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 mayalso be derived from discarded CCA lumber. At present, alkaline copper
quaternary (ACQ) and copper azoles (CA) are the most commonly used wood
preservatives for both commercial and residential uses. The most common formula
of ACQ (type D) and CA (type B) have 66.7% copper oxide and 96.1% Cu,
respectively.