It's Okay To Nit Pick

The Green Guide
Emily Main, August 1, 2006

Some head-scratching new evidence has come to light out of the U.K. via the June 2006 Archives of Disease in Childhood. In a study of 231 school children with head lice, researchers in Wales found that 80 percent of the lice tested had genes that had grown resistant to pyrethroids, synthetic versions of a naturally occurring pesticide found in chrysanthemums and the class of chemicals most commonly used in over-the-counter lice-treatment shampoos and lotions, such as Nix Cream Rinse Lice Treatment.

The study didn't examine pyrethroid resistance in lice in the U.S., but Daniel Kass, director of environmental surveillance and policy for the New York City Department of Health & Mental Hygiene, says he has heard anecdotal evidence that pyrethroid-based lice treatments are losing effectiveness, which amounts to unnecessary chemical exposures if the products aren't working. Permethrin, the most common pyrethroid used in these treatments, is toxic to aquatic life after it's washed down the drain and can cause skin, respiratory and eye irritation in small doses. Even so, Kass says that treatments aren't intended to come in contact with skin long enough to cause any major health effects; most remain on hair for a maximum of 10 minutes and need, at most, two applications to kill the insects. "There is very little data suggesting that a typical and correct treatment—following labels carefully and judiciously—can put kids at risk," says Kass. "These treatments are only dangerous to the extent that lice become more resistant as people use more product," he adds.

While pyrethroid shampoos wash out, pyrethroid home pesticides such as Raid linger in indoor air and can produce ill health effects such as headaches, nausea and, in severe cases, neurological damage, on top of contributing to resistance in pests. So instead, discourage roaches and other insect invaders by using integrated pest management techniques, such as caulking foundation cracks to prevent them from entering and depriving them of food and moisture by sealing food containers and fixing leaks. (See our Pest Control Product reports at for more ideas).

Another chemical examined in the Welsh study, the organophosphate malathion, was found to be more effective than pyrethroids in killing lice. But that doesn't make it a safer shampoo for humans. Malathion, currently under safety reassessment by the U.S. Environmental Protection Agency, can react with chlorine in water to form malaoxon, a nervous system toxin. Fortunately, its use in head lice treatments in the U.S. is not widespread. The Food and Drug Administration has only approved one malathion-based treatment, a prescription-only product called Ovide Lotion.

Although not named in the study, parents should avoid prescription products containing lindane if a child's head lice isn't responding to over-the-counter treatments. An organochlorine chemical classified by the International Agency for Research on Cancer as a possible human carcinogen, lindane has been known to cause seizures and death upon ingestion of large amounts. It also persists in the environment for long periods of time and builds up in the fatty tissue of fish.

The most effective way to control head lice, says Kass, is through diligence and mechanical controls, such as nit combs. "Mechanical controls that involve removing lice from hair one by one are very effective if done properly and consistently," he says, adding that schools must be vigilant about detection, as well. "If schools treat each single case as an example of an emergent outbreak, you prevent the outbreak before it gets serious," he says. To put your mind at ease, speak to your child's school to see what protocols are in place should head lice pay a visit. If they don't yet do so, suggest that the school purchase nit combs and teach children how to use them.

The most effective nit combs have closely spaced, metal teeth that are about 1 ½ inches long. The National Pediculosis Association recommends the LiceMeister comb ($9.95;, 888-542-3634).