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Using
Pressure Treated Wood in Vegetable Gardens Pressure treated wood is commonly used in building decks, retaining walls, and play structures in landscapes but is it safe to use for raised beds in vegetable gardens? Many biological organisms, such as insects and decay fungi, can destroy wood. The purpose of incorporating preservative chemicals into wood products is to make the wood toxic to the organisms that would ordinarily consume it, and thus increase its useful service life. Some preservative chemicals can be purchased by the consumer and then applied to the wood by dipping, brushing, or spraying. Other preservative chemicals, such as chromated copper arsenate (CCA) are intended only for use in pressure-treated wood and cannot be purchased separately. CCA-treated wood is typically light green in color, but it sometimes contains a dye that turns it brownish. Each piece of lumber contains a stapled-on tag or a stamp that indicates the type of preservative used plus other pertinent information. Most CCA-treated lumber in California is either hemlock or white fir. Human health, food safety, and environmental concerns regarding the use of treated wood in general, and CCA-treated wood in particular, have developed in recent years. The wood treating industry has voluntarily withdrawn CCA-treated lumber intended for use in residential applications, although it is still available for industrial uses. This withdrawal took place effective January 2004. Alternative treated wood products are currently available, such as amine or alkaline copper quat (ACQ) and copper azole (CA), and more will be offered in the future. Still, questions regarding the proper use and safety of CCA-treated lumber remain. There is no evidence that copper and chromium in treated wood are carcinogenic, however the United States Environmental Protection Agency (U.S.EPA) has classified arsenic as a human carcinogen. Long term exposure to low levels of arsenic can also result in non-carcinogenic toxicity. It is well documented that arsenic can be toxic by ingestion, inhalation, and by absorption through the skin. Studies have proven that most of the elements used in CCA are fixed in the wood, however some amounts of arsenic, chromium and copper is dislodged from the lumber with exposures to rain and water. The chemicals that leach from CCA-treated lumber are rapidly adsorbed by soil particles. The amount in the soil decreases rapidly with distance from the treated lumber. Vegetables grown immediately adjacent to treated wood may incorporate limited amounts of arsenic, but based on US Public Health service standards, these vegetables would still be safe for human consumption. Over time, soil adjacent to treated lumber in a raised bed garden will likely be mixed and distributed throughout the garden area, thereby diluting the effect of the arsenic that is leached. Coating CCA-treated lumber with paint reduces leaching for a time until the paint eventually erodes away. Inserting a layer of plastic sheeting between the wood and the soil also reduces the risk of exposure. An independent toxicology assessment firm reviewed the available animal toxicity and human occupational studies regarding exposure to arsenic associated with CCA-treated wood. They considered a range of exposure routes and levels for both adults and children and concluded that the use of CCA-treated wood in residential and playground settings does not pose significant health risks to children or adults. CCA-treated lumber is not considered a hazardous waste and can
be disposed of in municipal waste landfills. This lumber should
not be burned in fireplaces because small amounts of arsenic will
become airborne in the smoke and the smoke particulates can be
inhaled and can be toxic. The ash from burned lumber will contain
some arsenic, copper, and chromium, so it should not be composted
into the garden.
March 25, 2004 |