The proof is in — slather on the DEET

The best-known insect repellent is the chemical-based DEET. Most plant-based insect repellents contain essential oils from a combination of citronella, cedar, eucalyptus, peppermint, lemongrass, geranium, and soybean.

In a study presented in the New England Journal of Medicine (July 2002), Mark S. Fradin, M.D., and John F. Day, Ph.D., compared the efficacy of 16 products in a laboratory environment. Volunteers inserted their repellent-treated arms into a cage with a fixed number of unfed mosquitoes and recorded the elapsed time to the first bite. They tested each repellent three times on 15 individuals. Here is what they found.

Products containing DEET provided the longest-lasting protection, correlating positively with the concentration of DEET. However, the duration of protection tended to plateau at a concentration of 50 percent. The formulation containing 4.75 percent DEET (OFF! Skintastic for Kids) provided an average of 88 minutes of complete protection; 23.8 percent DEET (OFF! Deep Woods) protected for an average of 301 minutes and protected significantly longer than the controlled-release formulation containing 20 percent DEET (Sawyer Controlled Release).

No non-DEET repellent provided protection beyond 1.5 hours. Only the soybean-oil-based repellent (Bite Blocker for Kids) provided protection for a period similar to that of the lowest-concentration DEET product: 94.6 and 88.4 minutes, respectively.

The IR3535-based repellent (Skin-So-Soft Bug Guard Plus) protected against mosquito bites for an average of 22.9 minutes. The citronella-based repellents (Natrapel, Herbal Armor, Green Ban for People, Buzz Away, and Skin-So-Soft Bug Guard) protected for 20 minutes or less. There was no significant difference in protection time for the slow-release formulations. Skin-So-Soft Bath Oil provided a mean of only 9.6 minutes of protection. Repellent-impregnated wristbands containing DEET or citronella protected the wearer for only 12 to 18 seconds on average.

After the original study was completed, a new botanical repellent containing oil of eucalyptus was marketed under two names: Repel Lemon Eucalyptus Insect Repellent (WPC Brands) and Fite Bite Plant- Based Insect Repellent (Travel Medicine). The repellent had a mean protection time of 120 minutes. DISCUSSION

Most alternatives to topically applied repellents have proved to be ineffective. No ingested compound, including garlic or thiamine (vitamin B1), has been found capable of repelling biting arthropods.

Multiple factors play a part in determining how effective any repellent will be (the species, the user’s age, sex, level of activity, biochemical attractiveness, the ambient temperature, humidity, and wind speed), so a given repellent will not protect all users equally. Yet the study shows that only products containing DEET offer long-lasting protection after a single application. Certain plant-derived repellents may provide short-lived efficacy, which may be sufficient when arthropod bites are primarily a nuisance. Frequent reapplication of these repellents would partially compensate for their short duration of action. However, when one is traveling to an area with prevalent mosquito-borne diseases that could be transmitted through a single bite, the use of non-DEET repellents would be ill-advised. However, DEET is not perfect; it may be washed off by perspiration or rain, and its efficacy decreases dramatically with rising outdoor temperatures.

DEET has a remarkable safety profile after 40 years of use and nearly 8 billion human applications. DEET is a plasticizer, capable of dissolving watch crystals, the frames of glasses, and certain synthetic fabrics. However, in humans, fewer than 50 cases of serious toxic effects have been documented, and three quarters of them were easily resolved. Many involved long-term, heavy, frequent, or whole-body application of DEET. No correlation has been found between the concentration of DEET used and the risk of toxic effects. The Environmental Protection Agency has concluded that “normal use of DEET does not present a health concern to the general U.S. population.” Until a better repellent becomes available, DEET-based repellents remain the gold standard of protection.