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.
Concerns over the possible release of chromium and arsenic to the environment have resulted in it being banned for timber destined for residential use in the early 2000s. 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 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 CrAsO
4 and/or copper(II) arsenate Cu
2, 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.
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).
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 healh problem in some areas of the world.
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.
CCA treated wood has relatively low toxicity, and animals would need to ingest unlikely amounts (like 28 g daily for a month, for an adult horse) in order to become poisoned. However, ashes from burned timber are much more toxic, and cattle have been poisoned in this way.
Releases to the environment
The amount and rate of arsenic leaching varies considerably depending on numerous factors, such as local climate, acidity of rain and soil, age of the wood product, and how much CCA was applied. A study has found that soil contamination due to the presence of CCA-treated wood after 45 years was minimal.
Many studies in less aggressive soil types show leaching to be as low as 0.5 ppm (red pine poles in service,) or up to 14 ppm (treated pine in garden beds). Should any chemicals leach from the wood they are likely to bind to soil particles, especially in soils with clay or soils that are more alkaline than neutral.
CCA manufacture and use are regulated by various national and international standards, such as AWPA P23-10 for the US and SANS 673 for South Africa.
In the US, the use of CCA to treat timber for residential use has been banned since December 2003. Timber treated before that date was still allowed to be sold, but retailers were required provide warning labels. Treated timber products already in use, including playsets and decks, were allowed to remain in place. Exceptions to the restrictions were allowed, including the treatment of shakes and shingles, permanent wood foundations, and certain commercial applications. The EPA has also issued regulations for the industrial application of CCA to wood.
In 2003, the Environmental Risk Management Authority in New Zealand decided to not restrict CCA use for any applications, but notes that few well-designed studies have been carried out of those using CCA or CCA-treated timber.
CCA-treated wood should not be used where it may come in contact with food, such as in kitchen tops, cutting boards, or beehives. It should not be exposed to chemicals such as bleaches, acids, soda, etc.
Disposal of scrap and waste CCA-treated timber should be done only in approved incinerators or controlled landfill sites, which are designed to handle potentially toxic wastes such as paints, insecticides, batteries, etc.
- Hunt and Garratt, Wood Preservation, 1938, p. 127
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- Case Studies in Environmental Medicine - Chromium Toxicity
- National Pollutant Inventory - Chromium (III) compounds fact sheet