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Q.Clients have been asking us about ACQ, a new pressure-treated wood that is supposed to be "nontoxic." Does this product pose fewer hazards than CCA lumber? How does the new product perform?

A.John Wagner responds: In addition to copper, conventional CCA preservative contains chromium and arsenate, which the Environmental Protection Agency (EPA) has labeled "hazardous." Though these two toxins bind almost entirely with the wood, CCA’s manufacturing process creates several hazards. First, it’s dangerous to touch CCA residue (chemicals that have not become "fixed" in the wood during the treating process). It’s also dangerous to touch or breathe the sawdust. Many homeowners rightly wonder if CCA is safe for decks, playgrounds, and picnic tables, especially where children will have direct contact with the wood.

These fears and dangers inspired Chemical Specialties Inc. (CSI, One Woodlawn Green, Suite 250, Charlotte, NC 28217; 704/522-0825) to develop ACQ Preserve, a new product that has several preservative qualities identical to CCA, but that has a much lower toxicity, both during its manufacture and with its subsequent handling. ACQ uses a water-borne preservative that contains ammonia, copper, and "quat" — quaternary ammonia, a disinfecting detergent — as an insecticide and fungicide treatment. It doesn’t use any chemical compounds listed by the EPA as hazardous, and it comes with a "green" label from Scientific Certification Systems (a commercial environmental labeling group).

Performance. Wood performance isn’t affected by preservative chemicals, whether you are using ACQ or CCA, except when the water used in the preservative-impregnating process dries unevenly during storage or shipping. This causes or accelerates splitting and checking. However, wood performance and finishing characteristics are greatly affected by the wood species, or — more specifically — the wood’s grain patterns. The species typically used in pressure-treating with CCA and ACQ — southern yellow pine, fir, and hemlock — are fast-growing flat-grained woods with wide grain bands that readily soak up the water-borne preservative. On the downside, the grain patterns in these species are unstable, so weathered decks built with pressure-treated woods commonly suffer from cupping, splintering, splitting, cracking, warping, twisting, and nail pull-out.

There are two ways to help ensure a more stable wood, whether it’s treated with ACQ or CCA. First, use pressure-treated wood that’s factory-treated with a water repellent. The alternative is to immediately apply a penetrating finish, such as a water repellent (often listed as WR on the label), water repellent preservatives (WRP) with a mildewcide additive, or a semi-transparent stain (an oil-based, pigmented WRP). None of these penetrating finishes flake, crack, or peel. (Film-forming finishes, such as paint and solid-color stains, are not recommended.)

Regardless of whether you choose CCA or ACQ, and whether you use factory-sealed material or apply a sealant soon after installation, make sure you inform your clients that the wood must be refinished every year with a penetrating stain or water repellent — not every other year as some manufacturers claim.

John Wagner is a freelance writer in Montpelier, Vt., who often writes about building technology.