Chromated copper arsenate
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Chromated copper arsenate (CCA) is a wood preservative used for timber treatment, in use since 1930's. It is a mix of copper chromate and copper arsenate. It preserves the wood from rotting due to insects and microbes. It is also known under trade name Tanalith. The chromium acts as a bactericide, the copper acts as fungicide, and the arsenate acts as insecticide. It became used as an alternative to creosote, pentachlorophenol, and coal tar. Sodium silicate is one of the modern alternatives to CCA.
CCA-treated wood is often referred to as CCA wood or pressure treated wood.
In CCA treatment, copper is the primary fungicide, arsenic is a secondary fungicide and an insecticide, and chromium is a fixative which also provides ultraviolet (UV) light resistance. Recognized for the greenish tint it imparts to lumber, CCA is a preservative that was extremely common for many decades, however it contained arsenic. The chemicals may leach from the wood into surrounding soil, resulting in concentrations higher than naturally occurring background levels. A study cited in Forest Products Journal found 12–13 percent of the chromated copper arsenate leached from treated wood buried in compost during a 12-month period. Once these chemicals have leached from the wood they are likely to bind to soil particles, especially in soils with clay or soils that are more alkaline than neutral. In the United States on 1 January 2004 the Environmental Protection Agency (EPA) began restricting the use of CCA in treated lumber in residential and commercial construction, with the exception of shakes and shingles, permanent wood foundations, and certain commercial applications. This was in an effort to reduce the use of arsenic and increase environmental safety.
The CCA-treated wood causes concern to the general public. CCA timber is still in widespread use in many countries, and was heavily used during the latter half of the 20th century as a structural, and outdoor building material, where there was a risk of rot, or insect infestation in untreated timber. Although widespread bans followed the publication of studies which showed low-level leaching from in-situ timbers (such as children's playground equipment) into surrounding soil, the most serious risk is presented by the burning of CCA timber. Recent years have seen fatal animal poisonings, and serious human poisonings resulting from the ingestion - directly or indirectly - of wood ash from CCA timber (the lethal human dose is approximately 20 grams of ash - roughly a tablespoon). Scrap CCA construction timber continues to be widely burnt through ignorance, in both commercial, and domestic fires. Safe disposal of CCA timber remains patchy, and little practiced, there is concern in some quarters about the widespread landfill disposal of such timber.
[edit] External links
- 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
See also:
- Timber treatment
- Paris Green
- Scheele's Green
- Ammoniacal copper arsenate (ACA)
- Ammoniacal copper zinc arsenate (ACZA)
- Acid copper chromate (ACC)
- Arsenic-treated wood to be phased out
