Pressure-Treated Wood: How bad is it and what are the alternatives?
We use approximately 3.8 billion board feet of preservative-treated lumber per year in the building industry in North America, and treated wood accounts for over 21% of all dollars spent on timber products (figures from the American Wood Preservers Institute—AWPI). What is the impact of these materials on the environment? Do they release harmful materials into our buildings? How safe are treated woods to work with?
While the safety concerns about living with and using treated wood are sometimes overblown, there are legitimate concerns about its use. The following information will help you decide when and where it is appropriate to use pressure-treated lumber, and what alternatives might be available.
How preservatives work
To understand how wood preservatives function in protecting wood, we need to understand what types of stress wood is up against. Physical agents such as heat, ultraviolet light, abrasion, movement, and chemicals can decrease the strength of wood over time. Far more damage, however, is done by biological agents: decay fungi, bacteria, insects, and marine borers.
For these organisms to enter and degrade wood, four basic conditions are required: moisture, free oxygen, a temperature between 50° and 90°F, and food (namely, the wood). Of these conditions, moisture is the most easily controlled in most wood applications. By keeping the wood dry (within frame walls, for example), or by allowing the wood to dry between wettings (as with wood siding and shingles), we prevent most rot. In situations where prolonged moisture exposure is unavoidable, rot can be prevented only by eliminating the wood as a food source. We do this by making the wood poisonous to the insects and fungi that want to eat it.
Wood preservatives were used in North America as long ago as 1730, when wood was dipped in a solution of arsenic salts to protect against insect attack. Wood soaked in a solution of copper sulfate was also found to be resistant to decay. Because of the solubility of these metals in water, however, the treatments only worked if the wood stayed dry. Creosote, patented in 1831, was the first wood preservative effective in ground-contact and high-moisture areas. Derived from coal tar, this highly complex mixture of chemicals was the most widely used wood preservative for many years, but the surfaces of creosote-treated woods were oily, smelly, and unpaintable. It is used today primarily for treating railroad ties and utility poles—rarely for lumber. Pentachlorophenol, or penta, was developed in the 1930s as the first synthetic pesticide. The easily synthesized compound could be produced in large supply, and it quickly became popular. Because of trace contaminants, however, (including dioxins) penta use is now greatly restricted.
In the 1930s researchers succeeded in “fixing” arsenic salts in wood using sodium dichromate. Copper sulfate, an effective fungicide, was added to the formulation to create today’s most common wood preservative: chromated copper arsenate, or CCA. CCA, with its characteristic greenish color, was approved for treating wood in the late 1940s, but did not come into widespread use until the 1960s when demand rose for treated wood that could be painted or stained.
In the past two decades the salt formulation, made with sodium dichromate, has been replaced with an oxide formulation based on hexavalent chromium. Of the three CCA manufacturers, only Chemical Specialties, Inc. (CSI) still produces the salt version, and it represents less than 5% of its CCA sales, according to Jim Saur, Vice President for Marketing at the company. Once CCA is fixed to the wood it is no longer soluble, so it will continue protecting wood for many years, even in water contact.
No matter what the type of preservative, for effective treatment it must penetrate deeply into the wood. This is accomplished by pressure treating. The lumber is loaded into a cylindrical pressure chamber, and high pressure forces the preservative into the wood cells. For maximum penetration and retention of preservative, the pressure chamber is sometimes first
depressurized to pull air out of the wood cells. (This technique is often used with creosote.)
With CCA, the amount of preservative retained in the wood also depends on the concentration of preservative in the liquid. For non-ground-contact applications, where a retention level of only .25 lbs per cubic foot is required, a .75% to 1% solution of CCA is typically used in the treatment process. For marine pilings, where up to 2.5 lbs. per cubic foot retention is required, a 7% to 10% solution of CCA is used. Note that retention of preservative in the wood is not the same as fixation. Retention refers to that amount of preservative left in the wood when the wood leaves the pressure treating vessel. Fixation refers to the chemical reaction of CCA components within the wood that renders them insoluble.
