When people think of treated wood, quite often they are thinking about pressure-treated lumber. At one time, pressure-treated lumber was quite dangerous to both livestock and humans. Recently, however, a new method of treating lumber has reduced the risks associated with pressure-treated lumber. This new process makes the lumber relatively harmless to livestock -- including chickens -- and making it available as a resource to use in building chicken coops.
Copper Chromium Arsenate
At one time, all pressure-treated lumber was processed using a chemical called copper chromium arsenate. The process used to treat the lumber involved removing some of the water contained by the wood, under extreme pressure and replacing it with the fluid containing the copper chromium arsenate. Arsenic would leach from the wood into the soil and the groundwater, contaminating them and making them unsuitable for consumption. When chickens scratched the ground, they could poison themselves with arsenic. They also could ingest arsenic or inhale the fumes from the treated lumber if their coops or perches were made from treated lumber.
By contrast, lumber treated with copper is believed to be environmentally friendly. Lumber treated with copper azole can be toxic, but this treatment is considered less hazardous than copper chromium arsenate. Copper azole-treated lumber is not recommended for "animal enclosures where the animals are known to gnaw on and consume the treated wood." Specific warnings when using copper azole-treated lumber include using mulched lumber as animal bedding and using such pressure-treated lumber to build structures or containers that hold animal feed. No data are available on copper azole-treated lumber's animal toxicity, however. While no recommendation exists to avoid using this lumber for chicken pens or other livestock pens, its safety as a building material for chicken coops or runs is still questionable.