2 74.8 116.8 139.1 -34.6 79.7

a

Negative values occurred as a result of calculating child-specific estimates for multiple days. For example, negative

estimates of soil ingestion occurred when an individual child had low, but positive, soil ingestion, but the standard

deviation was large.

Source: Calabrese et al. (1997). 
5-38

Table 5-3. Dust Ingestion Estimates for the Median of Best Four Trace Elements Based on Food/Dust Ratios for 64

Anaconda Children (mg/day) Using Al, Si, Ti, Y, and Zr

Category

Soil Ingestion (mg/day)a

Min P5 P10 P25 P50 P75 P90 P95 Max Mean SD

Median of

best 4

-261.5 -186.2 -152.7 -69.5 -5.5 62.8 209.2 353.0 683.9 16.5 160.9

Best tracer -377.0 -193.8 -91.0 -20.8 26.8 198.1 558.6 613.6 1499.4 127.2 299.1

2nd best -239.8 -147.2 -137.1 -59.1 7.6 153.1 356.4 409.5 1685.1 82.7 283.6

3rd best -375.7 -247.5 -203.1 -81.7 -14.4 49.4 406.5 500.5 913.2 25.5 235.9

4th best -542.7 -365.6 -277.7 -161.5 -55.1 52.4 277.3 248.8 6120.5 81.8 840.3

a

Negative values occurred as a result of calculating child-specific estimates for multiple days. For example, negative

estimates of dust ingestion occurred when an individual child had low, but positive, dust ingestion, but the standard

deviation was large.

Source: Calabrese et al. (1997).

Table 5-4. Mean and Median Soil Ingestion (mg/day) by Family Member

Participant Tracer Element Estimated Soil Ingestiona

 (mg/day)

 Mean Median Std Maximum

Childb Aluminum 36.7 33.3 35.4 107.9

Silicon 38.1 26.4 31.4 95.0

Titanium 206.9 46.7 277.5 808.3

Source: Davis and Mirick 2006

a

 For some study participants, estimated soil ingestion resulted in a negative value. These estimates have been set to

0 mg/day for tabulation and analysis.

b

 Results based on 12 children with complete food, excreta, and soil data.
5-39

Table 5-5. Estimated Daily Soil Ingestion Based on Aluminum, Silicon, and Titanium Concentrations

Estimation

Method

Mean

(mg/day)

Median

(mg/day)

Standard

Deviation

(mg/day)

Range

(mg/day)

95th

Percentile

(mg/day)

Geometric

Mean

(mg/day)

Aluminum 181 121 203 25-1,324 584 128

Silicon 184 136 175 31-799 578 130

Titanium 1,834 618 3,091 4-17,076 9,590 401

Minimum 108 88 121 4-708 386 65

Source: Binder et al. (1986).
5-40

Table 5-6. Calculated Soil Ingestion by Nursery School Children

Child

Sample

Number

Soil Ingestion as

Calculated from Ti

(mg/day)

Soil Ingestion as

Calculated from Al

(mg/day)

Soil Ingestion as

Calculated from AIR

(mg/day)

Limiting Tracer

(mg/day)

1 L3

L14

L25

103

154

130

300

211

23

107

172

-

103

154

23

2 L5

L13

L27

131

184

142

-

103

81

71

82

84

71

82

81

3 L2

L17

124

670

42

566

84

174

42

174

4 L4

L11

246

2,990

62

65

145

139

62

65

5 L8

L21

293

313

-

-

108

152

108

152

6 L12

L16

1,110

176

693

-

362

145

362

145

7 L18

L22

11,620

11,320

-

77

120

-

120

77

8

9

10

11

12

13

14

15

16

17

18

L1

L6

L7

L9

L10

L15

L19

L20

L23

L24

L26

3,060

624

600

133

354

2,400

124

269

1,130

64

184

82

979

200

-

195

-

71

212

51

566

56

96

111

124

95

106

48

93

274

84

-

-

82

111

124

95

106

48

71

212

51

64

56

Arithmetic

Mean

1,431 232 129 105

Source: Adapted from Clausing et al. (1987).
5-41

Table 5-7. Calculated Soil Ingestion by Hospitalized, Bedridden Children

Child Sample

Soil Ingestion as

Calculated from Ti

(mg/day)

