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Safety of timber treated with CCA preservative
CCA Public or regulator concerns, principally about CCA's arsenic content, have led to restrictions on its use in countries including Japan, Indonesia, Sweden and Germany. In February 2002 the United States Environmental Protection Agency (EPA) announced a voluntary decision by industry to stop supplying CCA-treated timber for residential uses by the end of 2003. This means wood for applications such as playground equipment, timber decking and house fences will be treated with alternative preservatives in the future. Uses by utilities, for example in power poles, and in industry and agriculture are not affected. In its announcement the EPA stated: 'EPA has not concluded that CCA-treated wood poses unreasonable risks to the public for existing CCA-treated wood being used around or near their homes or from wood that remains available in stores. EPA does not believe there is any reason to remove or replace CCA-treated structures, including decks or playground equipment. EPA is not recommending that existing structures or surrounding soils be removed or replaced.' In Australia, CCA preservatives are approved and regulated by the National Registration Authority (NRA) with input from environmental agencies. Treatment levels are set down by Australian Standards, and by State legislation in Queensland and NSW. Only dedicated commercial treatment plants have access to CCA. Unlike preservatives such as creosote, liquid CCA is not available to the public at hardware stores. The approved CCA loading depends on the 'hazard' to which the timber will be exposed, expressed in the 'hazard classes' H1 to H6. The timber used most commonly for domestic purposes is in classes H3 (outdoor above-ground timbers such as decking, fence palings, fascia etc) and H4 (outdoor in ground contact, e.g. posts). Wood treated according to the Standard should carry a stamp or brand, indicating the hazard level to which it was treated. Other numbers in the brand are codes for preservative type and treatment plant number. The NRA at this stage has not announced any intention to change or restrict the approval status of CCA in Australia. Any such change in the future would be through a process of dialogue with industry, the public and other stakeholders. In CCA treatment, timber is impregnated with the preservative solution - a mixture of copper, chromium and arsenic compounds - using controlled vacuum/pressure processes. The roles of the ingredients are: copper to control fungi, arsenic to control termites, and chromium to fix the copper and arsenic in the wood. More recent research at CSIRO has shown that arsenic is not needed for the control of termites or marine borers (copper chromate does this). Instead, the main benefit of the arsenic is to inhibit certain copper-tolerant fungi. The correct combination or ratio of these elements as described under Australian Standards is critical to the performance of the product. The fixation process ensures that virtually all the CCA becomes chemically bonded within the wood structure. The rate of fixation is dependent upon temperature. In warm conditions fixation may be complete within a few days, while in cold conditions it can take up to several weeks. Well-proven accelerated fixation methods are in use, such as kiln drying and steam conditioning. Producers are obliged to ensure that the product is adequately fixed before dispatching it from their sites. The CCA solution is orange, but turns green on fixation to give treated timber its familiar light green colour. Arsenic is the twentieth most abundant element in the earth's crust, with soil containing it at concentrations between about 0.2 and 20 parts per million. The arsenic used in CCA is in a form - arsenate or pentavalent arsenic - that is five to ten times less toxic than the most toxic form, arsenite (trivalent). Fixation modifies the arsenate into metal-metal complexes and organo complexes with wood. Ingestion studies with animals have shown that this greatly reduces its toxicity. For example, no evidence of toxicity was found after beagle dogs were fed 10 grams of CCA-treated sawdust per day for 5 days in food. Seventy percent of the arsenic passed with wood through the faeces. The rest was expelled with urine, having been extracted from wood in the stomach. The bulk (85%) of this urinary arsenic was detoxified to dimethylarsinic acid. In another experiment, researchers found no abnormalities after sheep and calves were fed 454 grams of CCA-treated pine once or 113 grams per day for 25 days. Studies have found slightly raised arsenic levels in the urine of some CCA treatment plant workers. However, no significant differences in medical history, compared with 'control' groups not associated with CCA, were detected. Nearly all the CCA fixed within timber remains there over its lifetime of service; if it did not, the wood would rot and fail in much less than the 30-50 year period for which it is often guaranteed. However, a small amount of leaching inevitably occurs. This can show up in small rises in arsenic levels in the soil close to posts and poles; studies have found levels return to normal within about 100 mm of posts and 100-200 mm of poles. Simple precautions, such as those described below, can be taken to minimise any perceived risks from the leaching.
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