What’s bad about pressure-treated wood?
Random core samples are extracted to monitor quality.
In the pressure-treating process, lumber is sealed in a tank, and air is extracted, creating a vacuum. Then a solution containing chromium, copper, and arsenic is added. Because of the vacuum, the chemicals are carried deep into the wood. Chromium is a bactericide, copper a fungicide, and arsenic an insecticide, and all arrest decay of some kind. All three are toxic, but chromium and copper don’t raise many concerns. If we don’t inhale it, chromium is not particularly harmful to us, and copper isn’t very toxic to mammals, although it is to aquatic life and fungi. It’s arsenic that is worrisome.
Arsenic is everywhere. If this gray, metal-like element is combined with oxygen, chlorine, and sulfur, it’s considered inorganic arsenic. If carbon is part of the combination, then the arsenic is organic. It’s the inorganic forms that worry people. The arsenate used in wood treatment is inorganic. Compared to organic arsenic, inorganic arsenic is much more likely to accumulate in living tissues, where it interacts with cell enzymes and impairs metabolism. Organic forms of arsenic don’t appear to do this, and are largely excreted before they can do us harm.
We’re exposed to arsenic—mostly organic forms—every day because very small amounts are present in all soil, water, and food. We typically eat 25 to 50 micrograms (a microgram is a millionth of a gram) of mostly organic arsenic a day. Low levels of arsenic are in everything we eat. The biggest source is shellfish.
Inorganic arsenic also may be present in foods due to residues in the soil from the days when arsenic was an approved pesticide. One reason root crops tend to accumulate arsenic is that minute particles of soil stick to the root’s skin, even after a brisk scrubbing. Peeling root vegetables before eating them gets rid of that arsenic. Still, there’s no need to worry about normal levels of arsenic in foods. The amount is so small it’s not harmful.
Soils contain both organic and inorganic arsenic. Background levels of arsenic in soil (amounts due to geological weathering, not to human contamination) typically range from 0.1 to upwards of 10 parts per million (ppm), and up to 40 ppm is considered tolerable, according to the U.S. Department of Agriculture. Above that level, detectable amounts start showing up in children’s urine, because kids ingest dirt.
Water contains background arsenic too, but there might also be arsenic from contamination. The current EPA limit for arsenic in drinking water is 50 parts per billion.
In large doses, inorganic arsenic is strong poison. Ingesting 1 to 3 milligrams per kilogram of body weight can be fatal. Lesser amounts can cause nausea and diarrhea, lower production of both red and white blood cells, and give you a pins-and-needles sensation in your arms and legs. Inorganic arsenic is also carcinogenic, increasing the risk of lung, skin, and other cancers.
But as USDA heavy metals expert Rufus Chaney points out, what constitutes an acute toxic dose isn’t really relevant to gardeners. What we want to avoid are chronic toxic doses, which can lead to disease. Chronic exposure means every day for a lifetime. 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’re talking about, not the organic types predominant in our diet. And, an ATSDR spokesperson points out, 0.3 microgram is a low estimate for the maximum tolerable dose.