How Mosquito Repellents Work

Think of your body as an airport and exposed flesh as the runway. What repellents do is make the runway invisible: Mosquitoes come in for a final approach and suddenly fog is covering their landing strip. Thus for repellents to work they must completely cover any exposed skin. Leaving a six-inch strip of skin free is like turning on the landing lights and inviting mosquitoes down for a Bloody Mary in the Admiral's Club.

The most powerful insect repellent available is N,N-diethyl-3-methylbenzamide, commonly known as DEET, available in various formulations (including aerosols, pump sprays, lotions, and gels). It is available in concentrations ranging from 5 percent up to 100 percent, but research has shown that it's effectiveness tends to plateau at about 50 percent strength. Depending upon the species of mosquito it's used against, 35- percent DEET can reach 12 hours of effectiveness. Dr. Mark Fradin, a dermatologist who has extensively studied mosquito repellents, suggests using 10-percent to 35-percent DEET for general use, and 50-percent DEET for jungle-like conditions.

The United States military's standard insect repellent is a time-release formula of 35 percent DEET produced by 3M and sold under the brand name Ultrathon. It can be purchased from SCS Limited, (800) 749-8425, or online from Travel Medicine, www.travmed.com. Over-the-counter products containing DEET in higher strengths include Off! Deep Woods for Sportsmen (100% concentration); Muskol Ultra 6 Hours (40%); Off! Deep Woods (23.8%); BugOut (15%); and Cutter Unscented (10%). DEET is remarkably safe when properly used. It has been used by hundreds of millions of people worldwide for over 40 years, with only a handful of adverse cases. Still, follow the directions carefully and never spray the stuff into your face or eyes. One cautionary note: The American

Academy of Pediatrics recommends a maximum concentration of 10 percent DEET for young children. DEET can be applied to clothing and is safe on natural fibers and nylon. On the minus side, it can damage rayons and other synthetic fabrics, melt plastic eyeglass frames and destroy plastic watch crystals. DEET also tends to break down at higher temperatures and can be sweated off or washed off in the rain.

Accept no substitutes

In a clinical study by Dr. Fradin and John F. Day, Ph.D. published in the New England Journal of Medicine, none of the various essential oils marketed as insect repellents, including citronella, lemon eucalyptus, and soybean oil offered protection longer than two hours. The effectiveness of citronella candles is particularly overhyped. In a field trial in Canada, citronella candles were shown to reduce biting by 40 percent-good for shooing the mosquitoes to the next picnic, but not an effective shield without repellent. Wristbands impregnated with citronella may be considered stylish in some quarters, but they do nothing to prevent mosquito bites. Also, forget about Avon's Skin-So-Soft Bug Guard, marketed as a combination skin lotion and repellent; it might make your skin supple as a baby's bottom, yet Fradin's study showed it will only save you from mosquitoes for about 10 minutes. (Avon disputes Fradin's findings and claims Skin-So-Soft Bug Guard Plus IR3535 was proven effective for up to three hours in field tests submitted to the Environmental Protection Agency.)