Utility poles treated with pentachlorophenol (penta) are the most common utility poles, accounting for almost 50% of the total market. Like other wood preservatives, penta is a pesticide, which acts against wood-destroying fungi and insects.
Produced commercially since 1936, penta's popularity for treatment of utility poles stems from the fact that it is a highly effective, economical preservative that controls virtually every type of wood-destroying fungus. Penta also provides protection against a variety of insects such as termites and powder post beetles. In stake tests and actual use, penta has been shown to preserve wood eight to twenty times beyond its normal service life expectancy.
Penta is carried into the pole in a petroleum oil solution. When properly applied, penta remains in treated wood products for many years. However, when present in low concentrations, penta is degradable by sunlight and certain bacteria. Thus, over time, treated wood surfaces become relatively free of penta while the preservative continues to work below the surface.