Insect Repellents

With cases of West Nile virus disease still being reported in the United States, questions about which insect repellent to use is continually being asked. Some may believe that mosquitoes disappear in the fall months with the coming of cold weather they have diminished, but you only need to take a walk along a lake to see that they are still prevalent.

A timely report appeared in The New England Journal of Medicine. Researchers at the University of Florida tested a variety of widely available insect repellents including some botanical products to determine which ones provide the best protection. Their study overwhelmingly showed that products containing N, N-diethyl3-methylbenzamide (DEET) are most reliable.

Products containing different concentrations of DEET were tested; researchers found that high doses resulted in longer-lasting protection. The only botanical repellent that came close to the efficacy of the lowest concentrations of DEET was the one containing soybean oil. Products containing eucalyptus oil were introduced after the study was completed and initial evaluation suggests that among botanical products, it may actually offer the longest-lasting protection.

Researchers concluded that products containing DEET rather than botanical preparations should be used in areas where diseases such as malaria, encephalitis, or West Nile virus, can be transmitted in a single bite. In the United States, DEET is sold in concentrations from 5% to 100% in lotions, solutions, gels, sprays, and impregnated towelettes. While higher concentrations provide longer-lasting protection for most uses there is no need to use the highest concentrations of DEET. Products with 10% to 35% DEET provide adequate protection under most conditions.

The American Academy of Pediatrics currently recommends that DEET-containing repellents used on children contain no more than 10% DEET. Persons adverse to applying DEET directly to their skin may get long-lasting repellency by applying DEET only to their clothing. Products with a DEET concentration over 35% are probably best reserved for when the wearer will be in an environment with a very high density of insects (e.g., a rain forest), where there is a high risk of disease transmission from insect bites or conditions which wash the repellent from the skin (e.g., rain or high temperature and humidity).