Beyond Pesticides Chemical Fact Sheet: Permethrin
Permethrin is in a class of compounds known as synthetic pyrethroids. Synthetic pyrethroids are synthesized derivatives of naturally occurring pyrethrins, which are taken from extracts of dried chrysanthemum flowers. Synthetic pyrethroids are more stable than natural pyrethroids, and therefore longer lasting in the field. Although synthetic pyrethroids are often thought of as "safe as chrysanthemums," they are chemically engineered to be more toxic than natural pyrethroids. Permethrin is widely used as an insecticide in agriculture, homes and gardens, and for treatment of ectoparasites (fleas, lice, scabies) on humans and animals. Worldwide, the dominant use of permethrin is for cotton, which accounts for over 60% of the permethrin used. In the U.S., almost 70% of the permethrin used in agriculture is used on corn, wheat, and alfalfa. Annually, over 100 million applications of permethrin are made each year in and around U.S. homes. Some common products containing permethrin as the active ingredient include: Nix, Elimite, Prelude, Combat, Ambush, Dragnet, Outflank, Pounce, Perthrine, Picket, and Astro. Permethrin comes in many forms, including sprays, dusts, fogs, emulsifiable concentrates, and creams. Additionally, in 2003, the EPA approved permethrin-impregnated clothing for public use.
Like all synthetic pyrethroids, Permethrin kills insects by strongly exciting their nervous system, a similar mode of action to DDT. In terms of its chemical arrangement, permethrin has four isomers. Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. On permethrin, the cis isomers are the most insecticidally active and therefore the most toxic. The half-life of permethrin in soil is 30 days, although it is less in soil with more organic matter in it. The half-life on foliage is 10 days. The trans-isomer has been shown to degrade more rapidly than the more toxic cis-isomer. Permethrin for agricultural uses is classified as a "Restricted Use" pesticide. Restricted Use pesticides are limited to licensed pesticide applicators or their employees, and only for the uses covered by the applicators certification or on the pesticide label. However, permethrin products labeled for spot treatments or other over the counter products are not restricted use.
According to the U.S. EPA, permethrin is a moderately to practically non-toxic pesticide, and falls into either toxicity class II or III, depending on the formulation. Products containing permethrin must bear the signal word WARNING or CAUTION. Permethrin may be readily absorbed from the gastrointestinal tract, minimally through intact skin, and by inhalation of dust and spray mist. The most severe synthetic pyrethroid toxicity is to the central nervous system, and seizures have been reported in severe cases of pyrethroid intoxication. However, according to the EPA, there are no reports of seizures in humans from exposure to Permethrin. At relatively high doses, neurotoxic symptoms in mammals include tremors, loss of coordination, hyperactivity, paralysis, and an increase in body temperature.
The LD50 (the lethal dose that kills 50 percent of a population of test animals) for permethrin is variable, ranging from 430-mg/kg body weight to over 4,000 mg/kg for rats. Some of this variability is due to varying proportions of isomers; the trans-isomers are hydrolyzed more readily and have a significantly lower toxicity in rats than do the corresponding cis-isomers, which are around 10 times more toxic than the trans-isomers. For example, the female rate acute oral LD50 of permethrin increases from around 220 mg/kg to 6000 mg/kg as the proportion of the trans isomer increases from 20% to 80% of the solution.
Long-term Health Effects
Effects on Reproduction:
Permethrin is highly toxic to fish, due to the sensitivity of their nervous systems. It is also highly toxic to many aquatic invertebrate animals; its effects on insects and crustaceans are particularly severe. Permethrin is practically non-toxic to birds, although there may be some long-term effects. Some endangered toads and salamanders may also be at risk from permethrin. Permethrin negatively affects many species of beneficial arthropods (those arthropods that are useful in agriculture). For example, permethrin is extremely acutely toxic to honey bees, even at very low doses. Although it is commonly thought that the potential for leaching into water is low because permethrin adsorbs strongly to soil particles and has a short half-life in water, the U.S. Geological Survey has found permethrin in ground and surface water in numerous locations. Furthermore, a very recent study of pesticides in bodies of water in the agriculture-dominated Central Valley in California found high levels of synthetic pyrethroids in stream sediments-levels high enough that they were toxic to freshwater bottom dwellers in almost 50% of the sampled locations. Permethrin was the most commonly detected pesticide in the study.
Regulatory Status and History
Permethrin was first marketed in 1977 for use on cotton. In October 1982, EPA began to allow an expansion of permethrin registrations to include use on livestock, poultry, eggs, vegetables, and fruit-a 500% expansion of the market for permethrin. This decision was quite controversial, and was opposed by EPA staff pathologist, M. Adrian Gross, who argued that permethrin presents an intolerable statistical risk of causing cancer. This controversy largely focused on the results and validity of the original EPA tests on permethrin. EPA's official stance at the time was that permethrin was not a carcinogen. John A. Todhunter, the assistant administrator for pesticides and toxic substances at the time, wrote that, "The likelihood of oncogenic effects in humans from exposure to low levels of permethrin is nonexistent or extremely low." However, Dr. Adrian Gross dissented on the EPA's evaluation, bringing to their attention that the Allowable Daily Intake of 0.05 mg/kg body weight/day was associated with cancer rates as high as between 5-10% of the population! He wrote, "I should think that risks of cancer of this order for a relatively new insecticide are unacceptable to any rational person." However, permethrin is not scheduled for re-registration until June 2006.
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