Treated lumber is wood that has been treated to protect against damage caused by rot and insects. Wood may be heat-treated or pressure-treated.
With heat-treating, lumber is exposed to high temperatures that destroy the wood's nutrients. Rot, fungi, and insects feed on these nutrients, so heat-treating creates wood that is not a suitable environment for these threats.
With pressure-treating, chemicals are used to shield wood from rot and insect invaders. To accomplish this, wood is placed in vacuum chamber. A preservative is then introduced, and it's driven deep into the body of the wood. In the past, wood was pressure-treated using chemicals such as arsenic, creating a product that could be dangerous to humans and pets. The chemicals used to treat lumber today are far less toxic.
Treated wood offers superior durability to wood that hasn't undergone this process.