Definition of Treated Wood as referenced in this article: Treated wood refers to wood industrially pressure-treated with arsenic and chromium (chromated copper arsenate or CCA). Copper acts as a fungicide, arsenate (a form of arsenic) acts as a pesticide, and cromium binds these chemicals to the wood. The pressure-treating process involves applying these chemicals to the wood and then subjecting the wood to high pressure. On December 31, 2003 the US wood treatment industry stopped treating residential lumber with CCA. This was a voluntary agreement the industry made with the EPA. CCA was replaced with copper-based pesticides except for some industrial uses.
Note: Loving Garland Green will not be using treated wood at the Garland Community Garden to make the frames for the raised beds as that is part of our agreement.
Those Most At Risk
From all the research that I've read, the greatest hazard from this treated wood is endured by workers who treat the wood day after day in factories where it is processed, and those who handle it at lumber yards day after day. Again, this is my opinion. Each individual will have to make their own determination. In case you haven't noticed, we live in a "buyer beware" environment that behooves all of us to pay close attention to the products we purchase and use.
Arsenic, one of the components of CCA, is a known human carcinogen. Thus it could be argued that reduction of any exposure to arsenic is advisable. But when estimating the risks that a chemical can pose, one should consider two main factors: toxicity and exposure. (Of course, the health and age of the person exposed are also key factors with children, the elderly and the infirm being more susceptible to harm from exposure to toxic chemicals.) Toxicity are the harmful effects the chemical may cause. Some chemicals are more toxic than others. Exposure is the dose received. Of course, the smaller the dose the less the harmful impact on the organism.
Types of Arsenic: Organic and Inorganic
If carbon is part of the combination, then the arsenic is considered organic. The arsenate used in wood treatment is inorganic and is more likely to accumulate in living tissues and impair metabolism. Organic arsenic (carbon based) doesn't appear to do this and we excrete most of it before it does any harm.
We typically eat from 25 to 50 micrograms of organic arsenic a day. (A microgram is a millionth of a gram.) Low levels of arsenic are in everything we eat. Shellfish is one common source of organic arsenic.
How Much Inorganic Arsenic Can we Ingest Safely?
According to the Agency for Toxic Substances and Diseases Registry (ATSDR) in Atlanta, we can ingest up to 0.3 micrograms of inorganic arsenic per kilogram of body weight per day and not be harmed. The average American woman, who weighs 132 lb. or 60 kg., would have to eat more than 18 micrograms daily all her life to see any ill effects. Before you get alarmed, remember this is inorganic arsenic we are talking about, not the organic types predominant in our diet. And, an ATSDR spokesperson pointed out, 0.3 microgram is a low estimate for the maximum tolerable dose.
Precautions to Take When Handling CCA Treated Wood
Refer to this EPA information sheet.
If you do choose to use pressure treated wood in your garden there are some precautions you can take:
- Once a year apply a penetrating type coating to the wood. Oil or water-based stains that can penetate the wood are recommended.
- Apply a ground cover such as sand or tire chips around a child's playground set built with pressure treated wood to minimize exposure to arsenic that leaches. Arsenic readily washes through ground covers to the underlying soil. (Of course, at some point in time the soil may need to be removed if the playset is removed and the soil is exposed again.)
Inorganic arsenic can also be in soil as a leftover from the days when arsenic was an approved pesticide. If you have any doubts regarding the quality of the soil in which you are growing food, get the soil tested. Here is the link to the Texas A&M Agrilife Extension Service--a great source for those of us who live in North Texas.