6.0 Topic #4: Additional Topics
The panelists raised several issues when responding to the following final set of charge questions: "What critical research needs should be addressed to provide ATSDR greater insight into the public health implications of soil-pica behavior? What is known about the causes of soil-pica behavior? Does the bioavailability of metals in soil change with the amount of soil that is ingested? Is soil-pica behavior "normal"? Please bring to ATSDR's attention any other topics relevant to soil-pica behavior that are not addressed by the aforementioned questions."
When responding to these questions, the panelists referred to their earlier discussions on the causes of soil-pica behavior and whether this behavior is considered "normal." Following is an overview of the panelists' comments on research needs and bioavailability.
Critical Research Needs. Several panelists thought the most critical data gap currently is the lack of a convincing account of the distribution of soil ingestion rates for various age groups, geographic regions, and selected subpopulations (SD, PS). The panelists thought that future research can best fill this gap, through a multi-faceted approach, possibly one that uses questionnaires, analytical studies, biomarkers, and observational studies to generate multiple lines of evidence of the distribution of soil ingestion rates. The panelists said that research is needed to validate the data currently reported in the literature and to generate additional data. Some panelists commented further on detailed study protocols that might help achieve this goal. These suggestions are not summarized here, but the panelists emphasized that research on the distribution of soil ingestion rates is needed so that ATSDR and other agencies can base their public health evaluations on more rigorously validated assumptions.
The panelists identified other data gaps, including the lack of soil ingestion data for older children (aged 7 and higher) and adults (JM), and the need for data characterizing long-term variations in soil ingestion (RW). One panelist also recommended that ATSDR determine whether the National Health and Nutrition Examination Survey (NHANES) is collecting data that can be used to characterize soil ingestion rates (RO). Another panelist noted that this survey is measuring blood lead levels and analyzing floor-wipe samples in certain homes, but he was not sure how these data might relate to soil-pica. Lastly, wondering if analytical studies that measure isotopic ratios of various tracers can generate robust estimates of soil ingestion rates, some panelists recommended that ATSDR investigate the feasibility of conducting such a study.
Bioavailability of metals in ingested soils. None of the panelists knew of studies showing that bioavailability of metals in soils decreases as soil ingestion rates increase, all other factors (e.g., particle size distribution) considered equal. Noting that pharmaceutical research has shown that humans absorb a smaller percentage of drugs when they are administered at higher doses, one panelist thought that bioavailability of metals in soils might decrease to a certain, but unknown, extent with higher doses (RW). He and other panelists (NF, DV) listed several other factors that likely play a greater role in bioavailability, such as alkalinity, particle size distribution, and composition of the soils, and the exposed individual's age, nutritional status, and dietary composition. One panelist added that bioavailability can vary considerably from metal to metal.
An observer noted that the issue of bioavailability varying with ingestion rates has important implications on how ATSDR evaluates acute health risks among soil-pica children. In such cases, a panelist thought the best approach to validate assumptions about bioavailability would be to conduct health studies to examine the prevalence of specific acute health outcomes (BL). The observer asked the panelists if they saw a need to research how bioavailability might vary with soil ingestion rate, possibly using recently developed in vitro models or EPA's swine in vivo models. A panelist recommended that ATSDR research bioavailability first by reviewing the results of soil ingestion tracer studies that collected biomarker data before considering conducting the modeling studies suggested by the observer (BL).
The need for reporting soil ingestion on a body-weight basis. One panelist asked if future research on soil ingestion should measure soil intake normalized to body weight (e.g., reporting "x milligrams of soil ingested per x kilogram of body weight"), consistent with how EPA is currently characterizing food and water ingestion rates (JM). Some panelists thought normalizing intakes to body weight would be consistent with other exposure metrics (BL), but others did not think this new approach added any insight to soil ingestion, especially considering that ATSDR already accounts for body weights in its exposure dose calculations (RO).