The California Office of Environmental Health Hazard Assessment (OEHHA) has listed the complex mixture of pentachlorophenol and by-products of its synthesis as carcinogenic under California Measure 65. The listing follows a notice of intent on October 30, 2015, which was followed by a comment period that closed on November 30, 2015. Pentachlorophenol as a pure substance was already listed under Measure 65 but the new listing includes multiple substances in the typical mixture that were not previously listed. The listing mechanism was given as AB(NTP), that is, the National Toxicology Program (NTP) of the US Department of Health and Human Services identified the mixture as carcinogenic.  Since the NTP is an authoritative body under Measure 65 that triggered the listing. Pentachlorophenol (with its byproducts) is used principally in treatment of wooden utility poles.  Its use is of particular concern near drinking water sources.
To know more click here.