Pressure treated lumber is wood that has been immersed in a liquid preservative and placed in a pressure chamber that forces the preservative into the wood fibers. This approach ensures that the chemical preservative makes it to the core of each piece of wood.
The most widely used chemical to treat wood is alkaline copper
The protection provided by the chemical depends on the amount of chemical that the wood absorbs. In the United States, the amount of chemical is measured in pounds of chemical per cubic foot of wood. For ground contact, 0.40 pounds per cubic foot is needed. For foundations, 0.60 pounds per cubic foot is the standard.