Proper fixation of CCA in wood can take several days, weeks, or months—depending on conditions such as temperature. CCA fixation is also influenced by the relative proportions of the chemical ingredients. Three types of CCA are recognized by the American Wood Preservers Association (AWPA)—Type A, B, and C—depending on the ratios of arsenate, chromium, and copper. The Type C formulation most effectively resists leaching and has become the standard within the industry. You should see a label on pressure-treated wood that identifies both the Type (A, B, or C), and the CCA retention in pounds per cubic foot (see illustration).
Along with CCA, there are several other water-borne preservatives. Ammoniacal copper arsenate (ACA) and ammoniacal copper zinc arsenate (ACZA) were developed to provide better penetration in woods that are difficult to treat, such as Douglas fir (these species, known in the trade as “refractory” species, are often scored with small incisions before treating to improve preservative penetration). Acid copper chromate (ACC) and chromated zinc chloride (CZC) are much less common water-borne preservatives. Another chemical, only recently approved for use by AWPA, is known as ACQ (“AC” for ammoniacal copper, and “Q” for quaternary ammonia, or quat).
Environmental and health considerations
Before addressing the environmental and health risks posed by pressure-treated wood, it should be noted that there are some environmental benefits as well. One obvious environmental benefit of preservative-treated wood is a savings in energy and resources resulting from increased longevity of the wood. The American Wood Preservers Institute (AWPI) claims that without treated wood “our requirements for railway, utility, construction, and marine industries would have exhausted our resources many years ago.” Had suitable wood preservatives not been developed, however, we would doubtless be using other materials. As for the wood resource that is used in producing pressure-treated wood, much of it is plantation-grown southern yellow pine. While this source is clearly preferable to depleting old-growth forests, the commercial tree plantations raise other environmental concerns. As with all natural resources, the less we use, the better.
Environmental and health concerns with wood preservatives include both dangers associated with concentrated chemicals and concerns about the wood after treating. Because more than 99% of the treated lumber used in the building construction industry is treated with water-borne arsenicals (CCA-treated southern pine treated is the most common), this discussion will focus primarily on CCA-treated wood.
From an environmental standpoint, this has always been one of the most important considerations with wood preservatives: whether the chemicals will leach out of the wood into the ground, groundwater, or (in the case of pilings and docks) into open water. Even if pollution were not an issue, leaching of preservatives would still be a primary concern of the wood preserver industry, since if the chemicals leached out, the wood would no longer be protected against rot. So, does leaching occur?
“Many in the treating industry have, at least up until recently, not been entirely up-front about the fact that some leaching does occur,” says Bruce Johnson, a wood preservation expert at the Forest Products Laboratory in Madison, WI. From the standpoint of effectiveness at protecting the wood, that was probably a fair statement, according to Johnson. “When you have a well-treated piece of wood handled in the recommended manner during and following treatment—whether it’s in a marine or aquatic environment, or in the soil—the amount of leaching that occurs has very little impact on the serviceability of the wood,” he says. But some leaching clearly occurs.
Both laboratory and field tests of leaching have generated numbers that range all over the place, notes Johnson. The amount of leaching depends on a number of factors: the formulation of the preservative (Type-C has better fixation than Type-A CCA), the type of wood, whether there is any finish on the treated wood, how the treatment was performed, how the wood was handled after treatment (whether it was properly conditioned), the type of exposure, and the configuration or geometry of the wood. Wood with a large surface-to-volume ratio, such as plywood, will leach more CCA than wood with a low surface-to-volume ratio, such as a 4x4 post.
