Head Lice (Pediculus humanus capitis) are a common parasite found in human hair. School-age children from age 3-10 are often the most effected by outbreaks of lice infestations because of their sustained proximity and propensity for sharing hats, combs and clothing. Non-prescription sales of head lice products and related school expenditures in the United States alone cost consumers an estimated $350 million yearly.[1] The two most successful conventional pyrethrum-based pediculicides are Rid©[2] and Nix©[3], averaging sales of more than $10 million each annually.

Users and researchers, however, have long reported that head lice are becoming drug-resistant and conventional pediculicides are losing their effectiveness. This is because if even only one in a thousand head lice survive a round of treatments, that one will have a natural resistance to the pediculicide and therefore a survival advantage, undoubtedly passing that advantage on to future generations. Since the life expectancy of a head louse is only about 30 days and they are capable of laying between 1-10 eggs per day[4], it doesn’t take long for enough generations of selection with this advantage to create strains of head lice highly resistant to the treatment.

Description. So just what ARE these annoying little pests? Head lice are small, six-legged, wingless insects about the size of a flea or sesame seed which feed solely on human blood. They have sharp little claws that are perfectly evolved for what they do best—crawling along and clinging tenaciously to the hair shaft in between meals.

Pets. Thankfully, head lice do not thrive on pets. And contrary to popular belief, they cannot jump, fly or magically transport from one host to the next…

Nits. Nits (the eggs of the adult head louse) are small, yellowish-white or brown, oval-shaped eggs that are glued by the louse to the side of a hair shaft at an angle. Head lice usually live for approximately 4 weeks on their hosts and a female louse in that time can lay as many as 100 nits. Nits will hatch whether or not the hair they are on is attached to the head so it is important to not share combs. Once laid, it takes about 7-10 days for a nit to hatch, and another 7-10 days for the female to mature and begin laying her own eggs.

Food Supply. Since the only place head lice can get a blood meal is on your head, if you apply an effective repellent you essentially cut off their only supply of food. Without a regular blood meal lice starve to death in 24-48 hours. Therefore your house cleans itself in 2-3 days and you don’t have to worry any more about vacuuming, running everything through the drier, etc.

Further Information:

Head Lice: Information and Frequently Asked Questions

A Statement by Dr. Richard J. Pollack, Ph.D., Harvard School of Public Health.

Head Lice Description and Biology

University of Maine Pest and Disease Management Lab

Citizen Petition Seeking to Ban the Use of Lindane as a Treatment for Lice and Scabies

Samuel S. Epstein, M.D., Cancer Prevention Coalition

Head Lice; University of Main Pest Management Lab Fact Sheet.

Permethrin – Identification, toxicity, use, water pollution potential, ecological toxicity and regulatory information. National Pesticide Telecommunications Network Pesticides Database.

Pesticide Action Network North America; Pesticide Registration (PR) Notice 94-6, Environmental Protection Agency.


[1]Jones KN, English JC III. Review of Common Therapeutic Options in the United States for the Treatment of Pediculosis capitis. Clin Infect Dis. 2003; 36 (11):1355-1361.

[2]Rid© Active Ingredients: Piperonyl butoxide (4%) Pyrethrum extract (equivalent to 0.33% pyrethrins)

[3]Nix© Active Ingredient: Permethrin 1%

[4]Downs, Anthony M. R., Managing Head Lice in an Era of Increasing Resistance to Insecticides.
American Journal of Clinical Dermatology; 2004, Vol. 5 Issue 3, p169-177.