Soil Ingestion as

Calculated from Al

(mg/day)

Limiting Tracer

(mg/day)

1 G5

G6

3,290

4,790

57

71

57

71

2 G1 28 26 26

3 G2

G8

6,570

2,480

94

57

84

57

4 G3 28 77 28

5 G4 1,100 30 30

6 G7 58 38 38

Arithmetic Mean 2,293 56 49

Source: Adapted from Clausing et al. (1987).

Table 5-8. Mean and Standard Deviation Percentage Recovery of Eight Tracer Elements

Tracer Element

300 mg Soil Ingested 1,500 mg Soil Ingested

Mean SD Mean SD

Al 152.8 107.5 93.5 15.5

Ba 2304.3 4533.0 149.8 69.5

Mn 1177.2 1341.0 248.3 183.6

Si 139.3 149.6 91.8 16.6

Ti 251.5 316.0 286.3 380.0

V 345.0 247.0 147.6 66.8

Y 120.5 42.4 87.5 12.6

Zr 80.6 43.7 54.6 33.4

Source: Adapted from Calabrese et al. (1989).
5-42

Table 5-9. Soil and Dust Ingestion Estimates for Children Ages 1-4 Years

Tracer Element N

Intake (mg/day)a

Mean Median SD

95th

Percentile Maximum

Aluminum

 soil

 dust

 soil/dust combined

Silicon

 soil

 dust

 soil/dust combined

Yttrium

 soil

 dust

 soil/dust combined

Titanium

 soil

 dust

 soil/dust combined

64

64

64

64

64

64

62

64

62

64

64

64

153

317

154

154

964

483

85

62

65

218

163

170

29

31

30

40

49

49

9

15

11

55

28

30

852

1,272

629

693

6,848

3,105

890

687

717

1,150

659

691

223

506

478

276

692

653

106

169

159

1,432

1,266

1,059

6,837

8,462

4,929

5,549

54,870

24,900

6,736

5,096

5,269

6,707

3,354

3,597

a

Corrected for Tracer Concentrations in Foods

Source: Adapted from Calabrese et al. (1989).
5-43

Table 5-10. Geometric Mean (GM) and Standard Deviation (GSD) LTM Values for Children at Daycare Centers

and Campgrounds

Age (yrs) Sex

Daycare Centers Campgrounds

n

GM LTM

(mg/day)

GSD LTM

(mg/day) n

GM LTM

(mg/day)

GSD LTM

(mg/day)

birth to <1 Girls

Boys

3

1

81

75

1.09

-

-

-

-

-

-

-

1 to <2 Girls

Boys

20

17

124

114

1.87

1.47

3

5

207

312

1.99

2.58

2 to <3 Girls

Boys

34

17

118

96

1.74

1.53

4

8

367

232

2.44

2.15

3 to <4 Girls

Boys

26

29

111

110

1.57

1.32

6

8

164

148

1.27

1.42

4 to <5 Girls

Boys

1

4

180

99

-

1.62

19

18

164

136

1.48

1.30

3 to <5c Girls

Boys

27

33

146

105

1.57

1.47

25

26

164

142

1.38

1.36

All girls

All boys

Total

86

72

162a

117

104

111

1.70

1.46

1.60

36

42

78b

179

169

174

1.67

1.79

1.73

a

Age and/or sex not registered for eight children.

b

Age not registered for seven children.

c

This age category is calculated from the previous two rows of data in order to conform to the standardized age

categories used in this Handbook.

