Straight Dope August 22 2009

''I've noticed a lot of my neighbors have built elevated gardening beds in their yards using wood that's marked as treated with arsenic. Will the arsenic get into the vegetables and fruits these people are growing? If so, is that a health concern?
— Ray Charlton, Corvallis, Oregon

Hard to say. Were these vegetables and fruits they were actually planning to eat?

Manufacturers treat wood with arsenic for the same reason you don't want it in food — it kills things, in this case the bacteria, fungi, and insects that would otherwise nibble on the wood. Although several wood treatments contain arsenic, the compound of greatest concern is chromated copper arsenate, or CCA, a trifecta of dangerous chemicals that at one time (like 1990) was used on almost all the pressure-treated lumber in the United States. Although CCA is supposed to stay put, small amounts can leach out when the wood is exposed to the elements. CCA-treated garden borders aren't the only thing leaking arsenic into the environment; the same can happen with treated-wood mulch or chips, decking, and traffic sound barriers.

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(illustration by Slug Signorino)''