Copper is an essential metal for many organisms, including humans. It is used in enzymes that modify redox reactions and in some oxygen-carrying systems. Its function in these is associated frequently with its ability to exist as both Cu(I) and Cu(II). The most common state is Cu(II). Like many essential metals, large amounts are toxic, and the USPHS limit in drinking water is set at 1 mg/liter. It is particularly toxic to lower organisms and has been used as an algicide in lakes (as CuS04) and is also used in wood preservatives, either associated with other metals such as chromium or arsenic, or complexed with organic ligands (e.g., 8-quinoline, dimethyldithiocarbamate). Other sources are erosion of copper pipes, industrial wastes, and weathering of rocks. The carbonate and hydroxide are of low solubility, so the natural level in water is low in the absence of complex-forming substances.

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