Permethrin

Permethrin is a common synthetic chemical, widely used as an insecticide, acaricide, and insect repellent. It belongs to the family of synthetic chemicals called pyrethroids and functions as a neurotoxin, affecting neuron membranes by prolonging sodium channel activation. It is not known to rapidly harm most mammals or birds, but is dangerously toxic to cats and fish. In general, it has a low mammalian toxicity and is poorly absorbed by skin.

In medicine, permethrin is a first-line treatment for scabies; a 5% (w/w) cream is marketed by Johnson & Johnson under the name Lyclear. In Nordic countries and North America, it is marketed under trade name Nix, often available over the counter.

Uses
Permethrin is used:
 * as an insecticide
 * in agriculture, to protect crops
 * in agriculture, to kill livestock parasites
 * for industrial/domestic insect control
 * as an insect repellent or insect screen
 * in timber treatment
 * as a personal protective measure (cloth impregnant, used primarily for US military uniforms and mosquito nets)
 * in pet flea preventative collars or treatment.

Pest control
In agriculture, permethrin is mainly used on cotton, wheat, maize, and alfalfa crops. Its use is controversial because, as a broad-spectrum chemical, it kills indiscriminately; as well as the intended pests, it can harm beneficial insects including honey bees, and aquatic life.

Permethrin kills ticks on contact with treated clothing. A method of reducing deer tick populations by treating rodent vectors involves stuffing biodegradable cardboard tubes with permethrin-treated cotton. Mice collect the cotton for lining their nests. Permethrin on the cotton instantly kills any immature ticks that are feeding on the mice. It is important to put the tubes where mice will find them, such as in dense, dark brush, or at the base of a log; mice are unlikely to gather cotton from an open lawn.

Permethrin is used in tropical areas to prevent mosquito-borne disease such as dengue fever and malaria. Mosquito nets used to cover beds may be treated with a solution of permethrin. This increases the effectiveness of the bed net by killing parasitic insects before they are able to find gaps or holes in the net. Military personnel training in malaria-endemic areas may be instructed to treat their uniforms with permethrin, as well. An application should last several washes.

Personal healthcare
Permethrin is used on humans to eradicate parasites such as head lice or mites responsible for scabies and as a pest-repellent clothing treatment. Permethrin is more effective in reducing itch persistence than crotamiton or lindane. The common prescription is a 5% concentration of permethrin for scabies and a 1% concentration for the over-the-counter (OTC) treatment for head lice or crabs. Pharmaceutical grade permethrin 99% is differentiated from pesticide grade 94% by a higher purity, well specified impurities, and lower content of the toxic CIS component at 25% as opposed to 40% in the pesticide grade. Pharmaceutical grade permethrin is used extensively by agencies working in forest areas such as the US Army.

Permethrin is also used in industrial and domestic settings to control pests such as ants and termites. It may be incorporated in formulations of wood preservative.

Military use
In order to better protect soldiers from the risk and annoyance of biting insects, the US and British armies are treating all new uniforms with permethrin.

Stereochemistry
Permethrin has four stereoisomers (two enantiomeric pairs), arising from the two stereocentres in the cyclopropane ring. The trans enantiomeric pair is known as transpermethrin.

Toxicology and safety
Permethrin acts as a neurotoxin, slowing down the nervous system through binding to sodium channels. This action is negatively correlated to temperature, thus, in general, showing more acute effects on cold-blooded animals (insects, fish, frogs...) over warm-blooded animals (mammals and birds):
 * Permethrin is extremely toxic to fish and aquatic life in general, so extreme care must be taken when using products containing permethrin near water sources.
 * Permethrin is also highly toxic to cats, and flea and tick-repellent formulas intended and labeled for (the more resistant) dogs may contain permethrin and cause feline permethrin toxicosis in cats.
 * Very high doses will have tangible neurotoxic effects on mammals and birds, including human beings.

Permethrin is listed as a "restricted use" substance by the United States Environmental Protection Agency due to its high toxicity to aquatic organisms.

Due to high toxicity for aquatic life, permethrin and permethrin-contaminated water should be properly disposed. Permethrin is quite stable, having a half life of 51-71 days in an aqueous environment exposed to light. It is also highly persistent in soil.

Human exposure
According to the Connecticut Department of Public Health, permethrin "has low mammalian toxicity, is poorly absorbed through the skin and is rapidly inactivated by the body. Skin reactions have been uncommon."

Excessive exposure to permethrin can cause nausea, headache, muscle weakness, excessive salivation, shortness of breath, and seizures. Worker exposure to the chemical can be monitored by measurement of the urinary metabolites, while severe overdosage may be confirmed by measurement of permethrin in serum or blood plasma.

Permethrin does not present any notable genotoxicity or immunotoxicity in humans and farm animals, but is classified by the United States Environmental Protection Agency (EPA) as a likely human carcinogen, based on reproducible studies in which mice fed permethrin developed liver and lung tumors. Carcinogenic action in nasal mucosal cells due to inhalation exposure is suspected, due to observed genotoxicity in human tissue samples, and in rat livers the evidence of increased pre-neoplastic lesions raises concern over oral exposure.

Studies by Bloomquist et al., 2002 suggested a link of permethrin exposure to Parkinson's disease, including very small (per kg.) exposures:
 * 2002 study – "Our studies have documented low-dose effects of permethrin, doses below one-one thousandth of a lethal dose for a mouse, with effects on those brain pathways [that are] involved in Parkinson's Disease [...] We have found effects consistent with a pre-parkinsonsian condition, but not yet full-blown parkinsonism."

However a more recent 2007 study by the same researcher concluded that there was "little hazard to humans"
 * 2007 study – "long-term, low-dose exposure to permethrin alone did not cause signs of neurotoxicity to striatal dopaminergic neural terminals, or enhance the effects of MPTP. We conclude that, under typical use conditions, permethrin poses little Parkinsonian hazard to humans, including when impregnated into clothing for control of biting flies"

A 2006 study in South Africa, found residues of permethrin in breast milk, together with DDT, in an area that experienced DDT treatment for malaria control, as well as the use of pyrethroids in small-scale agriculture.

Domestic animals
Pesticide grade permethrin is toxic to cats. Many cats die after being given flea treatments intended for dogs, or by contact with dogs having recently been treated with permethrin. Only the less toxic human, pharmaceutical grade permethrin with well defined impurities and a reduced CIS:TRANS ratio is considered safe for pet use.