Hand-to-mouth Transfer of Soil

Risk estimates attributable to soil contamination are frequently dependent upon assumed soil ingestion rates. Assumptions regarding soil loading on hands, frequency of hand-to-mouth contact, and efficiency of transfer often form the basis upon which estimates of ingestion rates are calculated. However, those assumptions do not explain the process by which soil is ingested. Quantitative characterization of hand-to-mouth transfer of soil does not yet exist. Findings from a laboratory-based study of this process are reported here. Four adult subjects performing three activities (thumb sucking, finger mouthing, and palm licking on soil-loaded hands) produced a total of 108 transfer measurements. Despite variability observed among subjects, transfers were typically on the order of 10 mg per activity. Volunteers participating in this study noted that the presence of roughly 10 mg of soil in the mouth is readily detected (and unpleasant). Therefore repeated unintentional ingestion of that mass of soil by adults appears somewhat unlikely. In light of this observation, historical high-end estimates of daily soil ingestion rates in the range of 500 mg/day would appear to be implausible, at least for non-smoking, non-geophagic adults.

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