The majority of pressure-treated lumber produced before 2004 was made with chromated copper arsenate (CCA), which manufacturers voluntarily discontinued after public outcry over leaching of the wood's arsenic compounds. The U.S. Consumer Product Safety Commission published a study showing that applying an oil-based stain to CCA-treated wood annually heavily reduces toxic leaching.

On the Other: Newer Pressure-Treated Wood is Safer for Humans

Since 2004, pressure-treated wood is made largely with either alkaline copper quaternary (ACQ) or copper boron azole, both of which are made without arsenic. This wood is more expensive than CCA-treated wood due to the higher cost of copper. While the copper and borates in the wood may leach, they present no dangers to humans but can harm aquatic life and plants.

Bottom Line

There are risks to using any type of pressure-treated wood. It should ever be burned, as inhaling the ash and smoke can be very hazardous. CCA-treated wood is still available for industrial and agricultural use, and wood treated with ACQ and copper azole are approved by the federal government for residential use, such as for playgrounds and decking. While the risks to humans are minimal, the gardener or landscaper should carefully consider the use before installing any kind of pressure-treated wood.

Source:

Consumer Product Safety Commission: Reducing Arsenic Leaching from Pressure Treated Wood

Consumer Product Safety Commission: Chromated Copper Arsenate (CCA) -Treated Wood

FineHomeBuilding.com: The New Pressure Treated Wood