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Piperonyl Butoxide (PBO)
Beyond Pesticides Rating: Toxic

Piperonyl butoxide, or PBO as it is most often called, is a pesticide synergist. A synergist is another chemical that is added to a pesticide product, in addition to the active and inert ingredients, to increase the potency of the active ingredient. While the increased potency make the pesticides more deadly to their targets, synergists may also compromise the detoxifying mechanisms of non-target species, including humans. A typical pesticide product contains 5-20 times more synergist than active ingredient. Many products from repellants and pediculicides (lice killers) to foggers and garden sprays contain synergists. Formulations of permethrin, resmethrin and sumithrin, including ScourgeTM and AnvilTM, used along the for mosquito control to combat the West Nile Virus, commonly contain the synergist PBO. Prethroids, pyrethrins, rotenone and carbamates are the pesticides most often formulated in combination with PBO (Gosselin et al., 1984).  

Health Effects

PBO affects humans by inhibiting important liver enzymes responsible for breakdown of some toxins, including the active ingredients of pesticides. Specifically, it has been shown to inhibit hepatic microsomal oxidase enzymes in laboratory rodents and interfere in humans. Because these enzymes act to detoxify many drugs and other chemicals, a heavy exposure to an insecticidal synergist may make a person temporarily vulnerable to a variety of toxic insults that would normally be easily tolerated (Gosselin et al., 1984). In addition to the symptoms induced by the active ingredients, signs of PBO poisoning include anorexia, vomiting, diarrhea, intestinal inflammation, pulmonary hemorrhage and perhaps mild central nervous system depression. Repeated contact may cause slight skin irritation. Chronic toxicity studies have shown increased liver weights, even at the lowest doses, 30 mg/kg/day. Animal studies have shown hepatocellular carcinomas, even treatments as low as 1.2% (Takahashi et al., 1994). EPA considers PBO to be a class C possible human carcinogen. 

PBO in the Environment

PBO is moderately toxic to most amphibians, crustaceans, fish and other aquatic organisms. However, study published by the U.S. Fish and Wildlife Service found piperonyl butoxide to be very highly toxic to bluegills and aquatic sowbugs (Johnson, 1980). Very little is known about the persistence of PBO in the environment. 

References:

Gosselin, R.E. 1984. Clinical Toxicology of Commercial Products. Williams and Wilkins. Baltimore, MD. 

Johnson,W.W. and M.T.Finley Handbook of Acute Toxicity of Chemicals to Fish and Aquatic Invertebrates, Resource Publication 137, Fish Wildlife Service, U.S.D.I., Washington, D.C. 1980 

Takahashi, O., et al., 1994. “Chronic toxicity studies of piperonyl butoxide in F344 rats: induction of hepatocellular carcinoma.” Fund. Appl. Toxicol. 22:293-303.