When wood is pressure treated, chemicals are forced into the wood to preserve it and make it resist rot, decay and insects. Three of the most common chemicals used to treat lumber are discussed below. Two of them--CCA and AC2 (also known as ACQ)--result in "green" pressure-treated wood. There also is a form of "brown" pressure-treated wood that is produced by Olympic and other stain manufacturers.
CCA stands for chromated copper arsenate. Until 2003, if you went to any lumberyard, this would almost certainly be the only pressure-treated lumber you would find. However, in 2003 the EPA banned the sale of CCA lumber because of concerns over the fact that it contains arsenic. You can identify CCA because it is stamped "CCA" or has a tag on the end labelling it as such. You can also check its colour; CCA lumber will turn a dull grey over time.
ACQ stands for alkaline copper quaternary, which is also known as AC2. This is the "replacement" treated lumber for CCA since the latter was banned by the EPA in 2003. You can identify AC2 by a stamp or tag on the end, or by colour; it will fade to a golden grey over time.
Brown-treated lumber is wood that is "prestained" with a special version of a company's wood stain. Both Olympic and Thompson's produce brown-treated lumber. This wood is brown, of course, and you can distinguish it from lumber that has merely been stained by the penetration of the stain. If you cut the lumber, the stain on the prestained lumber will penetrate deeper than wood stained at home.
CCA lumber is treated with chromated copper arsenate. That last word, arsenate, means arsenic, which is a poison. To protect yourself, make sure to use a mask and gloves whenever handling CCA lumber, and wash your hands thoroughly after working with this wood. It is also a good idea to wash your work clothes separately from your other laundry to avoid possible cross-contamination of the chemicals used to the treat the wood. Finally, the Environmental Protection Agency also recommends that CCA-treated wood not be used in areas where drinking water, food, animal food or beehives are present.