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It’s a question that comes up regularly in the lumber and plywood aisle: “Just what is in the treatments for preserved wood products?” That may be followed by: “And are they safe?”
Preservatives have evolved since the 1970s, when preserved wood products were first sold in retail locations. Despite this evolution, there are still many misperceptions about preservatives used in treating.
Before tackling those 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 infuse it with preservatives and 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.
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.
Copper serves as the chief ingredient. Today’s preservatives contain as much as 50% to 97% soluble or micronized copper. Most decay fungi, termites and other organisms don’t eat wood containing copper.
While copper is effective, some organisms are tolerant to the element. 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 aren’t used exclusively for wood preservatives; they 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. Quaternary compounds are found in cleaners and disinfectants, as well as hair and body products.
These biocides and others, such as DCOI, are effective in protecting wood on their own. Carbon-based wood preservatives, such as PTI and EL2, contain no copper and rely on the protective characteristics of the biocides.
In the Mix
These compounds aren’t simply mixed together and sold to treaters. Preservative manufacturers thoroughly research, test and monitor the formulations to ensure effectiveness when infused into wood.
Each preservative 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.
National consensus standards administered by the American Wood Protection Association 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.
So how much preservative is in the wood? In most cases, it is less than 1% of the weight of the wood.
Preservative treating also enhances the sustainability of wood. Over the decades the wood can remain in service, a new tree can be grown to replace it in the future. Compared to alternative materials, life cycle science shows preserved wood has far fewer impacts on the environment.
When taken together, preserved wood products are indeed safe. Of course, use common-sense precautions when handling preserved wood. The requirements are the same for both preserved wood and untreated wood: wear gloves and long-sleeved shirts, and avoid inhaling sawdust by wearing a dust mask when cutting or drilling.
Preserved wood today is a safe, reliable and environmentally responsible building product that can provide decades of protection and enjoyment.