Wood can be impregnated with a variety of preservative chemicals to prevent rot and insect infestation in exterior applications like decks. Several species of wood can be treated, but a regionally available species usually predominates in any particular locale.  Pressure treated lumber is ideal for outdoor construction as it has a long, useful life span and is much less expensive than alternatives. Treated wood can last more than 40 years. The treatment process involves placing a load of lumber in a huge cylindrical chamber called a retort with a door on one end that can be sealed airtight and then forcing waterborne chemicals into the wood under pressure.