While Organic Gardening's risk management response to a perceived hazard has some merit, I believe their lack of risk assessment has led to the untenable conclusion that existing treated wood borders need to be removed.
Treated wood has been saturated under pressure with chromated copper arsenate (CCA), a fungicide that prevents rot. The elemental constituents of CCA are chromium, copper, and arsenic. Every movie fan who has watched Cary Grant in "Arsenic and Old Lace" can tell you that arsenic is a poison with murderous consequences. Fortunately, the chemistry is much more complicated than that, and all toxicity is a matter of dose and exposure.
Chromium, copper, and arsenic are present in CCA in forms known as metal oxides. Arsenic pentoxide and cupric oxide are considered to have very low toxicity to mammals, but chromium trioxide is in its most toxic form. The hazard of chromium trioxide is tempered, however, by consideration of its behavior in the environment. The main concern is how much will leach from treated wood.
Some leaching of CCA indisputably occurs from treated wood, but the next question is its fate once in the soil (or compost). The metal oxides tend to bind to the soil particles, except in highly acid soils (pH of 4.0 or lower) and, therefore, are not easily absorbed by plants. Even if maximum amounts of CCA leach, little chemical actually enters the root and edible parts of plants. Furthermore, the plant itself is likely to show symptoms of toxicity at levels far below those considered toxic to humans.
Soils naturally contain copper, chromium, and arsenic because these elements and their oxides are natural constituents of rock from which soil originates. These elements are actually essential micronutrients, meaning plant and animal cells require very small amounts for normal functioning. Even food naturally contains measurable levels of chromium, copper, and arsenic. Because toxicity is related to dose, there is little argument that high enough doses can harm plants and people, but "small amounts" of these essential elements are normal and present everywhere.
The U.S. EPA and the wood products industry recognize that treated wood has some hazard. Thus, CCA-treated wood is sold strictly for outdoor use and will be phased out for residential and school uses by the end of 2003. A dust mask is recommended during cutting; scrap wood should not be burned; and sawdust should not be used in compost piles or as a mulch.
Gardeners can avoid treated wood by using rot resistant alternatives such as plastic lumber. If they decide to use treated wood for raised beds, they can further reduce any hazard by lining the inside of the bed with plastic before filling with soil.
(Update from Dr. Linda Chalker-Scott, WSU Extension:
Rather than arsenic, new pressure-treated lumber has copper as its active ingredient.
Though it also will leach out of the wood, there is not a human health hazard
associated with its uptake by plants or animals. You probably get more copper
leaching into the water carried through your plumbing (assuming you have copper,
and not lead, pipes).
What about plastic timbers? Though I've not seen any literature about leachates from plastic lumber, I've seen some older plastic timbers that haven't aged well - they can warp and twist. I would avoid those made of rubber, because decomposing rubber produces leachates that are quite hazardous ")
For furthern information contact your local WSU Extension Office.
From The Gardener Vo. 5, No. 4, Winter 1994-1995. Slightly revised 02/27/11, Dave Pehling, WSU Extension Snohomish County