While permethrin may be marketed as an insect repellent, it does not prevent insects from landing. Instead it works by incapacitating or killing insects before they can bite. In pet flea preventive collars or treatment (safe for use on dogs but not cats)
In timber treatment

Medical use
Permethrin is available for topical use as a cream or lotion. It is indicated for the treatment and prevention in exposed individuals of head lice and treatment of scabies.For treatment of scabies: Adults and children older than 2 months are instructed to apply the cream to the entire body from head to the soles of the feet. Wash off the cream after 8–14 hours. In general, one treatment is curative.For treatment of head lice: Apply to hair, scalp, and neck after shampooing. Leave in for 10 minutes and rinse. Avoid contact with eyes. Pest control / effectiveness and persistence
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, as well as cats and aquatic life.Permethrin kills ticks and mosquitoes 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 kills any immature ticks feeding on the mice. Permethrin is used in tropical areas to prevent mosquito-borne disease such as dengue fever and malaria.
Cats
Permethrin is toxic to cats; however, it has little effect on dogs. 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. In cats it may induce hyperexcitability, tremors, seizures, and death.Toxic exposure of permethrin can cause several symptoms, including convulsion, hyperaesthesia, hyperthermia, hypersalivation, and loss of balance and coordination. Exposure to pyrethroid-derived drugs such as permethrin requires treatment by a veterinarian, otherwise the poisoning is often fatal. This intolerance is due to a defect in glucuronosyltransferase, a common detoxification enzyme in other mammals, that also makes the cat intolerant to paracetamol (acetaminophen). The use of any external parasiticides based on permethrin is contraindicated for cats. Aquatic organisms
Permethrin is listed as a "restricted use" substance by the US Environmental Protection Agency (EPA) due to its high toxicity to aquatic organisms, so permethrin and permethrin-contaminated water should be properly disposed of. 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. See also
Benzyl benzoate
DEET
Methoprene
Pyrethrin
Pyrethrum

References
External links
Permethrin in the Pesticide Properties DataBase (PPDB)
Permethrin General Fact Sheet – National Pesticide Information Center
Permethrin Technical Fact Sheet – National Pesticide Information Center
Permethrin-treated Clothing Hot Topic – National Pesticide Information Center
"Health Effects of Permethrin-Impregnated Army Battle-Dress Uniforms", National Research Council (1994, US)
Permethrin Pesticide Information Profile – Extension Toxicology Network
"Permethrin". Drug Information Portal. U.S. National Library of Medicine.