Source: Adapted from Van Wijnen et al. (1990).
5-44

Table 5-11. Estimated Geometric Mean Limiting Tracer Method (LTM) Values of Children Attending Daycare

Centers According to Age, Weather Category, and Sampling Period

Weather Category Age

(years)

First Sampling Period Second Sampling Period

n

Estimated Geometric

Mean

LTM Value

(mg/day)

n

Estimated Geometric

Mean

LTM Value

(mg/day)

Bad

(>4 days/week precipitation)

<1

1 to <2

2 to <3

4 to <5

3

18

33

5

94

103

109

124

3

33

48

6

67

80

91

109

Reasonable

(2-3 days/week precipitation)

<1

1 to <2

2 to <3

3 to <4

4 to <5

3 to <5a

1

10

13

19

1

20

61

96

99

94

61

92

Good

(<2 days/week precipitation)

<1

1 to <2

2 to <3

3 to <4

4 to <5

3 to <5a

4

42

65

67

10

77

102

229

166

138

132

137

a

This age category is calculated from the available data in order to conform to the standardized age categories

used in this Handbook. Value is a weighted mean of the previous two rows.

 Source: Van Wijnen et al. (1990).
5-45

Table 5-12. Distribution of Average (Mean) Daily Soil Ingestion Estimates per Child for 64 Childrena (mg/day)

Type of Estimate

Number of

Samples

Overall

(64)

A1

(64)

Ba

(33)

Mn

(19)

Si

(63)

Ti

(56)

V

(52)

Y

(61)

Zr

(62)

Mean 179 122 655 1,053 139 271 112 165 23

25th Percentile 10 10 28 35 5 8800

50th Percentile 45 19 65 121 32 31 47 15 15

75th Percentile 88 73 260 319 94 93 177 47 41

90th Percentile 186 131 470 478 206 154 340 105 87

95th Percentile 208 254 518 17,374 224 279 398 144 117

Maximum 7,703 4,692 17,991 17,374 4,975 12,055 845 8,976 208

a

For each child, estimates of soil ingestion were formed on days 4-8 and the mean of these estimates was then

 evaluated for each child. The values in the column “overall” correspond to percentiles of the distribution of

 these means over the 64 children. When specific trace elements were not excluded via the relative standard

 deviation criteria, estimates of soil ingestion based on the specific trace element were formed for 108 days for

 each subject. The mean soil ingestion estimate was again evaluated. The distribution of these means for

 specific trace elements is shown.

Source: Stanek and Calabrese (1995a).

Table 5-13. Estimated Distribution of Individual Mean Daily Soil Ingestion Based on Data for 64 Subjects

Projected over 365 Daysa

Range

50th Percentile (median)

90th Percentile

95th Percentile

1 - 2,268 mg/db

75 mg/d

1,190 mg/d

1,751 mg/d

a

 Based on fitting a log-normal distribution to model daily soil ingestion values.

b

 Subject with pica excluded.

Source: Stanek and Calabrese (1995a).
5-46

Table 5-14. Summary Statistics and Parameters for Distributions of Estimated Soil Ingestion by Tracer Elementa

Soil Intake (mg/day)

A1 Si Ti Meana

Mean

Min

10th

20th

30th

40th

Med

60th

70th

80th

90th

Max

97

11

21

33

39

43

45

55

73

104

197

1,201

85

10

19

23

36

52

60

65

79

106

166

642

1,004

1

3

22

47

172

293

475

724

1,071

2,105

14,061

91

13

22

34

43

49

59

69

92

100

143

921

Lognormal Distribution Parameters

Median

Standard Deviation

Arithmetic Mean

45

169

97

60

95

85

--

--

--

59

126

91

Underlying Normal Distribution Parameters

Mean

Standard Deviation

4.06

0.88

4.07

0.85

--

--

4.13

0.80

a

 Using Binder et al. (1986) data with actual fecal weights.

b Mean = arithmetic average of soil ingestion based on aluminum and silicon.