A 1979 paper in the
Journal of Environmental Quality reported on CCA leaching from stakes that had been in direct soil contact for approximately 30 years at the Harrison Experimental Forest near Saucier, Mississippi—an area with an average of 63 inches of rainfall per year. Soil within six inches of the stakes was found to contain 10 to 15 times more arsenic than background levels, and similar increases in chromium and copper. Beyond six inches, however, the increases over background levels were insignificant. According to Johnson, there is little further migration because “the components that leach are rapidly adsorbed by soil particles.” Migration down to the water table would be unlikely in most conditions. Even this amount of leaching left enough chemicals in the wood to protect against rot for decades. In addition, the Type-C CCA currently used for almost all pressure treating has been shown to leach less than the formulation that was used in the stakes.
Concern about CCA leaching into the marine ecosystem has led the Township of East Hampton, NY (part of Suffolk County on Long Island) to consider a ban on CCA-treated wood 500 to 1000 feet from surface water. While the new zoning measure has not yet taken effect, it has generated a flurry of controversy, and the AWPI is leading a major campaign to defeat it.
The East Hampton effort to ban CCA-treated wood is supported in part by research conducted by Dr. Peddrick Weis (of the University of Medicine and Dentistry of New Jersey) and three scientists from Rutgers University. The researchers found that several common marine organisms were negatively affected or killed when placed in containers of seawater with small samples of CCA-treated wood. While the results seem compelling, this research has been criticized by the wood preservers industry since it was funded by the Center for Plastics Recycling Research at Rutgers, an institute promoting recycled plastic lumber, which is a potential competitor to CCA-treated wood in some applications.
Chemical spills and leaks
While there are some legitimate environmental concerns about CCA leaching from wood, environmental risks are probably more significant earlier in the process. The raw chemicals are water-soluble and 10 to 100 times more concentrated than what goes into the wood—and thus far more hazardous. “The risks and concerns increase as you go up the chain from the consumer,” noted Frank Kicklighter, Product Development Manager at CSI.
The chemical components of CCA, in their most concentrated and hazardous form, are handled only by the three preservative producers: CSI, Osmose Wood Preserving, Inc., and Hickson Corporation (owner of the Wolman trade name). These companies operate centralized chemical manufacturing plants and transport the pure chemicals by truck to their more dispersed blending plants. At the blending plants, the chemicals are mixed and moderately diluted, then transferred to treating plants, of which there are approximately 550 in North America. Some treating plants are independently owned, while others belong to the preservative producers. At the treating plants the CCA used to pressure-treat lumber is further diluted with water to a concentration of about 1%, though for some marine applications concentrations as high as 10% are used.
Chemical manufacturing plants, blending facilities, and wood treaters are all regulated by the EPA in the United States, and the transportation of hazardous substances is controlled by the Department of Transportation. Although no major spills or roadway accidents have been reported to date, the danger exists as long as hazardous substances are being used. And minor spills are apparently commonplace. An official at one of the three preservative producers told EBN that in trucking chemicals “there’s not anybody that hasn’t had a leak, with local environmental officials brought in, and a big stink made.”
Leaks and spills at production or treatment plants are rarely reported, and there is no reliable information on their frequency. Clearly the situation has improved greatly in recent years as tighter regulations and improved state and federal monitoring have taken effect. Some older plants, particularly those that have applied creosote and penta for decades, have become environmental nightmares. Sidney Jackson, the EPA’s Product Manager for Wood Preservatives, described one site where so much creosote had gotten into the groundwater that it was pumped back out of the aquifer for reuse! More recently, lumber exiting the pressure chambers was often allowed to drip-dry in unprotected stockyards. Regulations effective December, 1991 now require recovery basins, dikes, and impervious membranes to ensure that dripping chemicals are collected and disposed of properly.
At present, there is no environmentally sound way to dispose of treated wood once its usefulness is past. In landfills it takes up valuable space and won’t break down for many decades. While the heavy metals will presumably remain locked up in the wood for this time, ultimately the wood might release the chemicals into the landfill environment. Disposal in municipal incinerators releases some arsenic, chromium, and copper into the air, though—if the facility is properly designed—most is captured with pollution control equipment. If these heavy metals are successfully kept out of the air, however, they end up in the incinerator ash, where they are highly leachable. We may in the future see a prohibition on incineration of CCA-treated wood in municipal solid waste incinerators.
