DEET: It Can't Be Beat

Products with high DEET concentration, like Off, offers the longest protection. (Julia Ewan -- The Washington Post)

For all the counting of dead birds, the treating of ponds thick with mosquito larvae and the possibility of widespread spraying, the final line of defense against West Nile remains the human skin. Keep the insects away from that fragile barrier and the risk plunges to zilch.

The scientific consensus on the best way to do that remains clear: In the United States, nothing comes close to DEET. Since the government developed the chemical 50 years ago, it's become the gold standard for keeping all manner of creepy-crawlies at bay. Especially mosquitoes.

In a study published last year in the New England Journal of Medicine, 15 hardy volunteers from the staff at the University of Florida shoved their arms into a cage full of underfed mosquitoes after slathering on one of 13 different kinds of repellents and three repellent wristbands. Four concentrations of DEET were included, as well as four members of the Avon Skin-So-Soft product family, a few citronella-based products and one product based on soybean oil. All of the products provided near-100 percent effectiveness for some period of time. But the key difference among the repellents was the length of that effectiveness.

The results were unambiguous. DEET-based products claimed the top three spots, with Deep Woods Off, topping the DEET scale with a concentration of 23.8 percent DEET, keeping the bugs at bay for more than five hours, on average. No citronella product and no version of Skin-So-Soft-including Avon's Bug Guard Plus, with a new bug-repelling chemical, IR3535-worked for more than an hour.

"I think that if you're looking for the insect repellent that's going to last the longest, you're not going to find many people who will say anything is better than DEET," said Mark Fradin, a dermatologist at the University of North Carolina, who led the New England Journal study.

The study had a few detractors, including Avon, which said the arm-in-a-cage study design was flawed and that the company's own studies showed Bug Guard Plus worked for three hours or more. But Fradin's results match those in May's issue of Consumer Reports. That effort, also a cage test, put eight DEET-based repellents up against Avon's new formulation and, again, every DEET product outlasted Bug Guard Plus.

That's not to say that non-DEET repellents aren't effective. The Environmental Protection Agency (EPA) requires some proof that the stuff will keep the skeeters from biting before it will allow a company to market a bug repellent. But Consumer Reports found the benefits of most non-DEET products so limited that it didn't even bother to test them.

The problem, says Andrew Spielman, a professor of tropical public health at the Harvard School of Public Health, is that the protection offered is, at best, "transient."

Spielman said he sticks with the DEET-based products, though he said he uses the repellent cautiously. There are reports of medical problems, including seizures and deaths, linked to large doses of the chemical, though the EPA sees no significant risks from the normal use of the chemical. Still, Spielman advocates prudence.

"Just as a general principle," he said, "one should optimize for the minimum possible use of anything, be it aspirin or DEET. I use it only when I need it."

The American Academy of Pediatrics (AAP) recommends using only products with DEET concentrations of less than 10 percent on children, and pediatricians have recommended avoiding the use of the chemical on children under 2 years old. But those guidelines were put in place in 1999, before West Nile emerged, and a pediatrician writing in the AAP's house publication suggested that that babies older than 2 months can tolerate products as concentrated as 30 percent DEET without significant risk. Both the AAP and the EPA say the repellents should be kept off little hands and away from mouths and open wounds.

For the youngest babies, mosquito netting is likely to do the trick. For the rest of the population, putting a layer of clothing between them and the bloodsuckers might not be worth the discomfort, Fradin said. Nets "are not going to work if the fabric comes in contact with your skin," he said-the mosquitoes will simply bite through the cloth. "And if it's 105 degrees and you're hiking around, it's going to be uncomfortable."

The best protection against West Nile, however, might be to simply avoid the bugs, rather than wage chemical warfare. While the mosquitoes that emerge in the coming months will no doubt be bloodthirsty, Spielman said the West Nile-transmitting insects won't make their appearance until later in the season. A significant population of birds carrying the virus is required to support a number of infected mosquitoes sufficient to increase human risk.

"It's being outside in the garden at midnight late in the transmission season that's the risky situation. The need for protection in midsummer is pretty minimal because transmission is pretty rare," he said.

But it's probably worth slathering up for those harmless early season mosquitoes, anyway, he concedes. "A couple, three weeks from now, we'll be bitten," Spielman said. The pests may not carry disease, but "they are annoying." <

Brian Reid is a regular contributor to the Health section.