In fact, the toxicity of treated wood has nothing to do with its colour. Although the dangers of treated wood made headlines a few years ago,1 today’s products are far less harmful to our health. And they still do the job of protecting your structures against rot, fungus, mould, and insect infestations.

There are two types of treated lumber widely sold in Quebec: wood treated with alkaline copper quaternary (ACQ), and wood treated with copper azole (CA-B). Both are available in green or brown shades for the residential market, and both meet Health Canada standards.

Less toxic, but harmful to metal

Wood treated with either ACQ or CA-B, however, can cause assembly hardware to deteriorate. As a result, you should use hot-dipped galvanized steel nails, plated screws, or, best of all, stainless-steel fasteners. Take care to avoid direct contact between the treated wood and any metal element. For example, it’s a good idea to insert a piece of self-stick elastomer membrane under the base of aluminum handrail posts. 

Wood treated without solvents

Although it is a more expensive option, wood that has been pressure-treated using micronized copper azole (MCA), which is a darker brown in colour, is less prone to off-gassing, and the preservative is less likely to wash off. This is because of the specifics of the treatment and colour process: the preservative is in the form of microparticles, which are forced into the wood under pressure, with no solvent involved. As a result, it protects and retains its colour for longer. Another benefit is that its corrosion rate is comparable to that of natural wood, so it can be used with any type of fastener and can be in contact with aluminum in exterior applications.

Construction of freshwater docks

Ideally, a dock should be built using tree species containing natural preservative agents that are more rot-resistant as a result, like cedar, larch or hemlock. If a deck is to be built of treated wood, it must be of a type certified for this purpose by Health Canada, like ACQ, CA-B or MCA. The latter (wood pressure-treated with micronized copper azole) is especially recommended for freshwater dock construction.

To sum up, today’s treated-lumber products are less harmful to health and the environment—regardless of what colour they are!


1 - Production of wood treated with chromated copper arsenate (CCA) for the residential market was halted in January 2004 in Canada, because of the risk of arsenic poisoning. Structures made with these materials, however, may still be found outside older homes. Moreover, CCA-treated wood is still produced for institutional, industrial and agricultural use.