Permethrin

Authors

Overview


Permethrin is a synthetic pyrethroid pesticide used as an acaricide and insect repellent. It is employed not only in agriculture, but also in forestry, household settings, and public health programs. As a neurotoxin, permethrin affects neuron membranes by prolonging the activation of sodium, and is more effective against insects and aquatic life than mammals and birds. Due to this toxicological preference, permethrin especially poses risks to fish. For its effects on humans, permethrin has been classified as a type II or III toxin by the EPA, and studies of mice cells have demonstrated the chemical's potential to be carcinogenic.  

Just the facts


Physical Information

Common Name: Permethrin

Chemical Name: 3-Phenoxybenzyl (1RS)cis,trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate   

Molecular Formula: C21H20Cl2O3

Molecular Weight: 391.28

CAS Registry Numbers: 52645-53-1

Regulatory facts: Restricted Use Pesticide (RUP)

Chemical Structure












Structure received from Wikimedia Commons
 





Chemical Description


Permethrin presents in pure chemical form as either an odorless and colorless crystal solid or as a pale brown liquid. As with many pyrethroids, the compound is insoluble in water. However, unlike other chemicals in this group, permethrin is unique in having four stereoisomers, molecules that have identical formulas, atomic sequences, and arrangements, but differ in the way these molecules align in space.


Uses


Several common trade names of permethrin include Ambush, Dragnet, Eksmin, Qamlin, and Torpedo (EXTOXNET). The chemical has a long-standing and varied range of uses on landscapes. Permethrin was first marketed in 1973 and registered in the United States four years later, in particular for its use in the cotton industry (EPA). Since then, the dominant global use of this compound remains in this sphere, equating to roughly 60 percent of total permethrin usage (CAP).

However, permethrin is not confined to cotton alone. The chemical works against a multitude of pests on a myriad of crops, including, but not limited to, nuts, fruits, vegetables, ornamental plants, and mushrooms. As applies to commercial agriculture, permethrin is most commonly used on wheat, corn, and alfalfa. It is applied to a range of settings, on anything from home garden to agricultural fields to household pest control. These applications most often take the form of dusts, wettable powders, spray solutions, and emulsifiable concentrates (EXTOXNET). Permethrin may also be used on clothing and in everyday home uses, and is often prescribed as a skin treatment for diseases such as scabies and head lice (EPA). It may also be utilized, as it has been historically, in public health programs for insect vectors.

In the circle of tropical medicine, permethrin is best known for its success in controlling and limiting populations of mosquitoes, the insect vectors that transmit malaria and dengue fever. Mosquito nets, which are used to cover beds and hammocks to prevent insect-human contact, are often sprayed with a solution of permethrin in order to increase the effectiveness of the covering (EPA). In recent years, permethrin has also become a means to control the populations of ticks, which can transmit infections such as Lyme disease and Babesiosis. Used primarily in suburban settings such as backyards, forested areas, and public parks, this chemical kills the arthropods upon contact, and is especially effective when applied to clothing. However, the compound can additionally aid in reducing the transmission and spread of these diseases through the assistance of intermediate hosts, such as mice. Biodegradable cardboard tubes filled with permethrin-treated cotton are distributed in these areas noted above, and the mice collect this material to line their nests. As with permethrin treated-clothing, the chemical kills any of the ticks feeding on the mice at that time (CDH).


Pharmacology and Metabolism


As with all other synthetic pyethroids, permethrin affects an insect through the organism's nervous system. As a neurotoxin, this chemical makes the host nervous system hypersensitive to stimuli from the sensory organs (CAP), and affects this channel through binding to sodium ions, blocking their movement. This can lead to repetitive nerve impulses, the inhibition of certain enzymes, and increases in body temperature.

