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The geochemistry of selenium in soils on loess deposited as marine sediments involves the depletion by erosion of Se adsorbed on clays and iron oxides and its accumulation in lowlands. This depletion has affected the human heart-death rates (HDR) in Shaanxi Province, China, in western counties of the state of Wisconsin, and elsewhere. In Shaanxi province, a high age-adjusted, annual HDR of 140/ 100,000 people (due to cardiomyopathy or “Keshan disease”) was found to be accompanied by blood-Se values of 20 ng/cm3 for people living in eroded, clay-poor loess hills. HDRs persisted (without Keshan disease) to middle ages, but were accompanied by somewhat higher blood-Se values. For people living in areas where some clays had been deposited on the Wei River terrace, HDRs were less (66–72) and were accompanied by blood-Se values of 133-79 ng/cm3 (n = 85), respectively. These values are at the low end of the blood-Se levels found in many other countries. From equations derived from these data, a blood-Se level of 274 ng/cm3 can be projected for the onset of such heart diseases, a value still less than half of that considered to be the toxic level of Se.

In western Wisconsin counties, containing loess derived in part from seleniferous (2–40 µg/g) Cretaceous shale from Minnesota, the population had lower annual HDRs (283–315/100,000 people) than in counties in which low-clay (sandy) soils predominate (HDR = 343–406). A soil extraction by 0.1 N NaOH gave Se values of 57–91 ng/g for soils in areas having the highest HDRs and 102–120 ng/g in areas having the lowest HDRs. Thus, topography, erosion of clay, bedrock source, and crop depletion of clay adsorbed Se appear to affect human blood levels of Se and heart-death rates.

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