Arsenic and Pressure Treated Wood
Arsenic and Pressure Treated Wood
David E. Stilwell
Department of Analytical Chemistry
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8457 Fax:(203) 974-8502
Prior to January of this year, the most common wood preservative formulation in use was CCA (containing copper, chromium, and arsenic). Recent findings that arsenic can be released from this wood by leaching and physical dislodgement has prompted a phase out on its sale for most residential applications, effective January 1, 2004.
WHAT IS PRESSURE TREATMENT?
Pressure treatment is a process used to inject preservatives deeply into the wood. Typically, the wood is loaded onto a tram, placed into the treatment vessel, evacuated, and then the preservative is applied to the wood under pressure. Prior to the phase out, the most common preservative formulation was CCA. The amount of this preservative in the wood was typically 0.2% copper, and 0.3% chromium and arsenic; higher amounts are still employed for foundation and marine applications. Millions of board feet of this wood was produced each year and used for many outdoor applications such as decks, docks, playscapes, landscaping, fences, picnic tables, and traffic sound barriers. The use of wood preservatives extends the life of the wood which reduces the use of forest products.
WHAT ARE THE HEALTH AND ENVIRONMENTAL CONCERNS?
The arsenic used in the CCA formula is in the inorganic form which is a class A carcinogen, number 1 in EPA’s priority list (CERCLA), on the Z-list of toxic substances (OSHA), and is a potential endocrine disrupter. Moreover, this arsenic leaches from the wood, winding up in the soil, and it is dislodged from the surface upon physical contact. For example, elevated arsenic has been found near CCA structures in Connecticut, in soils under decks and highway traffic sound barriers, in Florida in soils under decks and playscapes, in New Jersey in soils under wetland boardwalks, and in Canada in soils near utility poles. In particular, in 85 samples under 7 decks in Connecticut the arsenic (in mg/kg) ranged from 3-350 and averaged 76; in 35 samples under 3 sound barriers located in Connecticut it ranged from 7-228 and averaged 67; in 65 samples under 8 structures in Florida it ranged from 1-217 and averaged 28; in 18 samples next to 8 utility poles in Canada it ranged from 150-410 and averaged 262; and in 18 samples under 2 boardwalks it ranged from 22-150 and averaged 65. All of these averages were well above the associated soil arsenic background levels (generally 1-5 mg/kg) and they all exceed the State of Connecticut guidance limit of 10 mg/kg for soil arsenic.
Arsenic can also be dislodged from CCA wood surfaces such as those commonly found on playground equipment, decks, and picnic tables. In this case the exposure is by hand to mouth contact. Evidence that there is physical dislodgement of arsenic from wood surfaces is illustrated by 2 studies, one done here at the station, the other by the US Consumer Products Safety Commission (CPSC). In the CAES study, 6 sets of 2.5-m-long CCA pressure-treated pine boards were purchased from three lumberyards. Each set consisted of three to four boards, and the total number of boards used was 19. Each board was cut into 30- or 60-cm test coupons. Either two or four coupons were used in the study for a total of 44 coupons. The coupons were placed outside to weather and were periodically sampled over a period of up to two years. A total of 316 wipe samples were analyzed for arsenic. The amounts of arsenic in the wipe samples were highly variable, but detectable arsenic was dislodged from all samples. Specifically, the amount of arsenic dislodged ranged from 5-122 µg/cm2, and averaged 34. Although we observed that the amounts dislodged decreased after multiple surface wipes, it was also noted that the surface could rejuvenate after weathering, and therefore we could not determine if there was any clear-cut trend in the overall amounts of arsenic dislodged over time. Another study on arsenic dislodged from surfaces was completed by the Consumer Product Safety Commission in early 2003. In this study 8 decks and 12 playscapes in the Washington DC area were tested for dislodgeable arsenic. The commission staff also compared wet and dry wipe methods to direct hand contact. Based on their study they determined that the amount of arsenic dislodged onto the hand, and subsequently ingested, could result in an increased risk of between 2-100 in a million of bladder and lung cancers.
Mounting evidence of arsenic exposure of the kind just sited, by government agencies, universities, public interest organizations and investigative reporters, prompted the US EPA to hold a series of advisory meetings. These meetings were held between October 2001 and December 2003. Groups and individuals from all sides of the issue were allowed to testify. Due in good measure to the recommendations stemming from the October 2001 meeting, the US EPA was able to negotiate a settlement, on February 12, 2002, which stated that all residential use of CCA wood would be phased out by January 1, 2004.
The EPA, CPSC and the CAES have recommendations on ways to minimize the potential risks associated with this arsenic containing CCA wood. These suggestions include:
· Wash children’s hands after playing on the wood.
· Do not eat while on CCA-treated playgrounds.
· No animal or children’s play areas under decks.
· Do not grow edible plants near the wood.
· Paint or stain CCA wood surfaces regularly.
· Use alternative materials for contact surfaces.
EFFECTS OF COATING
Exposure can be reduced by coating the wood with paint. Studies are underway to determine which coatings (paints or stains) are most effective. We have found that opaque coatings that are polyurethane or acrylic based can reduce the arsenic dislodged from surface as well as arsenic leaching to the soil more than 90%. Spar varnish, while forming an effective barrier to arsenic in the short term, tends to peel and crack after aging. Some penetrating coatings offer good reduction on surface boards but may not hold up well under soil contact situations. Penetrating finishes are typically clear to semi-transparent, and they tend to wear uniformly without peeling and chipping, which can be preferable when used on high wear areas such as foot traffic. Surface preparation may be advisable prior to coating the wood. We do not advise that the surface be sanded as it could generate fine particles. Only use the lightest possible power washing, scrub brush, and cleaners to prepare the surface. It is best to test the surface before, right after and then periodically after applying the coating. One source for a test kit is through the environmental working group http://www.EWG.org.
THE REPLACEMENT PRESERVATIVES
After January 2004 the CCA preservative will be replaced with ones that contain no arsenic or chromium. There are two major replacement formulations. One is ACQ, sold under the trademark Preserve Wood. This formulation is a combination of copper and didecyl dimethyl ammonium chloride (quat). The other is copper azole, sold under the trademark Natural Select. This formulation contains copper and tebuconazole. Borates can be added to the copper azole formulation, and may be approved for other uses, depending on the application.
These new preservative formulations do not contain EPA listed compounds, nor do they contain known or suspected carcinogens. The ACQ formulation was given the green chemistry award in 2002. Unfortunately, the toxicity of copper to some beneficial marine organisms presents problems for the use of these preservatives in marine applications.
There are many alternative products to consider in place of pressure treated pine. Wood polymer composite materials are now readily available. These materials are typically made from about 50% plastic (commonly recycled polyethylene) and about 50% sawdust (pine, cedar, rice hulls). The most common application of these materials is for decking. Pure plastic, either from polyethylene or polyvinyl chloride, is also on the market. Plastic products are most commonly used for fencing and railing, but plastic landscape timbers are also becoming available. Alternative woods that are resistant to decay include ipe, cedar, and pacific cypress.
ADDITIONAL SOURCES OF INFORMATION