ACQ-treated lumber is pressure-treated with alkaline copper quartenary, a copper-based chemical forced into the wood to preserve it against decay and insects. This preservative replaces the arsenic-based chemical preservative that was used for many years but was deemed too poisonous for use around children.
AC2-treated lumber reduces the risk of poisoning but causes regular nails, screws and deck hangers to corrode quickly, resulting in deck collapses in which people have been killed or injured. Building with this type of lumber requires care to select nails, screws and other fasteners that don't corrode. Special fasteners that are specifically designed for use with ACQ-treated materials are available from building material suppliers.