Preservative-treated wood is an economical, durable, and aesthetically pleasing building material; therefore, it is a natural choice for construction projects in our National Forests, National Parks, and other public lands. However, we need to ensure that the chemicals used in treated wood do not pose a threat to people or the environment. The purpose of this report is to provide a summary of the pertinent literature on leaching of wood preservative components and their mobility in the environment.

Mass timber structures have the potential to change wooden construction on a global scale. Numerous mass timber high-rise buildings are in planning, under development or already built and their performance will alter how architects and engineers view wood as a material. To date, the discussion of material durability and biodegradation in these structures has been limited. While all materials can be degraded by wetting, the potential for biodegradation of wood in a mass timber building requires special consideration. Identifying and eliminating the conditions that might lead to this degradation will be critical for ensuring proper performance of wood in these structures. This article reviews and contrasts potential sources of biodegradation that exist for traditional wood construction with those in mass timber construction and identifies methods for limiting the degradation risk. Finally, future research needs are outlined.

The release of arsenic from wood pressure-treated with chromated copper arsenate (CCA) can be decreased by application of wood finishes, but little is known about the types of finishes that are best suited for this purpose. This study evaluated the effects of finish water repellent content and ultraviolet (UV) radiation on the release of arsenic, copper, and chromium from CCA-treated wood exposed to simulated rainfall. Deck boards treated with CCA were either left unfinished or dipped in a finish prepared with 1%, 3%, or 5% water repellent. All specimens were exposed to leaching from simulated rainfall, and a subset of specimens was also exposed to UV radiation. The rainfall was collected and analyzed for total elemental arsenic, copper, and chromium. The water repellent significantly decreased the amounts of these elements in the runoff, but for the short duration of this study there was no difference among the three water repellent concentrations. It is possible that water repellent content would have a greater effect over a longer exposure period. Exposure to UV radiation caused a significant increase in leaching from both finished and unfinished specimens. This effect may be a result of increased surface area during weathering as well as loss of fibers caused by UV-induced surface erosion.

For decades chromated copper arsenate (CCA) was the primary preservative for treated wood used in residential construction. However, recent label changes submitted by CCA registrants will withdraw CCA from most residential applications. This action has increased interest in arsenic-free preservative systems that have been standardized by the American Wood Preservers' Association. These include acid copper chromate (ACC), alkaline copper quat (ACQ), copper azole (CBA-A and CAB), copper citrate (CC), copper dimethyldithiocarbamate (CDDC), and copper HDO (CX-A). All of these CCA alternatives rely on copper as their primary biocide, although some have co-biocides to help prevent attack by copper-tolerant fungi. They have appearance and handling properties similar to CCA and are likely to be readily accepted by consumers. Prior studies indicate that these CCA alternatives release preservative components into the environment at a rate greater than or equal to that of CCA, but because these components have lower mammalian toxicity they are less likely to cause concern in residential applications. As the treated wood industry evolves it is probable that a wider range of types and retentions of wood preservatives will become available, with the treatment more closely tailored to a specific type of construction application.

Preservative-treated wood is often used for construction of highway and foot bridges, wetland boardwalks, and other structures in or over water or sensitive environments. In these applications it is important that release of preservative from the wood into the environment is minimized. This publication addresses this concern by describing the various types of pressure-treated wood, reviewing recent research on the environmental impacts of pressure-treated wood, and discussing methods of minimizing potential environmental impacts. Recent research indicates that wood treated with these preservatives does release small amounts of chemical into the environment immediately adjacent to the treated structure, although no adverse biological impacts were observed. Environmental releases from treated wood can be minimized with appropriate treatment practices. These practices include fabricating members before treatment and specifying that the wood be treated using methods that ensure chemical fixation and prevent the formation of surface residues or bleeding. Guidance to specifying such treating practices are offered in this report and in sources such as the Best Management Practices developed by the Western Wood Preservers' Institute. Also, responsible construction practices such as storage of treated wood under cover and containment and collection of construction residue can further reduce the possibility of negative environmental impacts. As with any other construction material, careful specification and responsible use of treated wood will optimize its performance.