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Pentachlorophenol is a manufactured chemical with the molecular formula C6HCl5O.  It does not occur naturally. Pure pentachlorophenol exists as colourless crystals. Impure pentachlorophenol (the form usually found at hazardous waste sites) is dark grey to brown and exists as dust, beads, or flakes. Humans are usually exposed to impure pentachlorophenol (also called technical grade pentachlorophenol).