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Previous owners of our property used pressure-treated wood for their raised-bed vegetable patch. I have seen warnings about chemicals from PTW going into foods. How real is the danger?

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The danger is real enough for so-called "ground contact" applications, such as in your garden case. Over time, the constant exposure to moisture leaches out the chemicals from the lumber and into the soil or ground water (especially in soils high in clay).

Much before 2000, many pressure treating processes within the US involved some form of arsenic. Arsenic is still permitted in commercial and industrial applications.

Health Risks:

Arsenic

As a heavy metal, arsenic is quite toxic. Arsenic contamination of ground water (or well water) is not easily mitigated and will likely require long-term or permanent reverse-osmosis to be potable. {Reference CDC article}

However, your greatest risks from Arsenic exposure is from handling or working with treated lumber or from burning pressure treated wood (PTW) and inhaling the smoke. The CDC has pages dedicated to toxic substances like Arsenic: exposure, symptoms, and recommendations.

Chromated copper arsenate (CCA)

In CCA treatment, copper is the primary fungicide, arsenic is a secondary fungicide and an insecticide, and chromium is a fixative which also provides ultraviolet (UV) light resistance. Recognized for the greenish tint it imparts to timber, CCA is a preservative that was extremely common for many decades....On 1 January 2004, the Environmental Protection Agency (EPA) in a voluntary agreement with industry began restricting the use of CCA in treated timber in residential and commercial construction, with the exception of shakes and shingles, permanent wood foundations, and certain commercial applications. {Wiki - Wood preservation}, accessed 2-Jun-14.

CCA's health risks include Arsenic (see above) as well as Chromium. Chromium is a known carcinogen, and can also cause Immune, Renal, and Respiratory damage.

Copper

Other wood preservation techniques involved copper. Copper released into the environment usually attaches to particles made of organic matter, clay, soil, or sand. Low-levels of copper and actually required for good health. But high concentrations of copper can cause serious health problems.

  • Excellent answer! Thanks for taking the time to respond. – MercuryPlus Jun 3 '14 at 22:30
  • @MercuryPlus you're welcome! Please mark my answer as accepted if it meets your expectations. – Clayton Jun 3 '14 at 23:06

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