Lead aerosol pollution in the High Sierra overrides natural mechanism which exclude lead from a food chain.

Most of the lead contained in sedge and voles (mountain meadow mice) within one of the most pristine, remote valleys in the United States is not natural but came from smelter fumes and gasoline exhausts. In a food chain, natural mechanisms do not allow lead to accompany the bulk of the nutritive metals as they proceed to higher trophic levels. This exclusion can be expressed quantitatively by a comparison of lead/calcium ratios at successive trophic levels. This ratio decreased by an overall factor of 200 in proceeding from rock, to soil moisture, to sedge, to vole. This factor would have been 1200 if lead aerosols had not collected on sedge leaves and circtumvented the tendency by sedge to exclude lead from the nutritive metals it absorbed from soil moisture.