Source: Thompson and Burmaster (1991).
5-47

Table 5-15. Positive/negative Error (Bias) in Soil Ingestion Estimates in the Calabrese et al. (1989) Massbalance Study: Effect on Mean Soil Ingestion Estimate (Mg/day)a

Tracer

Negative Error

Lack of Fecal

Sample on Final

Study Day

Other

Causesb

Total

Negative

Error

Total

Positive

Error Net Error

Original

Mean

Adjusted

Mean

Aluminum

Silicon

Titanium

Vanadium

Yttrium

Zirconium

14

15

82

66

8

6

11

6

187

55

26

91

25

21

269

121

34

97

43

41

282

432

22

5

+18

+20

+13

+311

-12

-92

153

154

218

459

85

21

136

133

208

148

97

113

a

How to read table: for example, aluminum as a soil tracer displayed both negative and positive error. The

 cumulative total negative error is estimated to bias the mean estimate by 25 mg/day downward. However,

 aluminum has positive error biasing the original mean upward by 43 mg/day. The net bias in the original

 mean was 18 mg/day positive bias. Thus, the original 156 mg/day mean for aluminum should be corrected

 downward to 136 mg/day.

b

Values indicate impact on mean of 128-subject-weeks in milligrams of soil ingested per day.

Source: Calabrese and Stanek (1995).
5-48

Table 5-16. Daily Soil Ingestion Estimation in a Soil-Pica Child by Tracer and by Week (mg/day)

Tracer

element

 Estimated Soil Ingestion (mg/day)

Week 1 Week 2

Al

Ba

Mn

Si

Ti

V

Y

Zr

74

458

2,221

142

1,543

1,269

147

86

13,600

12,088

12,341

10,955

11,870

10,071

13,325

2,695

Source: Calabrese et al. (1991).
5-49

Table 5-17. Ratios of Soil, Dust, and Residual Fecal Samples in the Soil Pica Child

Tracer Pairs

Ratio Estimated Residual Fecal Tracers of

Soil Origin as Predicted by Specific

Tracer Ratios (%)

Soil Fecal Dust

1. Mn/Ti

2. Ba/Ti

3. Si/Ti

4. V/Ti

5. Ai/Ti

6. Y/Ti

7. Mn/Y

8. Ba/Y

9. Si/Y

10. V/Y

11. Al/Y

12. Mn/Al

13. Ba/Al

14. Si/Al

15. V/Al

16. Si/V

17. Mn/Si

18. Ba/Si

19. Mn/Ba

208.368

187.448

148.117

14.603

18.410

8.577

24.293

21.854

17.268

1.702

2.146

11.318

10.182

8.045

0.793

10.143

1.407

1.266

1.112

215.241

206.191

136.662

10.261

21.087

9.621

22.373

21.432

14.205

1.067

2.192

10.207

9.778

6.481

0.487

13.318

1.575

1.509

1.044

260.126

115.837

7.490

17.887

13.326

5.669

45.882

20.432

1.321

3.155

2.351

19.520

8.692

0.562

1.342

0.419

34.732

15.466

2.246

 87

100

 92

100

100

100

100

 71

 81

100

 88

100

 73

 81

100

100

 99

 83

100

Source: Calabrese and Stanek (1992).
5-50

Table 5-18. Daily Variation of Soil Ingestion by Children Displaying Soil Pica in Wong (1988)

Child subject number Month Estimated soil ingestion

(mg/day)

Glenhope Place of Safety

11 1 55

2 1,447

3 22

4 40

12 1 0

2 0

3 7,924

4 192

14 1 1,016

2 464

3 2,690

4 898

18 1 30

2 10,343

3 4,222

4 1,404

22 1 0

2 --

3 5,341

4 0

Reddies Place of Safety

27 1 48,314

2 60,692

3 51,422

4 3,782

Source: Calabrese and Stanek (1993).
5-51

Table 5-19 Key Studies Used to Derive Recommendations

Key Studies Sample Size Comments

Davis et al. 1990

Stanek and Calabrese,

1995a

Stanek and Calabrese,

1995b

Calabrese et al. 1997a

Davis and Mirick 2006

101

64

162

64

12

Primary analysis

Secondary analysis. This paper is a refinement of the data collected

by Calabrese et al. 1989

Secondary analysis. This paper is a refinement of the data

collected by Calabrese et al. 1989 and Davis et al. 1990 using the

best tracer methodology.

