From zappers to catchers to candles to sprays, mosquito repellents come in many forms. But which ones work?
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Mosquitos don't just whine in your ear and drive you mad with itching, but they also spread disease to more than 700 million people every year. Here's how to stay off a mosquito's menu.
Who's For Lunch?
Do you seem to get eaten alive when others are left alone? You're probably not just imagining it. Everyone's body chemistry is a little different, and some people are more likely to attract unwanted insect advances than others are.
Mosquitoes can sense your presence from far away. When you breathe out, you emit a plume of carbon dioxide that carries on the breeze, and CO2 also seeps from your skin.
Mosquitoes are attracted to carbon dioxide as well as the warmth and humidity you're giving off, says Renee Anderson, PhD, a medical entomologist at Cornell University in Ithaca, N.Y. They follow the trail, flying in a zigzag pattern, until they find the source. In addition, they are also attracted to certain chemicals in your sweat. And mosquitoes love a moving target -- it helps them zero in.
How to Stay Off the Menu
In general, mosquito repellent works by masking the chemical cues that welcome mosquitoes to dine.
DEET: Potent, But Safe
The most effective mosquito repellent is one of the oldest around. DEET (short for N,N-diethyl-3-methylbenzamide) was first developed for use by the U.S. Army in 1946, and it became available to the public in 1957. Many other products have hit the market since then, but none quite compare to DEET. In fact, it's the only mosquito repellent the CDC recommends for preventing mosquito-borne diseases.
In a study published in the New England Journal of Medicine in 2002, researchers compared several types of mosquito repellents head-to-head in laboratory tests. Fifteen brave study volunteers took turns sticking an arm treated with mosquito repellent into a cage full of hungry bloodsuckers. The researchers took note of how long it took a mosquito to bite.
"OFF! Deep Woods" repellent, a product containing about 24% DEET, fared the best. Its protection lasted an average of five hours.
The least effective products were wristbands treated with DEET or citronella, which offered almost no protection. According to the researchers, this wasn't a surprise. It's known that mosquito repellent only works on the surface to which it's applied directly. Mosquitoes are happy to bite skin only four centimeters away from the repellent slick.
DEET has an excellent safety record, despite some people's concerns. N,N-diethyl-3-methylbenzamide doesn't sound like something you'd want to spray on your skin, and perhaps its acronym reminds people of the dangerous and now banned insecticide DDT. They're nothing alike, however.
The Environmental Protection Agency must approve all pesticides used in the U.S., and although DEET isn't a pesticide by definition -- it does not kill insects -- it falls under the EPA's regulatory purview.
In 1998, the agency re-evaluated DEET, and found that it is very safe when used according to label directions, and it's not classified as a carcinogen (cancer-causing agent). The label directions on DEET products will say you should apply it only once a day to exposed skin and outerwear -- not under clothing. Under clothes, it can absorb into the skin more readily, and possibly cause irritation. DEET can also irritate the eyes.
"The most common complaint is when DEET gets in the eyes, and obviously, that's something to be avoided," says Ed Tate, a spokesman for the Consumer Specialty Products Association, an industry group that funds the DEET Education Program.
Young children shouldn't be allowed to apply DEET repellent themselves, but it is safe for them to use. This year, the American Academy of Pediatrics issued a statement saying that DEET is safe for children aged two months and older. Previously, the age limit was set at two years. The academy also raised the maximum DEET concentration in mosquito repellent for kids from 10% to 30%.
Tate says the changes were welcome, and overdue. "We always were a little vexed by the 10% restriction because we saw no basis in the science on DEET," he tells WebMD.
Bite-Blocking Botanicals
Nevertheless, some still wish for an alternative mosquito repellent. At least since the 1970s, many have sworn by Avon's Skin-So-Soft bath oil as an alternative.
"They believe there's some magic in the bath oil," says Andrew Pechko, a research and development manager at Avon. With apologies to the legions of devotees, there isn't.
The NEJM study shows that Skin-So-Soft worked as a mosquito repellent for an average of roughly 10 minutes, which hardly matched DEET products, or even soybean oil. In the study, a 2% soybean oil product called Bite Blocker for Kids protected against bites for an average of 94 minutes.
Avon does not market the original Skin-So-Soft oil as a mosquito repellent, but the company has come out with a formula containing IR3535, a new EPA-approved mosquito repellent. IR3535 belongs to drug maker Merck, and it has been used as a mosquito repellent in Europe for 20 years. Avon's products are the only ones with IR3535 available in the U.S.
In the NEJM study, Bug Guard Plus protected against mosquito bites for only about 23 minutes, on average. But Avon claims that their new "eXpedition" formula lasted as long as eight hours in outdoor tests.
"The EPA, as a matter of fact, does not recognize a mosquito cage test to establish product labeling claims," Pechko tells WebMD. "The EPA requires outdoor field studies."
Other Strategies
Besides all the sprays and lotions that contain mosquito repellent, there are many things that supposedly drive away mosquitoes in the surrounding area.
Citronella candles have been used since 1882 as a means of drawing mosquitoes away from people, but one study shows that they're not much more effective than plain candles, which also give off heat, carbon dioxide, and moisture.
You may have seen ads for ultrasonic mosquito repellent devices, which supposedly emit sounds that irritate or scare away the bugs. Organizations from the Federal Trade Commission to the American College of Physicians (in a review in Annals of Internal Medicine) cite numerous studies showing that these devices don't work.
Anderson says that people often ask her about two newer devices called Mosquito Deleto, made by the Coleman company, and Mosquito Magnet, made by American Biophysics Corp.
These things are portable traps that emit carbon dioxide and a chemical called octenol. They're supposed to lure mosquitoes away from people and into the trap.
"They do collect lots and lots of mosquitoes," Anderson says. But it's not yet known whether they really reduce the number of bites for people nearby.
"Right now the jury is still out," Anderson says.
As for the traditional electric bug zappers, don't use them. The violet light may be irresistible to some flying insects, but mosquitoes largely ignore it. "It's a lot of beneficial insects that are getting fried," Anderson says.
The best way to keep swarms of mosquitoes from descending on your backyard barbecue is to get rid of standing water, where mosquitoes lay their eggs.
Some mosquito species lay eggs directly in stagnant water. Others lay eggs in containers -- a tree hollow, a birdbath, a kiddie pool, etc. -- above the water line. Then, when it rains, the eggs are submerged and they hatch.
"You need to scrub out those containers," Anderson says. "Simply dumping the water out isn't going to dislodge those eggs that are attached along the side."
Originally published June 27, 2003.
Medically updated July 7, 2004.
SOURCES: Renee Anderson, extension associate medical entomologist, Department of Entomology, Cornell University. Ed Tate, spokesman, Consumer Specialty Products Association. Andrew Pechko, research and development manager, Avon, Inc. New England Journal of Medicine, July 2002. News release, American Academy of Pediatrics. Annals of Internal Medicine, June 1998. North Carolina State University, Department of Entomology web site. EPA web site. CDC web site.
© 2003 WebMD Inc. All rights reserved.
