Recent concerns surrounding asbestos exposure have extended from occupational settings into natural settings. These developments have caused us to examine the distribution of amphiboles, which are potential asbestiform minerals, within the soils of the U.S.A. Evaluation of mineralogical data from selected sand and/or silt fraction of soils from the USDA-NRCS National Cooperative Soil Survey database shows that soils in all states (except for Rhode Island) contain amphiboles. In 41 of the 50 states, 10% or more sampled pedons contain amphiboles. Overall, 4396 pedons out of the 34 326 pedons (about 13%) sampled in the U.S.A. contained amphiboles. Pedons containing amphiboles ranged from less than 1 to 49% of the pedons among all states. While amphibole asbestos deposits occur in mafic and ultramafic provinces, soil amphiboles occur evenly distributed across the U.S.A. The majority of the amphiboles found in the soils would likely not meet the mineralogical definition of asbestos (i.e., they would not have been derived from asbestiform amphiboles); however, the majority would likely meet a commonly used regulatory definition of a fiber (i.e., are over 5 μm in length with a greater than 3 to 1 aspect ratio). Based on the regulatory definition, 13% of soil pedons and 5% of soil horizons in the U.S.A. are “naturally contaminated.”

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