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Citizen’s Guide

July 2007


In 2007, the Paonia Mosquito Control District began fogging the town of Paonia with Biomist 3+15, of which the primary active ingredient is permethrin. In an effort to gather information on the potential impact of permethrin on human health, we conducted an extensive search of the scientific literature published in reviewed academic journals. The following is an overview and summary of our findings.

Permethrin belongs to a class of chemicals called pyrethroids, described by the Agency for Toxic Substances and Disease Registry (U.S. Department of Health and Human Services) as “manufactured chemicals that are very similar in structure to the [naturally occurring] pyrethrins, but are often more toxic to insects, as well as to mammals, and last longer in the environment” (ATSDR Public Health Statement, pg. 1). Permethrin is not a natural product. It is one of many synthetic pyrethroids developed for use as an insecticide based on the chemistry of the pyrethrum flowers. It is used in household insect foggers and sprays, flea dips and sprays for cats and dogs, ornamental garden and turf products, repellent/insecticide for clothing, mosquito abatement products, termite treatments, agricultural products, lice shampoos, and body lotions for scabies control. Pyrethroid insecticides are some of the most widely used pesticides in the world because they are believed to be less harmful to humans than other pesticides.

Permethrin can be breathed into the lungs, absorbed through the skin or ingested. The most dangerous route is inhalation through the lungs, as this allows the chemical to move directly to target sites (e.g., the brain) without being metabolized by the liver and other organs. The following recommendation is issued by the U.S. Department of Health and Human Services in their Toxicological Profile for Pyrethrins and Pyrethroids (September 2003). “Remaining indoors and closing your windows while your neighborhood is being sprayed will lessen your exposure.” (Toxicological Report, pg. 8). We also recommend turning off air conditioners and swamp coolers and keeping family pets indoors during this time. Because permethrin is odorless and weather conditions vary, individuals must use their judgment in deciding how long after spraying to keep these precautions in place.

Among the general population, ingestion (usually as residues on food) is the most common way that people are exposed to permethrin. The health department advises “Make sure you wash fruits and vegetables thoroughly before eating them” (Toxicological Report, pg. 8). Absorption through the skin is slow and it appears that only a fraction of the applied chemical penetrates the skin, thus reducing its toxicity via this route. Direct skin exposure may cause itching, tingling or burning sensations that peak within a few hours and resolve within 24 hours. “Pyrethroids might be able to penetrate the skin of infants and young children more easily than the skin of adults. ...pyrethroids that penetrate the skin may become more concentrated in internal tissues of the young.” (Toxicological Report, pg. 7). Thus, it is important to reduce children’s exposure to permethrin as much as possible.

Permethrin exerts its toxic effects directly on the nervous system. Studies in mice and rats show that sub-lethal intoxication leads to aggression, hypersensitivity to external stimulation, whole-body tremor, convulsions and paralysis. “If very large amounts of these chemicals were to enter your body, you might experience dizziness, headache, and nausea that might last for several hours. Larger amounts might cause muscle twitching, reduced energy, and changes in awareness. Even larger amounts could cause convulsions and loss of consciousness that could last for several days.” (Toxicological Report, pg. 5).

Although it does not accumulate in the body, permethrin (and/or its metabolites) has been detected in human urine, feces, and breast milk. It takes approximately one week to excrete 90% of it, primarily in urine.

Permethrin adheres strongly to soil (including household dust) and does not dissolve readily in water. The half-life in soil (time it takes for half of what is applied to break down) is approximately 30 days (shorter if exposed to sunlight) and on foliage it is 10 days. The department of health advises, “You should discourage your children from eating dirt. Make sure they wash their hands frequently and before eating. Discourage your children from putting their hands in their mouths or any other hand-to-mouth activity.” (Toxicological Report, pg. 8).

Permethrin is highly toxic to bees, fish, and other aquatic organisms. As stated on the Biomist label, it should not be applied over bodies of water, and water used to rinse spray equipment should be disposed of at an approved waste disposal facility.


In addition to permethrin, Biomist contains piperonyl butoxide, which inhibits the enzymes necessary to break down permethrin and eliminate it from the body, thus greatly increasing its toxicity. Formulated pesticide products typically contain solvents, emulsifying agents, petroleum distillates, and other ‘inert’ ingredients, many of which are known or suspected to have neurotoxic properties. Eighty-two percent of Biomist is inert ingredients, including petroleum distillates.


In our literature search we found 108 scientific journal articles on the potential health effects of permethrin. The graph below shows the number of findings on the most widely researched health-related endpoints. Shaded bars indicate whether or not there were significant effects of permethrin, effects only in combination with other chemicals or no effects.

The bulk of studies on the nervous system demonstrated that permethrin affects the brain in the way that it was designed to behave. That is, it causes repetitive firing of the electronic signals in particular regions of the brain. These effects were found in laboratory animals used for predicting what might happen in humans.

Behavioral studies of rats and mice showed reductions in learned behavior, as well as impairments in balance, strength, and speed. One study found changes in motor activity and social behavior in the offspring of mice who were given permethrin prior to mating.

Every one of the 15 studies that we found on the immune system showed significant effects. Results indicated that permethrin led to the death and/or reduced production of blood cells necessary to fight bacteria and viruses and remove waste products from the blood. Varied physiological changes, including DNA damage, were reported in other systems.

One of our primary concerns is recent research demonstrating that permethrin exhibits the characteristics of an endocrine disruptor. Endocrine disruptors are chemicals that interfere with the communication system of glands, hormones, and cellular receptors that control the body’s internal functions. A relatively unique feature of endocrine disruptors is that they exert their effects at extremely low doses, even when higher doses exhibit no adverse effects. Disorders that have increased in prevalence in recent years such as unusual male gonadal development, infertility, ADHD, autism, intellectual impairment, diabetes, thyroid disorders, and childhood and/or adult cancers are now being linked to prenatal exposure to endocrine disruptors.

In the permethrin studies of health effects on the female reproductive system, excessive cell growth was a common finding. In contrast, cell death and reduced tissue weight of male reproductive organs were among the findings on the male reproductive system. One study suggested that the products permethrin breaks down into may be 100 times more potent with regards to endocrine disruption than permethrin itself. Due to the potential endocrine disrupting effects of permethrin, pregnant women should take extra steps to avoid exposure.

Several studies have been conducted on the effects of combinations of chemicals used among U.S. military personnel during the Persian Gulf War, including permethrin (impregnated in battle dress uniforms) and DEET (used as an insect repellent). The toxicity of permethrin was shown to be greatly enhanced when used in combination with DEET. As shown in the graph, the adverse effects of chemical combinations were documented in several different systems. Given this, we recommend that serious consideration be given to the risks and benefits of using DEET as an insect repellent in areas where permethrin is used.

Very few studies have been performed in recent years on the cancer-causing potential of permethrin. A 1994 review by the U.S. Army concluded that permethrin is a “possible human carcinogen” based on early studies in rodents. In 2006 the EPA also classified permethrin as “likely to be carcinogenic in humans,” based on mouse studies showing evidence of lung and liver tumors related to permethrin.

Given the scientific evidence reported here, we urge people to weigh the risks and benefits of permethrin exposure carefully and to employ the precautionary measures recommended by the Department of Health and Human Services, as the real effects of permethrin on human health may not be recognized for many years.

If you have questions after reading this report, you may contact TEDX at 970-527-4082. Information on endocrine disruptors may be found on our web site. The National Pesticide Information Center at Oregon State University has a phone number for pesticide related questions from the general public: 1-800-858-7378. Toxicological profiles are also available on-line.