I found out today that pressure-treated posts (the stuff that is stained green with little slots all over it) are forbidden in Certified Organic farms. These posts are used in modern apple orchards to support the trellis wire of Tall Spindle systems, a method of planting small trees close together to quadruple yields.
The formula for the green stuff used to be CCA (Chromated Copper Arsenate). It is a poison containing copper and arsenic, which are deadly to fungus and termites respectively, two things that damage and decompose wood. It worked good and was in wide use.
But because of its wide use, studies were done on if the arsenic leached out into the soil. The research was inconclusive at best, but typical hysteria ensued as attorneys and plantiffs lined up to claim damages. The industry voluntarily eliminated its use after 2003.
The replacement formula is usually either Amine Copper Quat (ACQ) or Copper Azone (CA). It has twice the copper content of the CCA and thus is more expensive. Another drawback is the electrolysis between the copper in the preservative and steel fasteners and joist hangers causing corrosion. The industry has recommended that all fasteners and framing hardware be either double-dipped galvanized or stainless steel (more expensive again).
We won’t know for a few years if the new formula is any better or less of a hazard than the old. There was never any demonstrated danger from the old leaching into the soil, and no one has recommended tearing out your old pressure-treated deck; the trauma involved in the demolition would release far more arsenic than would ever leach out naturally.
Natural alternatives like Chestnut and Redwood are no longer viable options, and so treated wood is here with us to stay. I fear that hype and hysteria has replaced a decent product with a more expensive one that has not demonstrated a tangible benifit.