Pressure treated wood is a common material for outdoor structures such as bridges and decks. When wood treated with the new chemicals was exposed to metal parts, the metal corroded quickly, much more so in water-contact conditions. The problem can be avoided by using untreated wood or wood treated with the older methods. The old method of treating wood with chromium or some other type of acid won't protect it from corrosion. The wood will still decay over time. However, it won't harm any metal components that come into contact with it. The new method of treating wood with arsenic or lead compounds does provide some level of protection against corrosion. But because these chemicals are toxic, extra care must be taken not to release them into the environment. Toxicity levels may be reduced if treated wood is burned after treatment but emissions related to burning are toxic too. The best way to avoid this problem is to use only treated wood that has no metal parts within close proximity to the wood being built. This includes any metal fasteners used to assemble the wood structure instead of nails or screws which could cause corrosion to metal anchors or beams underneath the surface. If you have an existing structure made entirely out of pressure treated wood and want to add more wood to it without having to treat the whole thing again, go ahead.