Freshwater ecosystems are sensitive to metals since sediments may accumulate insoluble metal compounds that can eventually be released into the interstitial water, thereby adding to the amounts of soluble or suspended metals. The bioaccumulation of toxic metals in numerous aquatic species, including fish, is now well recognized. In arable soils, elevated metal concentrations are typically associated with the occurrence of metal-containing groundwater as well as the use of some fertilizers and agricultural chemicals. The resulting total loading rate often exceeds the carrying capacity of the soils for trace metals. Under some extreme conditions, arable soils may become unsuitable for growing crops for human consumption because these crops accumulate metals at levels higher than those deemed acceptable by the World Health Organization.
Some metals are essential for proper functioning of living organisms while others are nonessential.
Nonessential metals are suspected to enter metabolic pathways of chemically similar essential elements, thus having the potential to interfere with the normal functions of the latter. Reciprocally, essential metals can modify the adverse effects of toxic elements. As with all toxic agents, absorption at the site of contact, metabolism, distribution and elimination are major steps that determine the behavior of metals when these interact with living organisms. Skin exposure may represent a route of uptake for some organometallic compounds, especially those containing lead, but trace metals enter the human body primarily following oral or pulmonary exposure. The most hazardous volatile metal particles are those that are able to cross the pulmonary epithelium, although pulmonary uptake is significant only in highly polluted regions or in the case of heavy smoking.