July 4, 2002 CHAPEL HILL – In a new comparative study of insect repellents containing the chemical commonly known as DEET and plant-based repellents, products with DEET showed by far the greatest effectiveness in preventing mosquito bites, medical researchers say.
The study, appearing in the July 4 New England Journal of Medicine, found all products tested that did not contain DEET to be significantly less effective. Authors are Drs. Mark S. Fradin, a Chapel Hill, N.C., dermatologist and adjunct faculty member at the University of North Carolina at Chapel Hill; and John F. Day, professor of medical entomology at the University of Florida.
"We took 16 representative DEET and non-DEET products that were readily available to consumers as insect repellents and tested them carefully, repeatedly and in a way that eliminated as many variables as we could," said Fradin, clinical associate professor of dermatology at the UNC School of Medicine. "We controlled how many mosquitoes we had, their ages, how well-fed they were, what the temperature and humidity were, the levels of light and darkness and so on."
After volunteers applied various repellents to their skin according to package instructions, they placed their forearms in mosquito cages until they suffered their first bite, he said. Researchers recorded and analyzed the times that elapsed between arm insertion and the first failure of each repellent.
"We selected the time of first bite as a very stringent criterion for failure because the primary concern here is not nuisance bites but the possibility that diseases caused by mosquitoes could potentially be transmitted by a single bite," Fradin said.
Results varied widely, he said. DEET-based products fared best, depending on their concentrations. Wristbands impregnated with the chemical, however, offered no protection from mosquito bites.
OFF! Deep Woods, with 23.8 percent DEET, provided complete protection for an average of five hours, while Sawyer Controlled Release, with 20 percent DEET, lasted an average of four hours, the physician said.
OFF! Skintastic with a 6.65 percent DEET concentration offered complete protection for an average of almost two hours. Bite Blocker for Kids, in which the active ingredient is 2 percent soybean oil, lasted an hour and a half on average, he said.
Various Skin-So-Soft products, including those containing either the chemical IR3535 or plant based citronella protected, on average, far less than 20 minutes. Recently introduced oil of eucalyptus repellents Fite Bite and Repel lasted an average of two hours.
"If you are outside at a barbeque in the United States, this variability probably won't make much difference," Fradin said. "But if you are looking for prolonged protection that's reliable, especially in tropical and subtropical countries where mosquito-borne diseases are endemic, then your best bet would be to choose products containing DEET."
Worldwide, mosquitoes transmit diseases to 700 million people a year, he said. Malaria alone still kills up to 3 million people annually. Even in the United States, various forms of mosquito-borne encephalitis show up from time to time, and West Nile virus, which has been detected to date in 27 states, killed seven people in the New York City area two years ago.
DEET's toxicity gets a lot of news media attention every year, but the UNC dermatologist said concerns about that appear to be grossly exaggerated.
The "very safe" product has been on the market for some 45 years, and fewer than 50 cases of significant toxicity have been reported in the medical literature.
"In many of those cases, there was excessive, very inappropriate use of a DEET product in which someone was applying a high-strength formulation and covering large areas of skin for many days in a row," Fradin said.
Estimates are that DEET-based products have been used between 5 billion and 8 billion times with very few reported problems, he said.
"Until a better repellent becomes available, DEET-based repellents remain the gold standard of protection under circumstances in which it is crucial to be protected against arthropod bites that might transmit disease," Fradin said.
The State of Florida paid for part of the study, which received no support from industry, he said.
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