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CCA treated timber; often called 'tanalised' timber or 'pressure treated' timber. This treatment uses chemicals such as copper arsenate, which are very poisonous to people - so the treatment must be carried out in carefully controlled conditions - which is why the wood is sold to the public pre-treated.The method of application of the preservative is such that the chemicals should be very tightly held in the wood once dry - but research carried out in the USA suggests that arsenic may leach from treated wood into the soil, and traces of arsenic have been found on hands of kids who have been playing on equipment made from such timber. This may be particularly true where the wood has not been adequately dried after treatment. Gloves should be worn when handling this kind of timber, and care should be taken not to inhale the dust when sawing or working it in any way. It should not be burned.