Pentachlorophenol pesticide, molecular model. Pentachlorophenol is a synthetic chemical that was broadly used as a preservative for wood and as a pesticide. It was formerly widely available, but its sale to the public was restricted in the early 1980s. It is still used industrially to protect exposed wooden items, such as telegraph poles. Excessive exposure causes fever, organ damage and coma, and it is thought to be carcinogenic. In this molecule, atoms are colour-coded: carbon (grey), oxygen (red), chlorine (green) and hydrogen (white).