2. Relevance to Public Health 2.1 Background and Environmental Exposures to Zinc in the United States

Zinc is ubiquitous in the environment, constituting 20–200 ppm (by weight) of the Earth's crust. It is not found as elemental zinc in nature, instead being found mainly as zinc oxide or sphalerite (ZnS). Zinc is released into the environment as the result of mining, smelting of zinc, lead, and cadmium ores, steel production, coal burning, and burning of wastes. Ambient background air concentrations of zinc are generally <1 μg/m. Zinc is found in soils and surficial materials of the contiguous United States at concentrations between <5 and 2,900 mg/kg, with a mean of 60 mg/kg. The zinc background concentrations in surface waters are usually <0.05 mg/L, but can range from 0.002 to 50 mg/L.