For 3 decades, builders of outdoor decks, arbors, swing sets, and other unpainted structures have relied almost exclusively on the greenish wood known as pressure-treated lumber. Annual sales of some 7 billion board feet of this wood created a U.S. industry worth $4 billion per year. What makes the lumber so useful is what the pressure treatment forced into it: a toxic cocktail of arsenic and other pesticides that deters termites, other insects, fungi, and microbes.
Until this year, 90 percent or more of pressure-treated wood had been infused with chromated-copper arsenate, or CCA. In 2001 alone, CCA production devoured some 40 million pounds of arsenic and 64 million pounds of hexavalent chromium. Both arsenic and that form of chromium at relatively low concentrations are carcinogens, but arsenic is of greater concern because it leaches from the wood more readily.
The Environmental Protection Agency had approved pressure-treated wood decades ago, but the agency announced 2 years ago that it would begin reevaluating whether CCA’s ingredients posed a cancer risk to children. Wood-preservative makers responded by volunteering to phase out CCA for residential and almost all other uses where substantial human contact could be expected.
Indeed, as of Dec. 30, 2003, U.S. chemical companies no longer have EPA approval to sell CCA to treat wood for use around homes, though retailers have until May 16 to sell CCA-infused lumber still in supply pipelines. Because CCA production had accounted for 90 percent of domestic arsenic use, EPA notes that the treated lumber’s phaseout should “virtually eliminate” this poison’s U.S. market.
Several relatively nontoxic wood preservatives are already available, and they repel rot and bugs about as well as CCA does, notes Jim Jones, director of EPA’s Office of Pesticide Programs in Arlington, Va. The primary difference, he says, will be their 10-to-15-percent higher cost.
But what about risks from continuing exposures to the CCA in existing outdoor structures that will remain in place for years? Two months ago, EPA completed a draft risk assessment for CCA-treated decks and playground equipment. It concluded that some U.S. children, depending on where they live and how they behave, could indeed face an unacceptably high cancer risk from exposure to the treated wood.
The American Wood Preservers Institute of Reston, Va., an industry group, challenges that assessment. The group cites a California study on CCA-treated playground equipment as showing there is “negligible risk to children.”
At the other end of the spectrum is a Washington, D.C.–based advocacy organization, the Environmental Working Group. In May 2001, it petitioned the Consumer Product Safety Commission (CPSC) to require removal and safe disposal of treated wood from equipment in public playgrounds and to force CCA makers to refund to consumers the cost of old treated backyard play sets.
On Nov. 4, 2003, the commission unanimously rejected the petition. Chairman Hal Stratton explained that children already confront comparable exposures to arsenic from the diet and other sources, although he conceded that CCA-treated equipment “could be a significant source of [a day’s] arsenic for children” who play on it.
Stratton also argued that the chemical industry’s new voluntary phase-out of playground and backyard lumber would essentially “render the petition moot.”
Moreover, for structures already in place, some treatments may lock CCA within pressure-treated wood and thereby render it harmless. In fact, EPA and the U.S. Forest Service have recently launched independent investigations into the efficacy of water repellents and coatings on pressure-treated lumber. Although their preliminary data indicate that such finishes keep arsenic and chromium in place, neither agency has data yet that indicates how often treated wood would have to be resealed.
Kid stuff
Although CCA-treated lumber has been available for 60 years, it didn’t become the outdoor product of choice until the 1970s, when the United States’ love affair with home decks heated up.
Realizing the toxic nature of CCA’s ingredients, EPA has long warned against breathing sawdust from pressure-treated wood or burning the material.
Two years ago, as part of an ongoing safety reevaluation of long-used pesticides, EPA announced it was investigating cancer risks that CCA-treated wood might pose to consumers. The agency focused on children because they tend to spend much more time on decks and play equipment than adults do and because young kids frequently put hands, toys, and other items into their mouths.
EPA’s new hazard assessment proved tricky. The presumed route of most CCA exposure is from the wood to hands or other items that enter a child’s mouth. Because kids vary widely in how often they put nonfood items into their mouths, EPA needed to have researchers monitor and quantify this behavior in a large sample.
