Q. I'm building an outdoor deck for a client who wants me to use lumber treated with micronized copper, in the belief that it's less harmful to the environment than wood treated with other chemicals. I'm not very familiar with the material. Can it be used in ground-contact applications, including load-bearing posts?

A.Adam Taylor, the wood-products extension specialist at the University of Tennessee, responds: The short answer is yes, micronized-copper treated wood can be used in load-bearing and ground-contact applications. However, a little more background information might be helpful.

Treated lumber is widely used for decking and other exposed woodwork. The treatment process uses vacuum and pressure to force preservative liquids deep into the wood. After treatment the water evaporates, but the chemicals remain to protect the wood.

Until about 2004, the most common preservative used was copper combined with chromium and arsenic, or CCA. After CCA was withdrawn from residential use, it was replaced by alkaline copper quaternary (ACQ) and copper azole (CA), which include organic co-biocides instead of chromium and arsenic. Like CCA, both of these newer formulations leave the wood green in color (unless a dye is added), clean to the touch, paintable, and protected from insect attack and rot. ACQ and CA are very corrosive to metal, however, so it is important to use only ceramic-coated, stainless steel, or other approved screws and nails in lumber treated with them.

Micronized copper preservatives are variants of ACQ and CA. The key difference is that rather than being dissolved in the preservative, the copper takes the form of tiny solid particles -"micronized" copper - that are suspended in the treatment liquid. Wood treated with micronized copper is less corrosive to metal fasteners and lighter in color than wood treated with conventional ACQ or CA. You still need to use approved fasteners, but aluminum flashing can be used in contact with the treated wood. Brand names include Yellawood MCQ, MicroPro, and SmartSense.

All of the CCA replacements - including the micronized copper products - have much higher levels of copper than CCA itself, meaning that there's greater potential for the copper to leach out of treated wood into the surrounding environment. Fish are particularly sensitive to copper, and since test results have shown that micronized products resist leaching better than those that contain soluble copper, micronized lumber may be a more environmentally friendly choice where treated lumber will be in close contact with a pond, lake, or stream. That's the theory, anyway. But practically speaking, who knows? Chemicals in treated wood products are strictly regulated by the EPA and so far there has been no evidence of copper-treated wood causing environmental problems in use.

Micronized lumber is accepted by the I-codes, although its route to that acceptance has resulted in some confusion. Traditionally, wood preservatives have been evaluated and approved by the American Wood Protection Association, and AWPA approval - or "standardization," as it's called - is the usual avenue to acceptance by the codes. Micronized copper preservatives, however, have not been evaluated by AWPA; their code approval is based on an evaluation by the ICC Evaluation Service (ICC-ES).

Despite that technical difference, the traditional AWPA use classes - such as decking, above-ground use, and ground contact - are referenced in the ICC-ES evaluation report and on the lumber end tags. As with any type of treated lumber, you should make sure the designation on the end tag corresponds to your intended use. Ground contact increases the risk of rot and insect attack, and requires a higher level of preservative treatment.