DEET (N,N-diethyl-meta-toluamide) is arguably the most widely recommended active ingredient in insect repellents. Proven effective, DEET does not kill insects. Instead it prevents biting insects like mosquitoes and ticks from zeroing in on your skin.

Biting insects follow the scent of carbon dioxide gas to find a meal. Skin and breath naturally give off a carbon dioxide. By spreading a small amount of DEET on exposed skin and applying it to external clothing, insects cannot readily locate the source of the carbon dioxide.

Each year about 30 million Americans use products containing DEET. Its greatest benefit is in repelling disease-bearing insects, like deer ticks that can transmit Lyme disease, and mosquitoes that can transmit encephalitis and West Nile virus. It also benefits those exposed to insects that might be carrying malaria, dengue fever and other diseases.

Studies found that a concentration of 23.8% DEET successfully repelled mosquitoes for approximately 5 hours -- three times longer than the next leading product. It also found that higher concentrations extended that protection time. Other studies indicate ticks are repelled for 3-8 hours, depending on the concentration of DEET used.

The Environmental Protection Agency (EPA) registered DEET for public use in 1957. It re-assessed DEET in 1998 to ensure that it meets modern safety standards. It concluded that DEET is very safe when used as directed.

Older products containing DEET may have had misleading labels that included a child safety claim for concentrations of 15% or less. When the EPA reassessed DEET in 1998 it mandated that the child safety claim be discontinued, finding that the concentration of DEET was irrelevant; following directions was deemed to be critical. Concentrations of 100% DEET are safe for children when used as directed, while lower concentrations of 15% or less do not guarantee safety if the product is misused.

DEET has over 50 years of scientific research behind it, and the efficacy of widespread public use for several decades. This makes it the most vigorously tested repellent on the market. Although safe, make sure to read and follow all labels instructions that come with any product containing DEET.

April 2005 Update: Following years of recommending only DEET, the CDC (Centers for Disease Control) has announced two additional repellents: picaridin and oil of lemon eucalyptus. Both picaridin and oil of lemon eucalyptus are said to be relatively effective at repelling mosquitoes, and significantly more pleasant to use than DEET. Some brand names of picaridin include Bayrepel and Autan.