Leftover scraps of CCA-treated wood should never be burned on-site or in a wood stove. Without proper pollution control equipment, the toxic metals in the treated wood will be released directly into the air.
Health risks to building contractors
Of the various health concerns relating to CCA-treated wood, risks to installers are the most significant. These include inhaling or ingesting sawdust, and eye or skin contact with CCA surface residues or unfixed CCA.
It is well accepted that the chemical constituents of CCA—arsenic, chromium, and copper—are harmful to human health. There have been cases of people getting sick from working with CCA-treated wood, but we found no cases of acute arsenic toxicity in the literature. (Industry literature points out that arsenate, the form of arsenic used in CCA, is its least toxic form.) According to research reports, the quantity of CCA wood that would need to be ingested to cause serious health risk are quite large (a New Zealand study determined that a 220-pound calf would need to eat at least 1.5 pounds of CCA-treated wood to die from acute toxicity—the equivalent of eating an 18-inch piece of 2x4).
As we have found with other chemical toxins, however, acute toxicity is one thing; long-term poisoning another. With lead, for example, we have long understood the acute toxicity symptoms of lead poisoning for many years, but are only beginning to understand the health risks from long-term, lower-level doses. Because low-level health effects of arsenic, chromium, or copper are unknown, always follow recommended precautions when working with CCA-treated wood.
More significant than the risk of ingesting sawdust is the risk of absorbing CCA through skin contact with treated wood that has CCA surface precipitates (usually white crystals), or liquid CCA that has not been properly fixed into the wood. For the CCA to be properly fixed into the wood after pressure treating, temperatures must be high enough for the chemical reactions to occur. At 70°F fixation will probably be complete in three or four days, according to Bruce Johnson, while at 50°F complete fixation requires closer to a couple of weeks. “Once it gets close to the freezing point, there is virtually no fixation,” says Johnson.
CCA-treated wood is not usually kiln dried after pressure treating (except for some premium-grade material). It is often loaded directly from the pressure-treating vessels onto tractor-trailer trucks for delivery to building supply yards (AWPA standards for post-treatment “conditioning” are fairly lax). At the lumberyard, the wood is almost always stored outside. During the summer months this will not be a problem, but in cold weather, the CCA may not yet be totally fixed when you buy the wood. If this is the case, the CCA in the wood is still very leachable, posing both environmental and health risks. Johnson suggests that construction workers should exercise extreme care when working with CCA-treated wood during the winter months. Unless you know the wood was treated during the summer months, kiln-dried after treating, or by some means brought up to an adequate temperature for a long enough period of time, you should assume that the CCA in the wood is leachable and hazardous to work with.
Just because treated wood is wet when you buy it does not mean the CCA hasn’t been properly fixed into the wood. A great deal of water is added to the wood during treatment—remember the pressure treating solution is usually about 99% water—and the wood is rarely dried after treating. If the wood is wet, however—even in the summer when you know fixation is complete—you should exercise greater care working with the wood. CCA absorption through the skin is much greater when the wood surface is wet. Even perspiration from your hands can increase absorption of CCA through your skin. Always wear gloves when working with treated wood.
The primary occupant health issue with CCA-treated wood in buildings is whether the chemicals can get into the indoor environment or onto occupants’ skin where they could be ingested or absorbed. In a study of airborne arsenicals in basements of houses with all-weather wood foundations (constructed of CCA-treated wood) “virtually nothing was found,” says Bruce Johnson of the Forest Products Laboratory. But swipe tests of wood foundations, playground equipment, and decks (tests in which the wood is wiped with a cloth and the cloth then tested) can show arsenicals, depending on how the tests are conducted. Picking up small splinters of treated wood, or even wood cells, in the test samples will show CCA in the chemical assays. The ability to get CCA on your skin by walking on or touching treated wood may be reduced by surface coatings on the wood or use of premium treated wood that has a moisture repellant mixed with the preservative. Made by all three producers, these premium products cost more than standard material, but they should also last longer.
