human burden. Levels of lead in the atmosphere now are estimated to be 10 to 20 thousand times higher in some urban areas compared to more remote regions of the globe. Lead deposited on soils can bind to a number of other naturally occurring materials, including other dusts, clays, hydrous oxides, and humic and fulvic acids. Soil is logically the most common repository, or sink, for airborne lead, but it cannot be considered a permanent sink. As with other soil contaminants, once lead pollutes soil, opportunities are enhanced for lead to be absorbed and recycled into the human food chain through grazing animals, home gardening, and general agricultural activity. Also, lead exposure can occur directly from soil exposure to young children who play outdoors, mouth objects on the ground, or engage in extensive pica. In addition, millions of children are at risk of exposure to lead dust from old lead paint in their homes and schools. The amount of lead that can be toxic to a young child is quite small. The average toddler should consume no more than 100 micrograms per day of lead, according to the World Health Organization. In the 1970s, a person running her hand along a table top could acquire more than that amount on her fingertip because of deposits from ambient air. Because such small amounts of lead can be toxic, efforts are continuing to reduce exposures.

ATSDR is conducting a number of studies involving mining wastes. A study of residential surface soils in Leadville, Colorado, located near the California Gulch Superfund site, found that more than 60 percent of the residential soil levels were higher than 1000 ppm and more than 80 percent had levels higher than 500 ppm (CDOH et al., 1990). Soil lead levels of more than 500-100 ppm are associated with increased in blood lead concentrations, especially in young children (CDC, 1985). The Leadville study found that 41 percent of the children had blood lead levels greater than 10 µg/dl and 15 percent had greater than 15 µg/dl. The authors note that the Bunker Hill Superfund site in Idaho had similar findings of 41 percent of the children having levels of 10 µg/dl and above, and 14 percent had greater than 15 µg/dl; the Silver Creek (Utah) mine tailings exposure study found a mean blood lead level of 7.8 µg/dl compared to the 4.0 µg/dl of their control population (CDOH, 1990). EPA uses a level of 1.4 µg/dl as its estimate of background blood level. This indicated that at all these sites, children have elevated levels of lead.

Studies of community residents living near metal smelters have found that their blood lead levels correlate directly with the distance that they live from these facilities (Landrigan et al., 1975; Baker et al., 1977), as have studies of children living near major highways (Mahaffey, 1983). For those Superfund sites that involve previously operating

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