Primary analysis

Primary Analysis
5-52

Table 5- 20 . Summary of Estimates of Incidental Soil and Dust Ingestion by Children (1-7 years old) from Key Studies (mg/day)a

Sample Size Age

(yr)

Source Mean P25 P50 P75 P90 

101 2-7 Soil 61 42 

Soil and Dust 112 82

64 1-4 Soil 131 8 31 84 169 

162 1-7 Soil 104 10 37 80 156 

64 b 1-3

Soil 66 2 20 69 224 

Dust Only c 

127 - 27 198 559 

Soil and Dust d 97 24 134 392 

12 3-7 Soil 38 

Weighted

Average

1-7 Soil 90 8 35 78 174 

Soil and Dust 106 60 134 392 

a Using the average of Al and Si as tracers (except otherwise specified under note “b”)b Using the best tracer methodc Calculated assuming all the ingestion originated from dust.d Calculated by averaging the “soil” and “dust only” rows.
5-53

Table 5-21. Summary of Recommended Values for Soil Ingestion

Population Mean 95th percentile

Children (1-7 years old)

Incidental

Ingestion

Pica Incidental

Ingestion

Pica

100 mg/day 10 g/day 400 mg/day --

Note: See Section 5.4 for discussion of these values.
5-54

Table 5-22. Confidence in Soil Intake Recommendation

Considerations Rationale Rating

Study Elements

C Level of peer review All key studies are from peer review literature. High

C Accessibility Papers are widely available from peer review journals.

However, raw data were not available to do age specific

analysis

Medium

C Reproducibility Methodology used was presented, but results are difficult

to reproduce.

Medium

C Focus on factor of interest The focus of the studies was on estimating soil intake rate

by children.

High

C Data pertinent to U.S. Studies used children from specific areas of the U.S. Medium

C Primary data All the studies were based on primary data. High

C Currency Studies were conducted after 1980. Soil ingestion

behaviors are not expected to change with time.

High

C Adequacy of data collection period Children were not studied long enough to fully

characterize day to day variability. Most of the studies

were conducted during the summer months.

Medium

C Validity of approach The basic approach is the only practical way to study soil

intake, but refinements are needed in tracer selection and

matching input with outputs. The more recent studies

corrected the data for sources of the tracers in food. There

are, however, some concerns about absorption of the

tracers into the body and lag time between input and

output. 

Medium

C Study size The sample sizes used in the key studies were adequate for

some age groups, but not representative of the U.S. Data

are lacking for the very young children and children older

than 7 years old.

Medium

C Representativeness of the

population

The study population may not be representative of the U.S.

in terms of race, socio-economics, and geographical

location; Studies focused on specific areas.

Low

C Characterization of variability Day-to-day variability was not very well characterized. Low

C Lack of bias in study design (high

rating is desirable)

The selection of the population studied may introduce

some bias in the results (i.e., children near a smelter site,

volunteers in nursery school).

 Low

C Measurement error Errors may result due to problems with absorption of the 

tracers in the body and mismatching inputs and outputs.

 Low

Other Elements
Table 5-22. Confidence in Soil Intake Recommendation

Considerations Rationale Rating

5-55

C Number of studies There are 5 key studies. However, only four of those are

original data.

Medium

C Agreement between researchers Despite the variability, there is general agreement among

researchers on central estimates of daily intake for

children.

Medium

Overall Rating Studies were well designed; results were fairly consistent;

sample size was adequate for some age groups; accuracy

of methodology is uncertain; variability cannot be

characterized due to limitations in data collection period. 

Data at the upper end are highly uncertain. Distributions

provided may not be representative of long-term behavior. 

Medium (for

children - long-term

central estimate)

Low (for upper

percentile and

distributions of

long-term behavior)