Alternatives to CCA
Ultimately, CCA-treated wood is relatively safe stuff to work with if you follow manufacturers' recommendations, avoid lumber with a white surface residue, and avoid using fresh stock when temperatures are near freezing. In using it, however, you’re still supporting a large industry that produces and transports large amounts of hazardous materials, and is generating hazardous waste. Where feasible, therefore, you should consider safer alternatives.
Alternatives to pressure-treated lumber basically fall into three categories: substituting non-wood materials, substituting woods that are naturally decay-resistant, and using alternative treatments to protect wood at risk. Each of these strategies is outlined below in general terms. Whether any of them are, in the end, preferable to CCA-treated lumber will depend on the specifics of the job, such as cost, flexibility, structural demands, and aesthetics.
Pressure-treated lumber is used most often in structures unprotected from weather. Some people argue that wood simply isn’t a good choice for such applications. The ubiquitous suburban deck can, in many instances, be replaced by an equally satisfying stone patio or covered porch. Natural stone provided quality retaining walls long before treated lumber was used for that purpose. And “plastic lumber” is emerging as a viable option.
Of the alternative materials, locally collected or quarried stone, where available, is usually the best environmental choice. Manufactured materials, such as pavers and bricks of various types, can also be substituted for treated lumber in some situations. Most of these are energy-intensive to produce, however, and they are often transported long distances. Balancing such concerns against the concerns with CCA is tricky at best; blanket recommendations can rarely be made.
Treated lumber is often marketed as an alternative to concrete, especially in residential foundations and highway sound-control walls. While in some cases using wood instead of concrete may make sense due to the high embodied energy of concrete, there are other alternatives that you should consider as well. Slab-on-grade and pier foundations significantly reduce the need for concrete without having to rely on large quantities of treated wood. Foundation walls made from insulating cement block use less cement than poured foundations.
Plastic lumber is produced by over 20 companies in North America and is a potential alternative to treated wood for many applications. At present, both 100% polyethylene and 50/50 mixtures of polyethylene and wood fiber are available. All of these lack the tensile strength of wood, however, and are significantly more expensive than all treated wood. Some playground manufacturers have begun substituting plastic for handrails and other surfaces where the danger of splinters is great. Picnic tables and park benches are an increasingly popular use for recycled plastic, particularly as this use addresses concerns about treated lumber and food. The use of recycled plastic products for docks and piers is also growing. The quality of recycled plastic products varies greatly, so be sure to check out your source carefully before specifying them. Most plastic lumber is not yet code-approved for structural use in occupied buildings.
Traditionally certain decay-resistant woods have been used outdoors without treatment. Best-known among these are redwood and red cedar, though locust, cypress, white oak, and others have all been used in their native regions. Refer to table below for wood that might be available in your area. If these can be acquired from well-managed stands, they might be a good substitute for treated lumber. Even with decay-resistant species such as redwood and cedar, the sapwood has virtually no decay resistance, so be sure to use only heartwood when rot resistance is required. Also, there is some evidence that second-growth timber (with wider growth rings) has less decay resistance than old-growth timber.
Concerns about resource depletion and protection of endangered species in the Pacific Northwest have made redwood and cedar products environmentally suspect. The actual situation, however, is far from clear, as some of these woods may indeed come from forests that are well-managed and protected. Within the next few years wood that has been certified by independent groups for sustainable management should become available. When it does, using these decay-resistant softwoods for some outdoor applications may be a good environmental choice. A limited quantity of salvaged redwood and cypress are available in some parts of the country. These materials are environmentally benign and generally of very high quality. They also tend to be expensive and, therefore, more suitable for small details than for large areas of decking.
