"Treated wood" refers to lumber that has been altered by the addition of a preservative chemical. The wood is saturated in the preservative, and subjected to a cycle of high and low pressure, which helps to force the preservative deep into the cellular structure of the wood. Gardeners often use treated wood to make decay-resistant garden stakes, but the potential danger of treatment chemicals is a source of ongoing concern.
Some of the chemicals most likely to be found in residential applications of treated wood are alkaline copper quaternary, copper azole and a copper-based substance known as Cu-HDO. Treated wood is usually sold with tags that identify the preservative. These chemicals are essentially pesticides, and to some degree they are toxic to human beings as well as decaying organisms. Low-level exposure can cause eye, skin and respiratory irritation; and high levels of exposure are associated with a variety of maladies such as gastrointestinal distress and organ damage. Copper, a common ingredient in preservative chemicals, is also toxic on its own: excessive exposure can cause digestive problems and organ damage.
Types of Exposure
Numerous situations cause human exposure to the chemicals in treated wood. The most common concerns in the garden are direct handling of treated stakes, chemicals from stakes leaching into soil and chemicals being absorbed by plant roots. When you handle a treated wood stake, the preservative can enter your body directly through the skin, known as "dermal" exposure; or through the mouth if you touch your face, known as "incidental oral" exposure. If the preservative leaches from the stake, it could harm the beneficial organisms in the soil. The greater concern, though, is whether the chemical is absorbed into the vegetables that you will eventually consume.
Studies have shown that the preservative chemicals in treated wood can leach into soil and be incorporated into plant tissue. For example, researchers at the University of Minnesota found that common vegetable crops accumulated arsenic from lumber treated with chromated copper arsenate. The chemical was discontinued in 2003, but newer chemicals also pose leaching hazards. Research at the University of Miami demonstrated that significant amounts of copper leach from alkaline copper quaternary-treated wood.
With proper precautions, you can minimize the dangers of using treated wood stakes in your garden. Wear a dust mask when you are sawing or sanding treated wood, and wear gloves when handling it. Wash your hands if you handle it without gloves. If you must use treated wood stakes near vegetables or other food crops, wrap the wood in plastic sheeting to reduce the amount of preservative that will leach into the soil.