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.^[1]

CCA is known by many trade names and is the world’s most widely used wood preservative. It is manufactured to national and international standards depending on the country of intended use, including AWPA P23-10 for the USA and SANS 673 for South Africa, and each manufacturer needs to comply with these standards.

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, according to the United States' Environment Protection Agency website.^[2] Arsenic is also found naturally in the soil, food and water. The amount and rate of arsenic leaching from CCA 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. 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). Research in Volume 36 of Wood and Fibre Science shows that soil contamination due to the presence of CCA-treated wood after 45 years is minimal.^[3] 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.

A more serious risk than leaching is presented if CCA-treated timber is burnt in confined spaces such as a domestic fire or barbecue, and the smoke is inhaled. 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 restricted its use in residential situations. These restrictions were a precautionary move due to public pressure after the publication of some studies suggesting that CCA could pose a risk to children. Subsequent studies found this not to be the case.^[4][5] However, in response to the pressures at the time, the wood preservation industry in the USA and Canada volunteered not to use CCA for the treatment of residential timber. 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).^[6]

The US EPA advised that CCA-treated timber products already in use, including playsets and decks, could remain in place. Exceptions to the restrictions were allowed, including the treatment of shakes and shingles, permanent wood foundations, and certain commercial applications.

Following the USA and Canada actions in restricting CCA, similar actions were taken in other parts of the world, including the EU and Australia. The regulatory agencies in all these countries advised, however, that CCA-treated timber products already in use pose no significant threat to health if applied properly, and the restrictions are a precaution.^[7][8][9]

In 2003, the Environmental Risk Management Authority in New Zealand, 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.^[10]

The more recent research showing that CCA does not pose a risk includes a biomonitoring study by Lew et al. (2010), which concludes that there was no significant difference in the arsenic concentrations and speciation distributions in urine and saliva samples of children playing on CCA-treated and non-CCA-treated playgrounds.^[11]

CCA-treated 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. The chemical will continue to be used in the US and countries across the world in a wide variety of commercial and industrial applications such as poles, piling, retaining structures and many others.

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.

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)

See also

• Timber treatment
• Paris Green
• Scheele's Green
• National Pesticide Information Center

References

External links

• 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