The agency also considered how much time children play outside. For instance, Minnesota youngsters have a shorter outdoor-play season than do children in Gulf Coast states.
Further complicating the risk evaluation, EPA’s study of some 1,000 samples of pressure-treated lumber revealed that CCA leaches from weathered wood at widely varying rates. Because ultraviolet light and rain can accelerate CCA’s release, the EPA scientists have surmised that wood structures in the southern United States release more arsenic than do decks and swing sets further north.
These variables prompted agency scientists to make what’s called a probabilistic risk assessment for kids from CCA, Jones says. For CCA from lumber, the end result was cancer-risk estimates for children in the top 10 percent of projected exposures, the bottom 10 percent, and the groups in between.
Ordinarily, EPA considers a cancer risk as excessive when it’s higher than 1 in a million. On average, kids exhibiting extensive hand-to-mouth behaviors who live in warm environments face a 2.5 in 100,000 cancer risk—or more than 10 times the risk that triggers EPA concern. The agency now projects that for the top 5 percent of exposed children, the cancer risk could be 1.4 in 10,000, or more than 100 times the value that might be deemed acceptable. An EPA report dated Nov. 10, 2003, outlines the details of these calculations (http://www.epa.gov/oscpmont/sap/2003/december3/
shedsprobabalisticriskassessmentnov03.pdf).
Within a few months, Jones says, the agency plans to release a companion cancer assessment for adults who make or use CCA-treated lumber.
Parents’ dilemma
If CCA is a cancer risk to at least 5 percent of the nation’s youngsters, why doesn’t EPA advocate the removal of treated wood from yards and decks? The situation parallels the case of asbestos, a potent carcinogen present in many building materials. Currently, EPA recommends strongly against removing asbestos from buildings if it’s in undamaged ceiling or floor tiles, for instance. The agency argues that removal risks releasing dangerous concentrations of the currently sequestered, toxic fibers.
Similarly, EPA notes that removing all CCA-treated structures could release large amounts of now-interred pesticides.
The Environmental Working Group doesn’t accept that argument because its staff concludes that leaving the structures in place has a high cost. Last year in testimony before CPSC, the group’s vice president for research, Jane Houlihan, said that her organization’s tests indicate that homeowners with old CCA-treated decks, play sets, and picnic tables “remain at risk from high levels of arsenic . . . for 20 years, the entire useful life of the wood.”
For its study, Houlihan’s group measured arsenic residues on the surfaces of 598 treated-wood structures, including play sets, picnic tables, decks, and tree houses. Moist swabbing of 100 square centimeters of the surface—an area comparable to the size of a preschooler’s hand—picked up 0 to 2,813 micrograms of arsenic. The median value was 9 µg, though on 10 structures the amount exceeded 500 µg. In general, Houlihan observes, the swabbed value “typically far exceeds what EPA allows in a glass of water.”
On its Web site, the Wood Preservative Science Council in Mount Vernon, Va., criticizes these data. The industry group cites a CPSC study indicating that a child’s hand typically removes only a small fraction as much arsenic from a surface as the Environmental Working Group’s study picked up in its swabs. Therefore, the council argues that the group’s conclusion is grossly exaggerated. The critique also maintains that “it is incorrect to compare the form of arsenic in drinking water, which is typically soluble, to the form in treated wood, which is insoluble” and thus poorly absorbed by the body.
However, at the 2003 Annual International Conference on Contaminated Soil in Amherst, Mass., last October, CPSC scientists updated and increased their estimate of arsenic exposure for children playing on CCA-treated play sets. According to the new data, a child’s hand could pick up 7.6 µg arsenic from the wood. The scientists calculate that “a young child who plays primarily on CCA-treated wood playground structures in early childhood has an increased lifetime risk of 2 to 100 per million of developing lung or bladder cancer.”
EPA acknowledges that such a risk estimate, as well as its own recent data, concerns parents. Pending the final version of EPA’s assessment of CCA risks to children, the agency’s advice remains decidedly low tech. For now, Jones advises parents of children who have played on CCA-treated wood, “We strongly encourage that you always wash their hands.”
Wax on?