In the last few years certain tropical hardwoods with excellent decay resistance such as Greenheart have also become available. Unlike such overused tropical species as mahogany and teak, these “lesser-known species” are being actively promoted by environmental groups as a way of increasing the commercial value of standing rainforests. Even more than with domestic woods, reliable third-party verification of sustainable harvest is critical, as there are many complicating factors. The claim “plantation grown,” even if true, is not always a good criteria, as plantations do not provide the habitat and many other functions of healthy forests. Fortunately, there are many groups working to import tropical “good woods.”
Although CCA is environmentally preferable to creosote and penta preservatives, and has become the treatment of choice for most of the treated wood we use, there are some alternatives. Most of these alternatives cannot replace CCA in every application, but used appropriately they can considerably reduce CCA use.
Various formulations using boron, a remarkably safe and effective preservative, have been used for decades. Unfortunately, no reliable way has yet been found to keep the borates in the wood under wet conditions. There are some applications that use borate preservatives in combination with paraffin or surface sealants, but none has been shown to last nearly as long as CCA when exposed to weather. When the wood can be kept dry, however, and the primary function of the preservative is to protect against termites, borate preservatives can be an excellent choice. (See product spotlight, page 14.)
The newest product to come from one of the “big three” CCA producers is ammonium copper quat (ACQ). CSI, which manufactures ACQ, claims that it is as effective as CCA in both above-ground and ground-contact applications. Others argue that its effectiveness has not yet been proved, since exposure testing to date has been limited. It is generally considered to be significantly less toxic than CCA, and is subject to far fewer EPA regulations. ACQ costs about two and one-half times as much as CCA to wood treatment plants, and the treated wood should cost 10 to 20% more, according to Jim Saur at CSI. Wood treated with ACQ will also be a slightly different color—ranging from greenish to brownish, depending on species—and it may be blotchy.
Although ACQ and other less toxic alternatives to CCA have been in the works for years, they have not been promoted for fear of feeding consumer concerns about the dangers of CCA. ACQ is currently available on both coasts, and is becoming available in the heartland (contact CSI to find out about treaters using it in your area—address at end of article). Any lumber that has been treated with ACQ will be stamped or end-tagged with those initials. Now that a non-CCA product has entered the market, other companies will likely follow suit with less toxic alternatives of their own. Osmose is reportedly developing a copper citrate product that is similar to ACQ.
Another treatment option is to brush on or dip into a wood preservative on-site. The copper naphthenate solutions used to protect exposed endgrain in CCA-treated wood is one of several widely available preservatives. Penta and creosote were once used in this way as well, until the EPA restricted their use to certified treaters. Environmentally, this option is not attractive except for spot treating of cuts, because the preservatives, even if less toxic than CCA, are more likely to spill and drip onto unprotected ground, or to be disposed of improperly. In addition, all topical applications need to be reapplied periodically, further increasing the likelihood of pollution. Wood-pitch-based preservatives sold by Auro and Livos, two leading distributors of European non-toxic paints and finishes, have recently been discontinued, as has Auro’s borate-based preservative. An Auro spokesman cited the cost of meeting EPA pesticide regulations as the reason for taking the borate product off the market.
So what’s the bottom line? Properly fixed CCA-treated lumber is not the health hazard some would lead us to believe, and environmental damage from leaching chemicals is relatively minor. The industry does produce and use hazardous chemicals in huge quantities, however—chemicals that can severely damage both human health and the environment before they are fixed in the wood and after the wood is disposed of. Our recommendation is to try to minimize your use of treated wood by designing alternative details or using alternate materials. Create outdoor spaces and retaining walls with local stone. Consider treating framing lumber that will be kept dry with borates. When you need to use treated wood, follow common-sense safety precautions, especially when the wood is wet and when cold weather may have prevented proper fixing of CCA into the wood.