Dear EarthTalk: Is it true that the DEET used in most mosquito repellents is toxic? If so what problems does it cause? And what are some non-toxic alternatives for keeping mosquitoes at bay?

DEET is commonly known as the king of mosquito repellents, though not everyone is keen to slather it on their skin. A study conducted in the late 1980s on Everglades National Park employees to determine the effects of DEET found that a full one-quarter of the subjects studied experienced negative health effects that they blamed on exposure to the chemical. Effects included rashes, skin irritation, numb or burning lips, nausea, headaches, dizziness and difficulty concentrating.

Duke University pharmacologist Mohamed Abou-Donia, in studies on rats, found that frequent and prolonged DEET exposure led to diffuse brain cell death and behavioral changes, and concluded that humans should stay away from products containing it. But other studies have shown that while a few people have sensitivity to DEET applications, most are unaffected when they use DEET products on a sporadic basis according to the instructions on the label.

The upside of DEET is that it is very effective. A 2002 study published in the New England Journal of Medicine found that DEET-based repellents provided the most complete and longest lasting protection against mosquitoes. Researchers found that a formulation containing 23.8 percent DEET completely protected study participants for upwards of 300 minutes, while a soybean-oil-based product only worked for 95 minutes. The effectiveness of several other botanical-based repellents lasted less than 20 minutes.

But a number of new concentrations of botanical repellents that have hit the market since are reportedly better than ever. In 2005, the U.S. Centers for Disease Control (CDC) granted approval to two healthier alternatives to DEET--picaridin and oil of lemon eucalyptus--for protection from mosquitoes. Picaridin, long used to repel mosquitoes in other parts of the world, is now available in the U.S. under the Cutter Advanced brand name. Oil of lemon eucalyptus, which is derived from eucalyptus leaves and is the only plant-based active ingredient for insect repellents approved by the CDC, is available in several different forms, including Repel Lemon Eucalyptus, OFF! Botanicals, and Fight Bite Plant-Based Insect Repellent.

Some other good choices, according to the nonprofit National Coalition against the Misuse of Pesticides, include products containing geraniol (MosquitoGuard or Bite Stop), citronella (Natrapel), herbal extracts (Beat It Bug Buster) or essential oils (All Terrain). The group also gives high marks to oil of lemon eucalyptus, such as that found in Repel's Lemon Eucalyptus Insect Repellent.

Another leading nonprofit, Pesticide Action Network North America (PANNA), likes Herbal Armor, Buzz Away and Green Ban, each containing citronella and peppermint as well as various essential oils (cedar wood, lemongrass, etc.). PANNA also lauds Bite Blocker, a blend of soybeans and coconut oils that provides four to eight hours of protection and, unlike many other brands, is safe to use on kids.

CONTACTS: "Comparative Efficacy of Insect Repellents against Mosquito Bites," http://content.nejm.org/cgi/content/full/347/1/13; National Coalition Against the Misuse of Pesticides (NCAMP), www.beyondpesticides.org; Pesticide Action Network North America, www.panna.org.

--Tom Pollack, Oakland, CA