Wood in outdoor applications is usually pressure treated with preservatives. Since the 1970s, the majority of the woods used in outdoor residential settings have been treated with chromate copper arsenate (CCA). The most common formulation of CCA (type C) contains 47.5% as CrO3, 18.5% as CuO, and 34% as As2O5. Effective 31 December 2003, no wood manufacturer may treat wood with CCA for residential uses in the U.S. However, newly CCA-treated wood can still be used for commercial purposes and old CCA-treated wood may still be in service for residential uses. Wood mulch may also be derived from discarded CCA lumber.
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