There's no getting around the fact that preservatives are designed to kill organisms. When we choose preservatives, we need to find the right balance between protecting wood from decay and protecting our own health and the environment from chemicals. The earliest wood preservatives--salts of lead and arsenic--were plenty toxic to the problem organisms, but failed miserably when it came to being safe for humans and the environment.
Creosote, patented in 1831, was the first wood preservative to successfully protect wood from ground contact and high moisture. It is distilled from coal tar (a by-product of making coke from bituminous coal) and is toxic to fungi and most other decay and wood-boring organisms. Because it is oil-based, it stays mostly in the wood, rather than leaching out. By the 1920s it had become the treatment of choice for railroad ties. Unfortunately, creosote is smelly, ugly, unpaintable, and toxic to some plants. Furthermore, it is now classed as a known carcinogen.
Pentachlorophenol (penta), developed in the 1930s, is another preservative equally out of favor. It was the first synthesized pesticide and was widely used until the 1980s. Like creosote, it is an oil-based preservative. The state of California now recognizes it as a carcinogen, and studies show that it becomes concentrated in organisms on the food chain such as fish and birds of prey. Since 1986, use of both creosote and penta has been restricted to certified applicators only.
Today, preservative manufacturers are pursuing two different strategies for making their products safer. First, they are finding chemicals that are highly specialized--targeted just toward fungi, for example. Most of the surface-applied preservatives sold today include copper or zinc compounds, which are quite toxic to fungi yet relatively safe for humans.
For good reasons, wood is a highly popular material for outdoor projects. It's easy to cut. You can nail it. It's not too heavy to lug around. It comes in all sizes and shapes. And it looks good. The problem is that wood is also biodegradable. It's a food source for fungi, bacteria, insects, and other organisms. If we want to use wood outdoors--as fencing, sides for raised beds, retaining walls, or decking--we have to deal with these very real problems.
The most common option for controlling decay is to use wood treated with chemicals that make it more resistant. Most untreated wood in contact with the ground will last just one to four years, while preservatives can extend that life to several decades--or even longer--reducing pressure on our forest resources. It is estimated that use of pressure-treated wood saves a quarter of a million trees from harvest each year. Furthermore, pressure-treated wood comes from fast-growing trees on tree farms, not from very old or slow-growing trees.
This article addresses these preservative options. Other articles in this series will look at some alternatives to preservative treatment, including using woods naturally resistant to decay, and "lumber" made from recycled plastic.
Most home centers have a whole shelf of compounds used for treating outdoor wood surfaces. The most common active ingredients are copper naphthenate, zinc naphthenate, and 3- Iodo-2-propynyl butyl carbamate (IPBC). Of these three, copper naphthenate is usually the most effective (a 10 to 20 percent concentration is recommended). Zinc naphthenate is a little less effective in preventing decay and mildew, and IPBC is less effective still and is not recommended for ground contact uses.
These products can be brushed, rolled, or sprayed onto finished or unfinished wood to provide some level of protection against decay or degradation. Many of these surface treatments also stain the wood, protect it from degradation by ultraviolet light, or seal it against moisture penetration.
The effectiveness of surface treatments is limited both by the less toxic chemicals used and by the fact that the chemicals soak only a little way into the wood. How far they penetrate depends on the wood species, how dry the wood is, and the solvent used in the preservative, but they rarely penetrate the wood more than a quarter inch.
Generally, much more effective protection is achieved when waterborne preservatives are forced under pressure deep into wood. This type of treatment is essential to protect most types of wood in direct contact with soil. Wood intended for ground contact should be rated .40 pcf, or pounds of chemical per cubic foot of wood. Wood for above-ground use should be .25 pcf.
The most common pressure-treating chemical in use today is chromated copper arsenate (CCA). In the 1930s, researchers found that sodium dichromate would fix arsenic salts to wood. They then added copper sulfate to improve resistance to fungal decay, to form CCA. Unlike woods with oil-based creosote and penta, CCA-treated wood can be painted. It was approved for treating wood in the late 1940s, and came into widespread use during the 1960s, primarily for treating southern pine (including loblolly, longleaf, shortleaf, and slash pine). By 1995, 5.1 billion board-feet of CCA-treated lumber, timbers, and other wood products were being produced annually in the United States--roughly 17 percent of all softwood lumber. A similar compound, ammoniacal copper zinc arsenate (ACZA) is used in the West for treating more dense Douglas fir, for which CCA is not as effective. (Douglas fir requires incisement, or punctures, for preservatives to penetrate deeply.)
Because CCA becomes tightly fixed to the wood fibers, it is relatively safe. Exactly how safe and under what conditions, however, remain hotly contested. For instance, studies indicate that some leaching does occur and that the chemicals move several inches through the soil, a valid concern if you're building a playset, for instance.
But from a broader, environmental-impact point of view, the primary problem with CCA-treated wood comes at the disposal end of the wood's life cycle, especially if it is incinerated. Some of the CCA (particularly the arsenic) may become airborne, but most will end up in the incinerator ash, where it is highly leachable and the risk of groundwater contamination is significant. While incineration of municipal solid waste accounts for only 16 percent of total waste disposal nationwide, it accounts for much more in some states: 60 percent in Connecticut and 45 percent in Massachusetts, for example. Currently, more than 2.5 billion board-feet of CCA-treated wood are estimated to be reaching the waste stream annually, and that amount is expected to increase significantly over the next few decades. Because of this concern, the wood products industry and EPA have agreed to stop selling CCA-treated wood by the end of 2003.
The quest for safer wood preservatives has led to two very promising new products. Neither of these products contain the EPA-classed hazardous chemicals arsenic and chromium, so they are more environmentally sound than CCA. Both alternatives protect wood from decay and insect attack as effectively as CCA, according to their manufacturers, and both remain effective from 30 to 60 years or 5 to 10 times longer than untreated wood.
Chemical Specialties, Inc., introduced ammoniacal copper quaternary compound (ACQ) in the early 1990s, and markets it nationwide as ACQ Preserve. Like CCA, wood treated with it is a light green color and accepts stains readily. Also like CCA, hot-dipped galvanized or stainless steel nails, fasteners, and fittings are recommended because preservative chemicals are corrosive to standard hardware.
Another company, Kodiak, Inc., produces and markets wood that is pressure-treated with copper dimethyl- dithiocarbamate (CDDC). The treated wood is brown rather than greenish like CCA- and ACQ-treated wood. This chemical protects wood as well as the others, but is supposedly less corrosive to fasteners.
The other two CCA manufacturers--Hickson and Osmose Corporation--have developed copper-based CCA alternatives, but they are not marketing these actively in the United States.
Both ACQ- and CDDC-treated woods cost somewhat more then CCA-treated wood and are not yet available in all areas.
Always wear gloves when applying a preservative, or when working with pressure-treated wood. If sawing, wear a dust mask to prevent inhaling sawdust. After treating cut ends with paintable preservative, wipe away any residue or precipitate that is visible. Never burn scraps or sawdust of treated wood, and only dispose of waste wood at landfills.
Alex Wilson is the editor and publisher of Environmental Building News.
Photography by John Goodman.
Article published on June 23, 2008.