Penta has been used as a fungicide, molluscicide, insecticide, and as a wood preservative, but registration for its use in lumber in the USA was canceled in 1986. Gradually, other registrations for this agent have been canceled and it is now only registered for industrial purposes; agricultural and domestic uses are prohibited. It is rated by the World Health Organization as highly hazardous.

It can be absorbed through intact skin and lungs and is an intense irritant to the skin and mucous membranes. When absorbed, it increases metabolism by uncoupling cellular phosphorylation. Animals fed in troughs made of lumber treated with penta may salivate and have irritated oral mucosa. Vaporization or leaching of penta in pens, enclosures, homes, and barns has caused illness and death. Signs of poisoning include nervousness, rapid pulse and respiratory rate, weakness, muscle tremors, fever, and convulsions, followed by death. Chronic poisoning results in fatty liver, nephrosis, and weight loss. Additional problems reported when penta-contaminated shavings are used as bedding include “off flavors” in broilers, impaired immune response in chickens, and possibly decreased fertility in boars.

Commercial lots of technical-grade penta contain small but biologically significant amounts of highly toxic impurities (dioxins and furans), and material available today is manufactured so that these toxic ingredients are kept at as low a concentration as possible. Penta can cause residues in animal tissues. Also, a significant amount of hexachlorobenzene is metabolized in animal tissues to penta. Pentachlorophenol is considered to be a carcinogen and a tumor promoter, although studies have shown that the pure material does not increase the incidence of tumors in rats and mice. The technical-grade material has also been shown to be immunotoxic in laboratory studies. Penta must be handled very carefully and kept away from animal contact.

Whole blood analysis for penta may aid in the diagnosis of poisoning; diagnosis is usually made on the basis of the signs and the proximity of treated lumber in the animal's environment. Acute toxic doses of penta range from 27–350 mg/kg and the fetotoxic NOEL is 10 mg/kg in rats.

There is no known antidote. Termination of exposure, bathing dermally exposed animals, oral administration of activated charcoal, and supportive therapy may be indicated. Bathing should be done gently with cold water and detergent so as not to cause vasodilation and increased absorption. Cattle, pigs, and chickens exposed to wood treated with commercial grade penta that contained these contaminants had increased mortality, decreased productivity, and other less specific herd health problems. (see Persistent Halogenated Aromatic Poisoning.) Antipyretics, eg, aspirin and acetaminophen, should not be used. Treatment involves cooling the animal and administering fluids, electrolytes, and anticonvulsants, in addition to removing it from the source of poison.

Last full review/revision March 2012 by William D. Black, MSc, DVM, PhD