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Executive Summary

Ten expert panelists reviewed and discussed the state of the science on soil-pica behavior--an issue that is relevant to the Agency for Toxic Substances and Disease Registry's (ATSDR) ongoing work at sites with human exposures to contaminated soils. During a 2-day workshop in June 2000, in Atlanta, Georgia, the panelists thoroughly discussed and debated the prevalence of soil-pica behavior, ingestion rates for soil-pica, means for identifying people with soil-pica behavior, and additional topics. Common themes discussed throughout the workshop included the need for clear definitions of key terms, the lack of extensive research on the distribution of soil ingestion rates, and the need for additional research to fill data gaps.

After the workshop, the panelists drafted definitions of three key terms and prepared eight summary statements highlighting their most important findings, listed below. An overview of the panelists' discussion that led to these findings and recommendations for evaluating soil-pica behavior is documented throughout this report. The panelists used the following definitions to frame their discussions:

  • Soil ingestion is the consumption of soil. This may result from various behaviors including, but not limited to, mouthing, contacting dirty hands, eating dropped food, or consuming soil directly.
  • Soil-pica is the recurrent ingestion of unusually high amounts of soil (i.e., on the order of 1,000-5,000 milligrams per day). Groups at risk of soil-pica behavior include children aged 6 years and younger and individuals who are developmentally delayed.
  • Geophagy is the intentional ingestion of earths and is usually associated with cultural practices.

The panelists identified strengths and weaknesses associated with the different methods (e.g., observational, questionnaire, and analytical) that have been used to evaluate the prevalence of soil-pica. Given the limitations of individual methods, they agreed that a study that incorporates multiple methods would provide a means for validating the prevalence of soil-pica and the distribution of soil ingestion rates. Such validation is needed to provide confidence in ATSDR's approach for conducting future public health assessments.

Even with a definition of soil-pica, the panelists found it difficult to determine the prevalence of this behavior. The panelists agreed that soil-pica clearly exists, but the prevalence at a given soil ingestion rate has not been adequately characterized. Nonetheless, noting that soil ingestion is normal behavior among children, the panelists unanimously agreed that ATSDR should continue to evaluate the public health implications of all types of soil ingestion, including soil-pica.

The panelists agreed that the existing soil ingestion studies--nearly all of which evaluated children's behavior for durations of 2 weeks or shorter--are inadequate for determining the frequency of, and seasonal variations in, soil-pica.

The panelists noted that geophagy typically involves consumption of clay materials from known (and usually uncontaminated) sources at depth, rather than consumption of surface soils from residential properties. To address this type of ingestion, the panelists suggested that ATSDR survey communities of concern to determine if geophagy is practiced and where geophagical materials are obtained.

The panelists thought that soil-pica among adults was probably rare. Given anecdotal accounts of soil-pica adults, the panelists suggested that ATSDR consider the possibility that this behavior occurs, perhaps by conducting surveys or availability sessions with communities. However, it is sometimes unclear from anecdotal accounts whether the behavior reported was soil-pica or geophagy. because the literature on soil ingestion rates is extremely sparse, the panelists agreed that the distribution of age-specific soil ingestion rates had not been well characterized. Some panelists noted that ATSDR can estimate percentiles in this distribution by using a statistical analysis of existing soil ingestion data. However, the panelists stressed the need for field validation (including study of biomarkers and, where relevant, health effects) of any derived soil ingestion rate, as this ultimately would provide more confidence in ATSDR's public health evaluations of sites with contaminated soils.

After lengthy discussions, the panelists noted that ATSDR's assumption that soil-pica children ingest 5,000 milligrams (mg) of soil per day appears to be supported by only a few subjects in soil ingestion studies (i.e., two children in Massachusetts and several children in Jamaica). Referring to the soil ingestion rates presented in the literature, as summarized in EPA's Exposure Factors Handbook, some panelists thought that ATSDR's assumed ingestion rate for soil-pica children was high. Other panelists agreed, however, that ATSDR should err on the side of being protective and should use 5,000 mg until more data are collected. They also stressed the need for validating the 5,000 mg soil ingestion rate.

ATSDR views the contents of this report as advice for the agency to consider as it decides how to evaluate and address public health issues surrounding soil-pica. The contents of this report are NOT ATSDR policy.

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