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DEET Deemed Best at Repelling Pests

Study: Other products don't come close in keeping mosquitoes away

WEDNESDAY, July 3, 2002 (HealthDayNews) -- Want to keep mosquitoes away this holiday? Then there really is no substitute: Despite its infamously offensive odor, DEET works best, and it's perfectly safe when used judiciously.

A new comparison of 16 products that boast bug-banishing properties, from citronella oil to a soy-based brand, finds those with DEET typically last longest and give the most protection against bites.

Only the soy oil formulation came close to the synthetic chemical, which the researchers say has an undeserved reputation for being harmful to people. Citronella, spiked cosmetics and wrist bands fail to fend off mosquitoes for very long, if at all. The findings appear in tomorrow's issue of the New England Journal of Medicine.

"For complete protection for any significant duration of time, only the DEET-based products give you that benefit," said Dr. Mark Fradin, a dermatologist and lead author of the study.

"For a short period, the alternative repellents are fine but they need to be reapplied," says Fradin, who adds that even "natural" herbal bug repellants can be harmful.

Fradin cautions that DEET is now the only choice if the reason for repellents is to avoid diseases and not merely discomfort. In these cases -- such as a safari in malaria-infested Africa or an area with dengue fever -- a single bite can be lethal.

Fradin and Jonathan Day, a mosquito expert at the University of Florida's medical entomology lab in Vero Beach, tested the repellents on 15 men and women. The volunteers agreed to bear some bites for the sake of science by baring their arms repeatedly to a cage full of hungry mosquitoes.

To simulate a natural outdoor experience, and to avoid stacking the deck against weaker repellents, the cage contained only 10 mosquitoes at a time, as opposed to the hundreds that are often deployed in these "arm-in-cage" tests.

Deep Woods OFF!, which contains 23.8 percent DEET by volume, prevented the first bite the longest -- by an average of five hours but long as six. The two runners-up also contain DEET, in strengths of 20 percent and 6.65 percent.

The only non-DEET product in the top five was HOMS' Bite Blocker for Kids, which contains 2 percent soy oil and stayed effective for an average of a little more than 1.5 hours.

After the paper was submitted for publication, Fradin and Day also tested a eucalyptus oil-based product that staved off bites for about two hours.

However, the rest of the repellents fared poorly. None with citronella -- an oil derived from grasses that's touted as herbal approaches to pest control -- had an average staying power of more than about 20 minutes. Most delayed bites far shorter. And three wrist bands, including two with DEET and one with citronella, proved essentially useless. "It won't cloak you. It's not a force field of protection," Fradin explains.

DEET (N,N-diethyl-3-methylbenzamide) was developed by the U.S. Army in 1946, and has been available commercially since 1957. Some 230 DEET-containing products are now on the market, according to the Environmental Protection Agency (EPA), which oversees insect repellents.

In high concentrations, DEET can be extremely corrosive, irritating mucous membranes like those in the eyes and nose. It can even eat away plastic watch faces and eyeglass frames. However, despite some 8 billion applications on humans so far, there have been fewer than 50 documented cases of serious illness resulting from use of the repellent -- a track record that's far safer than that of, say, aspirin.

Richard Pollack, a Harvard University tropical disease specialist, says consumers remain wary of the substance thanks in large part to the marketing efforts of companies selling DEET-free products. "I have yet to see anything that holds a candle to DEET. It's the product that I would use," says Pollack, co-author of an editorial accompanying the journal article.

And a little DEET seems to go a fairly long way. Studies have shown that a 50 percent solution of the chemical is nearly as protective as a full-strength formulation, and a 30 percent dose is almost as potent as a 50 percent version. "There is some difference between 10 percent and 20 percent DEET, but if you apply twice as much [of the weaker solution] you have the same dosage," he says.

Fradin and Day used only Aedes aegypti mosquitoes, the kind that carry yellow fever. DEET may be less effective against other species of the pests, such as those that transmit malaria. Yet for any given species, Fradin says, "on average, DEET's the best choice."

Joseph Conlon, technical advisor for the American Mosquito Control Association, agrees. "We haven't found anything better than DEET," says Conlon, who has conducted mosquito control missions in 67 countries.

Conlon says the military has several new repellents in varying stages of development that might improve on DEET and other existing products. "You can find things that are almost as effective [as DEET] but they have to be reapplied, and people don't think of reapplying so they get bitten more often."

As for herbal extracts, Conlon says many, such as clove oil and spearmint oil, have proven to be effective at high concentrations. However, at those doses they're harmful to skin as well as mosquitoes. However, companies sell these products at minuscule doses of the active ingredient, and advertise that they work even though they're unlikely to keep an insect at bay.

David Degan, an EPA spokesman, says the agency is in the process of retooling its repellent review system. Currently, synthetic chemicals like DEET are considered under different rules from those covering plant extracts. EPA would like to make all products subject to a single standard.

"Efficacy is important because we're talking about potential public health implications," says Degan, who adds that companies will also have to demonstrate safety under the new rules.

Andra Mielnicki, a spokeswoman for Avon, defended the cosmetic company's Skin-So-Soft Bug Guard Plus lotion, which had an average time-to-first-bite of just under 23 minutes. The product contains the chemical IR3535, which in a 2001 study, by Thai researchers, proved effective against a variety of species in that country.

Press materials from Avon claim the lotion is as effective as S.C. Johnson's OFF! Skintastic, which has DEET.

However, Fradin and Day found the opposite. The OFF! Skintastic for Kids product lasted four times longer than Avon's cream, and OFF! Skintastic, which has a higher concentration of DEET, lasted nearly five times longer. Mielnicki said her company based its assertion on tests from "independent dermatologists."

Therese Van Ryne, a spokeswoman for S.C. Johnson of Racine, Wisc., called DEET the "gold standard" for repellents. S.C. Johnson has also introduced an herb extract product with lemon eucalyptus oil that trumpets a less noxious scent than DEET but somewhat shorter protection against pests.

"If a sportsman is heading off into the woods for an extended period of time, we would definitely recommend Deep Woods OFF! But if a mother is taking her child out for a walk in their neighborhood, OFF! Botanicals would be suitable for that occasion," Van Ryne says.

Although this study looked only at mosquitoes, previous studies have found that DEET is also effective at warding off ticks, which can spread Lyme disease.

What To Do

There are other ways to avoid mosquitoes that don't involve using chemicals. Wear shoes and long-sleeved clothing if it's comfortable enough, and try to avoid outdoor activities around dusk, when the critters are most active.

To find out more about DEET, visit the Environmental Protection Agency. Learn more about the pests from the

SOURCES: Mark Fradin, M.D., clinical associate professor, University of North Carolina, Chapel Hill; Richard Pollack, Ph.D., instructor, Harvard University School of Public Health, Boston; Joseph Conlon, technical advisor, American Mosquito Control Association, Orange Park, Fla.; David Degan, spokesman, Environmental Protection Agency, Washington, D.C.; Andra Mielnicki, spokeswoman, Avon Products Inc., New York City; Therese Van Ryne, spokeswoman, S.C. Johnson, Racine, Wisc.; July 4, 2002, New England Journal of Medicine
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