EPA also has been coordinating a multicenter research program to see whether various deck sealants are effective barriers to CCA release. Jones notes, “All of the preliminary results that we have are encouraging.” However, he acknowledges, that “what people really will want to know is if [sealants] are going to last 1, 2, or 3 years . . . or need to be reapplied every few months.”
In the Sept. 15, 2003 Environmental Science & Technology, Stan Lebow and his U.S. Forest Service colleagues in Madison, Wis., report tests simulating a year’s worth of CCA leaching by rain. The researchers cut 10-inch-long pieces of 2-by-6-inch pressure-treated lumber. They applied a clear, penetrating, water-repellent sealer—a lab-concocted “bare-bones” formula of just 1 percent to 5 percent paraffin—to some pieces. The other wood remained unsealed.
For 6 weeks, the boards sat in the equivalent of a continuous drizzle for 9 hours on each of 5 consecutive weekdays. Water draining off each board was separately analyzed for CCA leaching.
To evaluate solar ultraviolet (UV) light’s effect on wood weathering, the scientists also exposed some cut boards to UV light between the first and second weeks of rain exposure. UV weathering, essentially a breakdown and sloughing off of wood fibers, repeatedly exposes surfaces to CCA leaching. Indeed, Lebow’s team reports, UV exposures tripled arsenic’s release from uncoated wood.
All paraffin concentrations sealed the CCA into the wood comparably. However, none proved as effective a barrier as had paint or pigmented stains that the researchers tested in a related, unpublished experiment. Moreover, both the paint and colored stain proved better than the clear water repellent at limiting UV weathering. Though the sealant limited leaching, UV exposure quintupled the arsenic that rain removed from sealant-treated wood.
Paint proved the best barrier, at least in the short term, Lebow notes. However, he suspects that consumers may ultimately reject paint as an option because it tends to scuff off and requires extensive surface preparation before it can be repainted. Pigmented sealers may offer a good compromise.
In fact, commercial sealants and stains come in a wide variety of formulations containing many types of additives. Those extras might improve weatherproofing, adhesion, or durability over the recipes tested in his lab, Lebow observes.
Houlihan, however, remains doubtful that CCA-treated lumber can be made safe. She notes that in her group’s study, wood sealed with commercial products more than 6 months earlier showed surface arsenic residues that were “statistically indistinguishable” from those on never-sealed wood. She suspects that scuffing and other activities quickly degrade the sealants’ CCA-trapping efficacy.
What’s next
Eventually, even regularly sealed outdoor structures made from CCA-treated lumber will reach the end of their useful life. For now, EPA recommends sending retired CCA-treated wood to landfills, where most leached compounds will bind to soil. What people should not do, Jones says, is recycle the lumber into garden edging or any place where wood might leach its toxic contents into the root zone of edible plants.
A 2001 study at the University of Minnesota found that arsenic leached from CCA-timber edging in raised-bed gardens, although it didn’t elevate crop-arsenic concentrations to worrisome levels.
However, the Minnesota team did recommend that gardeners grow plants at least 15 inches from the treated wood and, where plants develop extensive root systems, install plastic liners inside the wood frames.
EPA also notes that retired CCA-treated lumber should never be burned. An Australian study reported in the Sept. 15, 2003 Environmental Science & Technology that burning this wood not only releases arsenic into the air, but also creates copious amounts of dioxin, another human carcinogen. The chemistry of the dioxin formation isn’t clear.
There are several good alternatives for people replacing CCA or looking into new outdoor construction, according to Ed Gulick of the Berkeley, Calif.–based Green Resource Center.
This nonprofit group assists architects, contractors, and homeowners.
There are maintenance-free composites made from roughly equal parts of recycled plastic and sawdust. However, most contractors and architects have switched from CCA lumber to pressure-treated wood incorporating agents known as alkaline copper quaternary or copper boron azole. The components of these chemicals are nontoxic to people, EPA says.
Indeed, EPA awarded Chemical Specialties of Charlotte, N.C., its 2002 Presidential Green Chemistry Challenge Award for developing alkaline copper quaternary. The agency said that replacing CCA with alkaline copper quaternary is “one of the most dramatic pollution-prevention advancements in recent history.”
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