ANALYTICAL APPROACH
Childhood Soil Ingestion: How Much Dirt Do Kids Eat? Ramon M. Barnes Department of Chemistry Lederle Graduate Research Center University of Massachusetts Amherst, MA 01003-0035
Every parent knows that toddlers and kids eat dirt just by seeing how dirty their hands and faces are at the end of a day, but they don't know how much dirt actually gets swallowed. Apparently many environmental scientists and public health officials don't know either, although children's "hand-tomouth activity" and the amount of dirt on kids' hands have been studied. Based on observation, we know that some children only suck their fingers whereas others put everything in sight into their mouths, including toys, grass, and handfuls of playground soil. This normal mouthing activity is a major way in which children consume nonfood materials from their environment. Pica, on the other hand, is an abnormal craving to eat clay, paint, chalk, or similar substances. An estimate of the
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amount of nonfood material ingested by children is an appropriate indicator in many environmental risk assessments (1). Researchers in our plasma spectrochemical analysis laboratory were asked by the university's School of Public Health Environmental Health Sciences Program a few years ago to participate in a study to quantitate childhood soil ingestion. A contract was received from a California pharmaceutical company involved in lawsuits pertaining to dioxin contamination of soils in Times Beach, MO. This article summarizes some of the factors considered in conducting this study from the analytical chemistry viewpoint. Results of the investigation have been published (2, 3) along with details of the analytical methodology (4, 5) and secondary information (6). Early estimates of soil ingestion by children range from 10 to 10 000 mg/ day (2, 3). During the past few years detailed studies have been performed by Binder et al. (7), Clausing et al. (8), and Davis et al. (9). Although in each of these investigations the accuracy was
limited to some extent by the experimental design (e.g., food intake was not measured), all of the studies used a soil tracer element approach that represents a significant improvement in the estimate of soil consumed compared with earlier approximations. For example, Binder et al. (7) applied inductively coupled plasma atomic emission spectrometry (ICP-AES) to determine Al, Si, and Ti concentrations in infant stool specimens and yard soil samples for 59 children. Clausing et al. (8) measured Al and Ti along with an acidinsoluble residue in the feces of 18 children and in soil by ICP-AES. Approach How do you measure the amount of nonfood material ingested by toddlers while they play at home or at their daycare centers? Although a number of direct methods are conceivable, careful consideration led us to a mass balance technique in which human excreta (i.e., feces and urine) and diet-duplicating meals and snacks were collected for a number of weeks and analyzed. The differences between the selected ele-
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990
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ANALYTICAL APPROACH ment content found in food, beverages, and medication ingested and the feces and urine excreted were then assumed to result from nonfood materials. Some minor error might be introduced because alternative excretion pathways (e.g., sweat, hair) are ignored. Dust and soil samples from each child's play environment also were collected, and the nonfood material balance was converted into an equivalent amount of local dust and/or soil based upon a timeweighted average of estimated contact. Eventually, a comprehensive study to quantitate soil ingested by 64 toddlers in the Amherst area, based upon ICP-AES and ICP mass spectrometry (ICP/MS) determinations of 8 selected elements in dust, feces, food, soil, and urine (2, 3), was established. The approach and methods were verified based on the soil ingestion recovery measured for six adults assuming that the metabolic absorption of elements by adults is the same as that for children (2,5). A distinctive element of this study design compared with earlier or concurrent investigations was that the actual dietary intake for each child and adult was duplicated and evaluated. Selection of elements, development of sample preparation techniques, and evaluation of measurement procedures were based on our experience with children's ingestion of lead-contaminated soils (10) and on literature guidelines. Tracers In selecting possible soil tracers or markers, we had to consider whether an artificial or natural marker could be used; whether the marker was chemical or physical; and, if a chemical marker was used, whether it was an element or one or more isotopes (stable or radioactive). In examining the first category, natural rather than artificial chemical or physical materials were preferred. We did not want to alarm parents or be involved with extensive biological safety testing. Few parents would consent to the exposure of their children to radiotracers or physical markers, such as microglass beads in a sandbox. Second, natural elements rather than stable isotopes were selected for diagnostic purposes because alteration of the natural isotopic abundance for almost all elements would require systematic addition of enriched isotope spikes in the child's home environment. Finally, the ideal commonplace chemical marker should be present in soil at high concentrations yet exist at low concentrations in air, beverages, dust, food, medicines, and water. The chemical marker must be easy to mea1024 A
sure reliably and have a low human bioavailability. For this latter specification, a high fecal-to-urine excretion ratio was preferred so t h a t analyses could be simplified by examining one but not both types of specimens. A limited number of elements exist in soil but not in foods that meet these requirements. Elements known to be essential to humans (i.e., major elements: C, H, N, 0 , S, Ca, P, K, Na, CI, Mg; trace elements: As, Co, Cr, Cu, F, Fe, I, Mn, Mo, Ni, Se, Si, Sn, V, Zn) were not considered because diet is the primary source for many of them. Only part of the ingested material in the gastrointestinal tract is absorbed, and the unabsorbed portion is excreted in the feces, keeping the body in a steady state of metabolic equilibrium. Some elements (e.g., Al, Cd, Cr, Mn, Ni, Sn, and V) are poorly absorbed (i.e.,