Studies have demonstrated that this process, of slowing down the nervous system, is negatively linked with temperature, and therefore permethrin more strongly affects cold-blooded organisms such as insects and frogs, than it does warm-blooded animals. The fact that permethrin is rapidly metabolized by mammalian livers is proof of this chemical's preference (EXTOXNET). In mammals, the breakdown products of permethrin, the metabolites, do not persist in tissues. Instead, the animal quickly excretes these by-products. In studies of oral administration to rats, within several days the mammals almost completely eliminated the compound from their bodies, with only three to sex percent of the original doses remaining. The only exception is that permethrin persists slightly longer in fatty tissues, and has a half-life of four to five days in the brain and in body fat (EXTOXNET).


Health Effects


As mammals then, humans are not significantly affected by permethrin use. Depending on the stereoisomer formulation of the chemical, permethrin can be classified within class II or class II toxicity (EPA). Regardless which of these two toxicity classes a permethrin product falls within, all must contain a label of "Warning" or "Caution" due to the chemical's potential to cause eye and skin irritation (EXTOXNET). Out of all permethrin's isomer formulations, the cis-arrangement has been found the most toxic.

Acute effects of permethrin vary in accordance with the route of exposure. Through the oral route, permethrin is mostly harmless; studies in rats demonstrate a LD50 of 430 to 4000 mg/kg. The same can be said of dermal exposure, although the chemical has been found to cause mild skin irritation in rabbits (EXTOXNET). In some cases, however, dermal exposure to permethrin has lead to eye and skin irritation in humans (PAN). Chronic effects of permethrin have not yet been proven.

The potential carcinogenicity of permethrin is so far inconclusive. The EPA has classified the chemical as potentially carcinogenic through oral exposure, mainly due to several studies where mice developed two types in benign tumors (EPA). In laboratory tests, the ingestion of permethrin had the ability to reduce the ability of T-lymphocytes, vital components of our immune system defense, to recognize foreign protein bodies. Using mouse cell cultures, researchers found that permethrin also inhibited the abilities of another two kinds of lymphocytes (CAP). Therefore, cancerous risks can be associated with this chemical.


Environmental Effects


Permethrin does not persist long in the physical environment. Its half-life in soil has been reported at 30 to 38 days, as the chemical is rapidly processed and degraded by various microorganisms (EXTOXNET). Additionally, because permethrin has a strong chemical bond to soil particles, there is little risk of it leaching into and thus contaminating groundwater. Its half-life in water is even shorter than that in soil.

However, although permethrin does not persist long in aquatic environments, it does pose a threat to organisms in this setting. As noted above, permethrin has a more powerful impact on cold-blooded organisms than on mammals. Fish, especially commercial species such as rainbow trout and salmon, are particularly vulnerable to the chemical. There is also evidence that the compound has between a low and moderate potential to bioaccumulate in these organisms (EXTOXNET). Because of these effects, any permethrin product used in agriculture has become marked as a "Restricted Use Pesticide."

An additional environmental risk presents in the targets of permethrin, insects. A variety of insects have developed resistance to permethrin, namely, cockroaches, head lice, and certain agricultural parasites. On the same topic, by eliminating larger insects from agricultural fields and other settings, permethrin may also indirectly cause a rise in mite populations due to this loss of predatory species (EXTOXNET).

Permethrin poses similar risks to the production of honey and other agricultural activities through its negative influence on bee populations.


References




Coalition for Alternatives to Pesticides. 1998. Permethrin. http://www.safe2use.com/poisons-pesticides/pesticides/permethrin/cox-report/cox.htm. [accessed July 9, 2010]


Connecticut Department of Health. 2005. Tick Bite Prevention. http://www.ct.gov/dph/cwp/view.asp?a=3136&q=395590 [accessed July 9, 2010]


Extension Toxicology Network - Pesticide Information Profiles. 1996. Permethrin. http://extoxnet.orst.edu/pips/permethr.htm [accessed July 9, 2010]


PAN Pesticides Database. 2010. Permethrin. http://www.pesticideinfo.org/Detail_Chemical.jsp?Rec_Id=PC35397 [accessed July 9, 2010]


United States Environmental Protection Agency. 2006. Permethrin Facts. http://www.epa.gov/oppsrrd1/REDs/factsheets/permethrin_fs.htm [accessed July 9, 2010]

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