Background Soil Mercury: An Unrecognized Source of Blood Mercury in Infants?

Introduction: During the past four decades, mercury (Hg) research focused on fish consumption has explained less than 22% of Hg in human blood. One overlooked exposure pathway for infants and young children is the concentration of Hg in soils. Although 75% of the US population lives in urban areas near industrial facilities, minimal data exist regarding the concentration and speciation of Hg in residential soils. Chronic exposure through ingestion of low concentrations of Hg in soils may explain a portion of the blood Hg levels noted in infants. Methods: Three relatively unexposed residential sites in a suburban community were selected. The primary route of contamination was atmospheric deposition. Soils were digested in a nitric acid-hydrofluoric acid solution and analyzed by cold vapor atomic absorption spectrophotometry. Measured concentrations of total Hg in local suburban soil samples were compared to levels measured in a national study of 27 remote and rural sites. The Al-Shahristani pharmacokinetic model, developed after the 1971 Iraqi Methyl Hg poisoning incident, was used to calculate the blood Hg concentration in a hypothetical year-old infant. Results: Soil samples contained Hg concentrations ranging from 0.01 to 0.24 ppmw. The distribution of Hg in the soil samples was non-linear and non-normal. The mean soil Hg concentrations at the three locations were 0.08, 0.05 and 0.08 ppm. Calculated blood Hg concentrations for a 10 kg, year-old infant due to ingestion of soil (200 mg/day) containing 0.2, 0.4 and 0.8 ppm Hg were 0.08, 0.17 and 0.26 μg/L, respectively. Conclusions: The pilot study data appear to support the hypothesis that chronic, low-level soil ingestion may be a significant source of Hg for infants. Further study is warranted.