Extension Communications 3614 Administrative Services Building Ames, Iowa 50011-3614 (515) 294-9915
2/26/01 Contacts: Mark Shour, Extension Entomology, (515) 294-5963, mshour@iastate.edu Elaine Edwards, Continuing Education and Communication Services, (515) 294-5168, eedwards@iastate.edu Yard and Garden Column for the Week Beginning March 2 Toxicity Concerns about Raised Bed Construction Materials By Mark Shour Extension Program Specialist Iowa State University Extension Are you planning on building or modifying a raised garden or flowerbed this season? Treated lumber is one type of framing material that you can use in your landscape project; however, it has both advantages and disadvantages. Let's take a look at treated lumber from both sides of the issue. Wood preservation efforts History books record Egyptians preserving wooden funerary objects with cedar oil. Ship builders coated wood with resins (pitch) of certain conifers or charred wood to discourage pests. A patent for "oyle or spirit of tarr" was granted in 1716 in England to protect wood planks. A coal tar product (creosote) has been used in the United States since 1850 for utility poles and railroad cross ties; pressure treatment for these applications began in 1875. Types of wood preservatives Several chemicals can be used to preserve wood, but three are used in most commercial processes. Creosote is the primary preservative for railroad cross ties and also is used for highway bridges and marine structures (bulkheads, pilings, docks and seawalls). Pentachlorophenol (penta), developed in the 1930s, is the most widely used of the oil-based preservatives and is used for utility poles, fence posts and glue-laminated timbers. Many uses of both creosote and penta have been withdrawn during the past decade. Chromated copper arsenate (CCA), first used in India in 1933, is a waterborne preservative and is the predominant material used to treat lumber intended for residential use and for some nonresidential applications. Wood with this preservative is sold as green-treated lumber or Wolmanized lumber. The chromium component in CCA helps hold the copper and arsenic in the wood. Copper salts are fungicides and the arsenic component has a dual role, acting both as an insecticide and as a fungicide for copper-resistant fungi. Advantages of treated lumber Benefits of preserving wood used for landscaping projects include extending the useful life of the wood; protecting the wood from insects, fungi and other wood-destroying organisms; and reducing the demand for limited forest products, thus conserving a renewable natural resource. Treated wood can last for more than 30 years in the environment. Ongoing studies in Mississippi and Florida show treated wood intact after 50 years in the soil compared with the failure of untreated wood within a 4-year period. Another benefit is the lower cost for treated lumber compared with other types of landscaping materials. Disadvantages of treated lumber There are several concerns regarding the use of treated lumber in garden applications. Creosote-treated lumber gives off an objectionable odor, has the potential for soiling clothes from the oozy black chemicals, has vapors that are toxic to plants growing nearby and is difficult to finish (saw, sand and paint). Direct contact with penta-treated wood can cause human skin irritation and plant damage or death when grown in proximity to the wood. Although CCA-treated lumber is cleaner, free from objectionable odors, not toxic to plants and can be painted compared with creosote- and penta-treated products, the preservative chemicals may not be as leach-resistant as once asserted. Copper and arsenic can slowly leach from CCA-treated lumber if the wood is not properly sealed. This activity has been confirmed by EPA, industry and university sources. Because arsenic is a human carcinogen, much attention has been focused on the potential hazards to children and adults from contacting structures built with this wood. Burning, mechanical abrasion, direct contact with wood, sawdust and acidic rainfall can release arsenic in CCA-treated lumber. Arsenic has been found in soil beneath or directly adjacent to structures built with CCA-treated lumber. Keeping perspective Arsenic is not a stranger to our lives. We breathe in small concentrations of it in the air, drink it in water, inhale it in tobacco products and consume it in foods, especially shellfish. Independent and university-based studies examining the addition of arsenic to the environment due to CCA-treated lumber have not found a consistent, significant change in background soil levels or acceptable dietary intake levels. Product misuse is the primary factor for excessive exposure, based on five widely publicized accounts of livestock or human harm from exposure to CCA-treated wood, sawdust or wood ashes. One researcher from the University of Toronto estimated the chances of getting cancer from arsenic leaching from CCA-treated wood are less than 10 people in a million, compared with the skin cancer incidence of tens of thousands in a million. Raised bed construction Let's wrap up with some recommendations about the use of treated lumber in landscape projects. It is best to avoid creosote- and penta-treated lumber. Consider using a heavy plastic liner between the sealed treated wood and the garden soil. If you are not comfortable using treated wood, consider using recycled plastic lumber, stones or concrete blocks as framing material. You also could consider using naturally decay-resistant wood such as eastern red cedar, walnut, Osage orange, white oak, black locust or redwood for the project. If you decide to use CCA-treated lumber, follow these recommendations to minimize exposure to the preservative chemicals. 1. Avoid using treated boards with a powdery white surface residue. 2. Wear a dust mask and goggles when working with the wood. 3. Put a drop cloth down before sawing or sanding wood. Put collected sawdust and wood scraps in a trash bag. 4. Do not burn treated wood, scraps or sawdust. Dispose of refuse through ordinary trash collection or burial. 5. Wash hands and other exposed skin immediately following work. Wash work clothes separately before reuse. 6. Do not use treated wood chips or sawdust as mulch or compost or in playground areas. 7. Seal treated wood with an oil-based product before installation. Inspect exposed surfaces of treated wood structures every year for evidence of flaking or splintering. Apply fresh sealer as needed. -30- ml: isugarden
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