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.
Composition and application
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.
Mechanism of action
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. The copper acts primarily to protect the wood against decay, fungi, and bacteria, while the arsenic is the main insecticidal component, providing protection from wood-attacking insects including termites and marine borers. It also improves the weather resistance of treated timber and may assist paint adherence in the long term. These compounds are toxic to the human system when inside the bloodstream, usually from burning wood treated with these compounds which is very dangerous.
Alternatives
Alternative heavy-duty preservatives include creosote and pentachlorophenol. Similar water-borne preservatives include alkaline copper quaternary (ACQ) compounds, copper azole (CuAz), ammoniacal copper zinc arsenate (ACZA), copper citrate, and copper HDO (CuHDO). Usually more expensive options, but safer, are pressure and heat treated lumber which contains no chemicals. Usually they lack the long-term robust qualities and resistance of chemically treated lumber. Some may be pressure treated with water alone, which while much more long lived than normal wood, will not repel insects as much as chemically treated but will resist mold and water damage much more than non-treated lumber.
Safety
Concerns over the safety of CCA have focused on its chromium and arsenic contents.
Arsenic is found naturally in the soil, food and water, and is still used to treat some medical conditions However it has a long list of negative health effects, especially in inorganic form, by contact or by ingestion, and was designated a human carcinogen by the U.S. Environmental Protection Agency (EPA) in 1986 (even though its actual risk remains unclear). Arsenic in drinking water is a serious public health problem in some areas of the world.
Handling
Itching, burning rashes, neurological symptoms, and breathing problems have been associated with handling unmarked chromated arsenical wood preservatives, including contact with the sap draining from treated wood.
Regulatory action was motivated in the 1990s by studies suggesting that CCA could pose a risk to children in playgrounds built with CCA-treated timber. Later studies however found that, while concentrations of arsenic in the soil and hand rinses were considerably higher among children who played on CCA-treated playground toys than in the control group, there was no significant difference in the arsenic concentrations in urine and saliva samples.