When people look for wood for their outdoor projects, one common question is: “What do they put in the wood to make it last?” It’s often followed by another important question: “And is it safe?”
Preservatives for wood products have evolved since the 1970s, when the products were first sold in retail locations. Despite this evolution, there are still many misperceptions about preservatives used in treating.
To answer these questions, it is helpful to understand why we pressure treat wood products.
Wood, of course, is a natural material and has unique qualities that make it an excellent building material. But Mother Nature can be a cruel mistress, seeking to decompose organic materials and return them to the Earth to support new growth.
Decay fungi and insects are part of the cycle of breaking down wood fiber once it’s no longer part of a living tree. While that may be great for nature, you don’t want that to happen to wood that is a structural element in your home or outdoor living area.
The most effective protection for wood is to integrate preservatives into the wood to keep fungi and insects from eating it. Pressure treating the wood extends the service life from a few years to decades, helping ensure sustainable forests.
Preservative Ingredients
One common misperception is that preservatives are poison. In reality, preservatives prevent fungi or insects from degrading the wood by creating a long-lasting disinfectant barrier.
Today’s preservatives may appear to be an alphabet soup of acronyms. However, they really convey what is in today’s preservatives. For preserved wood sold in retail locations, they are typically treated with MCA (Micronized Copper Azole), CA-C (Copper Azole) or ACZA (Ammoniacal Copper Zinc Arsenate).
For these preservatives, copper serves as the chief ingredient. They contain as much as 50 percent to 97 percent soluble or micronized copper. This copper provides an effective deterrent to decay fungi, termites and other organisms that want to eat the wood.
Some organisms, however, are tolerant to copper. So mixed with the copper are biocides and fungicides to enhance the protection. All of these are suspended in water, which carries preservatives into the wood when pressure is applied.
Two types of biocides are commonly used in preservatives: azoles and quaternary compounds. These chemicals are also found in a host of consumer products.
Azoles are antifungal chemicals used with a number of natural products. They include propiconizole, also used on fruits, vegetables and nuts, and tebuconazole, used on flowers, shrubs and other agricultural products.
There are some preservatives that don’t contain copper. These carbon-based preservatives, such as PTI and EL2, contain no copper and rely on the protective characteristics of the biocides.
So how much preservative is in the wood? In most cases, it is less than 1 percent of the weight of the wood.
In the Mix
Each preservative used for wood must be approved by the U.S. Environmental Protection Agency (EPA), which requires extensive toxicological reviews to determine any potential human health or environmental risk. The American Wood Protection Association (AWPA) determine the amount of preservative required in the wood to protect it for its intended end use.
This process helps promote the safety of preserved wood by providing a balance between protecting the wood and minimizing the amount of preservative that may move into the environment. Preserved wood today is a safe, reliable and environmentally responsible building product that can provide decades of protection and enjoyment. For more information on preservatives, go to the online technical libraries at PreservedWood.org and SouthernPine.com.
