Does more preservative mean a better product?

Does more preservative provide better protection of wood products or does it disproportionately increase the potential for introducing pesticide into the environment? In an era of sensitivity about misuse of pesticides in the environment, it seems prudent to examine the dose-response benefits from wood preservatives for treated-wood products exposed in natural settings. Results from “graveyard tests” of preservative-treated wood stakes exposed in the ground are regarded as reliable models of durability in treated-wood products. In this paper, failure distribution patterns of preservative-treated stakes in long-term field plots are discussed with reference to early failures, to first quartile and median failure times, and to distribution about medians. Evidence supports a hypothesis that for inorganic wood preservatives, retention levels above a certain upper bound do not yield proportionate durability gains. Environmental factors do not appear to extensively change the upper bounds of the dose-response relationship but can significantly affect the lifespan at any retention within that dose-response range. A similar relationship is suspected for metal-organic preservatives. With organic systems such as creosote, there is a direct relationship between dose and increased durability. If this hypothesis about a truncated dose-response relationship proves to be a fundamental performance characteristic of inorganic and organo-metallic wood preservatives, then the upper bounds of the dose-response relationship should be an important benchmark in standards that seek to define that optimum point at which product reliability is maximized with minimal potential environmental impact.

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