Chromated copper arsenate
Chromated copper arsenate (CCA) is a wood preservative that has been used for timber treatment since the mid-1930s. It is a mix of chromium, copper and arsenic (as Copper(II) arsenate) formulated as oxides or salts, and is recognizable for the greenish tint it imparts to timber. CCA was invented in 1933 by Dr. Sonti Kamesam, an Indian scientist, and was awarded its first patent (British) in 1934.
CCA is known by many trade names including the worldwide brands "Ascu", "Tanalith", "Wolman", "Permacure" and "Celcure".
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 of CCA, 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.
Releases to the environment
Over time small amounts of the CCA constituents, mainly the arsenic, may leach out of the treated timber. This is particularly the case in acidic environments. The chemicals may leach from the wood into surrounding soil, resulting in concentrations higher than naturally occurring background levels. A study found that during a 12-month period 12–13 percent of the CCA leached from treated wood buried in compost. On the other hand there have been many other studies in less aggressive soil types that 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). Soil contamination due to the presence of CCA-treated wood after 45 years is minimal.
Although widespread restrictions (see below) followed the publication of studies which showed low-level leaching from in-situ timbers (such as children's playground equipment) into surrounding soil, a more serious risk is presented if CCA-treated timber is burnt in confined spaces such as a domestic fire or barbecue. Scrap CCA construction timber continues to be widely burnt through ignorance, in both commercial, and domestic fires.
Notwithstanding this, disposal by burning, e.g. in approved incinerators, is an acceptable option, and some energy may be captured in the process.
Limitation of human exposure
A number of countries have reviewed CCA during recent years and have looked at limiting the public exposure to CCA-treated timber by restricting its application in residential situations. These reviews have resulted from increasing public pressures and the possibility of CCA-treated timber posing a health hazard. In response to these pressures the preservation industry in the USA and Canada volunteered not to use CCA for the treatment of residential timber, and on 31 December 2003 the production of CCA-treated wood for such applications became a violation of the manufacturers' labels approved by the United States Environmental Protection Agency (EPA). Exceptions were allowed, including the treatment of shakes and shingles, permanent wood foundations, and certain commercial applications. The regulatory agencies advised however that CCA-treated timber products already in use pose no significant threat to health. Indeed CCA will continue to be used in North America in a wide variety of commercial and industrial applications such as poles, piling, retaining structures and many others.
Following the USA and Canada actions in restricting CCA, similar actions have been taken in other parts of the world, including the EU and Australia. In New Zealand the Environmental Risk Management Authority, reviewing the same data that prompted the actions elsewhere, concluded that there was no reason to 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 timber is still in widespread use in many countries and remains an economical option for conferring durability to perishable timbers such as plantation-grown pine.
Disposal of large quantities of CCA-treated wastes or spent timber at the end of its lifecycle has been traditionally through controlled landfill sites. Such sites are lined to make them impervious in order to prevent losses to the water table and they are covered to prevent rainfall washing out any contained potential toxicants. These controlled sites handle a range of waste materials potentially more noxious than that posed by CCA-treated timber, e.g. paint-stuffs, car batteries, etc. Today, landfill sites are becoming scarcer and disposal of waste materials is becoming economically unattractive. The wood preservation and timber industries are therefore researching better ways of dealing with waste treated timber, including CCA-treated material.
Alternative heavy-duty preservatives include creosote and pentachlorophenol. Similar water-borne preservatives include alkaline copper quaternary compounds (ACQ), copper azole (CuAz), ammoniacal copper zinc arsenate (ACZA), copper citrate, and copper HDO (CuHDO)
- Hunt and Garratt, Wood Preservation, 1938, p. 127
- Forest Products Journal
- Wood and Fibre Science Vol 36 pp 119-128, 2004
-  - Timber Treatment Chemicals
- National Pesticide Information Center - Chromated Copper Arsenate (CCA): Guide to selected resources
- CPSC Test coatings to reduce arsenic emissions from pressure treated wood
- Case Studies in Environmental Medicine - Arsenic Toxicity
- Case Studies in Environmental Medicine - Chromium Toxicity
- National Pollutant Inventory - Chromium (III) compounds fact sheet