Humans can be exposed to soil contaminants through soil ingestion and through dermal uptake following soil contact with skin. For metal contaminants such as cadmium, the amount of intake via these pathways is typically less significant than the amount resulting from inhalation, water intake, and food-consumption pathways (McKone and Daniels 1991). Levels of cadmium in soil vary widely. In nonpolluted areas, concentrations in top soil are about 0.25 mg/kg (ppm) (EPA 1985); whereas in polluted areas, levels of up to 800 mg/kg have been measured (IARC 1993).
Indoor dust may be contaminated by deposition of particles from the air (originating from an emission source, or from suspension of contaminated soil), or by tracking of contaminated soil from outside. Friberg et al. (1974) report that the concentration of cadmium in the dust within houses was related to cadmium concentrations on air particles more than to soil concentrations.
Food is the main source of cadmium for non-occupationally exposed individuals. The gastrointestinal uptake of cadmium from food is generally less efficient than from water or by the lungs, because cadmium binds to food constituents (IARC 1993). The average daily intake of cadmium through food varies among individuals and by geographical area. An assessment using a Total Diet Study estimates the daily dietary intake of cadmium to be almost 15 µg/day (Gunderson 1995). Chaney et al. (1999) report that when zinc is present with cadmium at a ratio that is typical of geological materials (i.e., 100:1); zinc inhibits plant uptake, transfer to edible tissues, and absorption of cadmium in the intestine. However, when cadmium is present without zinc, food-chain mobility is much greater.
During waste incineration, arsenic (As) can be mobilized and emitted to the air as various inorganic compounds or in elemental form. Arsenic has valance states of -3, 0, +3, or +5, and is generally found in waters as H3AsO4, H2AsO4-1, and HAsO4-2, as well as H2AsO3-1, and H2AsO4-1. The principal arsenic-bearing minerals include arsenopyrite (FeAsS), niccolite (NiAsS), cobaltite (CoAsS), tennantite (Cu12As4S13), enargite (Cu3AsS4), and native arsenic.
By 1990, 70% of U.S. consumption of arsenic became attributable to the wood preservative industry and 20% to agricultural uses (ATSDR 1998b). Arsenic is also used in glass, nonferrous alloys, and electronics. Arsenic is released into the environment by human activities including arsenical pesticide and preservative use, metal smelting, waste incineration, and coal combustion.
The metal arsenic is insoluble in water (Weast et al. 1986). Trivalent arsenic compounds are quite soluble at ambient temperatures. Pentavalent arsenic