Chromated copper arsenate

Chromated copper arsenate (CCA) is a wood preservative containing compounds of chromium, copper, and arsenic, in various proportions. It is used to impregnate timber and other wood products, especially those intended for outdoor use, in order to protect them from attack by microbes and insects. Like other copper-based wood preservatives, it imparts a greenish tint to treated timber. CCA was invented in 1933 by Indian chemist Sonti Kamesam, and patented in Britain in 1934. It has been used for timber treatment since the mid-1930s, and is marketed under many trade names. In 2003, the United States Environmental Protection Agency and the lumber industry agreed to discontinue the use of CCA-treated wood in most residential construction. This agreement was intended to protect the health of humans and the environment by reducing exposure to the arsenic in CCA-treated wood. As a result of this decision, CCA-treated wood can no longer be used to construct residential structures such as playground equipment, decks, picnic tables, landscaping features, fences, patios, and walkways. Acute intoxication due to mishandling of treated products, e.g. by burning, is also a serious concern. Nevertheless, CCA remains a popular and economical option to make perishable timbers, such as plantation-grown pine, viable for applications like poles, piling, retaining structures, etc. The composition of CCA products is usually described in terms of the mass percentages of chromium trioxide or "chromic acid" , arsenic pentoxide , and copper(II) oxide . The preservative is applied as a water-based mixture containing 0.6–6.0% (by weight) of chromic acid, copper oxide, and arsenic acid (USDA, 1980), with pH 1.6–2.5. The mixture is infused into wood at high pressure. In the treated wood, arsenic is believed to be in the form of chromium (III) arsenate and/or copper(II) arsenate , or fairly stable chromium dimer-arsenic clusters. The chromium acts as a chemical fixing agent and has little or no preserving properties; it helps the other chemicals to fix in the timber, binding them through chemical complexes to the wood's cellulose and lignin.