Learn how insect repellents really work to protect you

Honestly, spraying a liquid all over your body to keep away bugs seems pretty crazy. But, remarkably, bug sprays work wonders in protecting us from mosquitoes, flies, ticks, and other pests in the outdoors. How does insect repellent work? What can bug spray protect me from? How should I use repellent? These questions and more will be answered.

Frequently Asked Questions

Q: How do repellents work?

A: This is a topic of ongoing interest among researchers. Two 2008 studies, for instance, arrived at slightly different conclusions.

The prevailing belief asserts that active ingredients in repellents create a vapor barrier at the skin surface that disorients or inhibits mosquitoes. Mosquitoes have chemoreceptors on their antennae that are stimulated by carbon dioxide (detectable in human breath) or lactic acid (in sweat). These same receptors, it appears, are inhibited by active ingredients such as DEET and picaridin.

At Rockefeller University in New York, researchers reinforced this view, showing DEET acts like a chemical cloak, masking human odors that blood-feeding insects find attractive.

Later in 2008 scientists at the University of California at Davis published research that shows mosquitoes avoid DEET simply because they have an intense dislike for its smell. "DEET doesn't mask the smell of the host or jam the insect's senses," said Walter Leal, professor of entomology at UC Davis, as reported in the online news service Science Daily. "Mosquitoes don't like it because it smells bad to them."

Whichever conviction you accept, wearing repellent underneath clothing is not advised because a layer of clothing reduces the efficacy of repellent, either because it diminishes the potency of the repellent's smell or it interferes with the vapor barrier repellents attempt to form on skin surfaces.

Q: Wearing repellent on my skin or permethrin-treated clothing is unappealing to me. Am I really at risk if I avoid using such measures?

A: Very likely, yes. West Nile virus has been detected in almost every state, with 4,261 cases (and 174 fatalities) reported in 2006. In 2005, 23,305 cases of Lyme disease were reported in the U.S. With West Nile virus, the number of cases continues to rise and the geographical distribution of the disease now reaches across the continental United States. In 1999, only one state (New York) had reported a human case.

In contrast, the health risks associated with repellents and permethrin, according to health authorities, range from tiny to nonexistent. This is why an agency such as the CDC advocates their use.

"Each year some areas of the U.S. experience intense outbreaks of West Nile virus transmission," says Dr. Emily Zielinkski-Gutierrez, a behavioral scientist with the CDC. "If you hear reports of human cases, infected mosquitoes or dead birds, it makes sense to raise your level of concern and take more precautions.

"Likewise with Lyme disease, if you live on the East Coast or an area of intensive transmission in the upper Midwest, the chance of getting infected is something to take very seriously.

"The risk of some mosquito- or tick-borne disease is present in nearly every state," says Zielinkski-Gutierrez. "Taking a few minutes to apply a repellent can prevent infection with a serious or life-threatening illness. That seems like a pretty reasonable trade-off to me."

Q: What's the proper amount of repellent to apply?

A: It varies by product, and may be influenced by the activity level (and corresponding perspiration rate) of the wearer. Each individual will likely discover the right dosage for their skin and conditions after a little experimenting.

Kurt Avery, president of Sawyer Products explains the goal is supply the correct number of DEET molecules per skin surface area. For a nonscientist to achieve this balance, Avery recommends a rational approach: Avoid overkill, yet don't try to rely on very tiny spritzes or repellent dotted randomly on exposed skin. A too-thin application could leave some skin vulnerable.

"In general, repellents yield a 3" 'spatial' coverage, so there can be a gap of up to 6" and still have you protected," he says. "This is why you can stay away from sensitive areas such as eyes, mouth, wounds, etc. "Permethrin [on clothing] yields an even better spatial protection."

Q: How about malaria and international travel?

A: Malaria, also transmitted by mosquitoes, is a serious and potentially fatal disease concern in some countries. To defend yourself against it, the CDC recommends:

• DEET (reapplied as needed, according to product instructions)
• clothing and bed-netting treated with permethrin
• taking a preventative antimalarial drug.

The species of mosquito that transmits malaria is more aggressive than North American species, meaning you may need to reapply repellent more often than you would back at home.

Q: Can insect repellent and sunscreen be worn at the same time?

A: Little hard evidence on this topic exists, though it is generally accepted that applying both insect repellent and sunscreen topically (to the skin) will diminish the efficacy of both.

Our advice: Generously apply sunscreen to cool, dry skin 20 to 30 minutes before activity. Wait until insect activity demands repellent, then apply it. A few sunscreen/repellent combos exist.

Q: Can children safely use DEET and other repellents?

A: According to the CDC: "No definitive studies exist in the scientific literature about what concentration of DEET is safe for children. No serious illness has been linked to the use of DEET in children when used according to manufacturer's recommendations."

Pediatricians typically suggest avoiding the use of repellents on infants younger than 2 months and advise DEET concentrations up to 30 percent. Repellents containing oil of lemon eucalyptus often carry a label warning that they should not be used on children under the age of 3.

Q: What are the advantages of controlled-release formulas?

A: Molecules of DEET are encapsulated in controlled-release lotions and gradually released over time, holding the "available DEET" dosage on the skin at a constant rate.

First on the market was a product now owned by 3M, Ultrathon. The highly effective repellent (estimated duration: up to 12 hours) encapsulates DEET (at 34% concentration) in a polymer base, which some wearers reporting finding greasy and odiferous. Avery says Sawyer responded by creating a controlled-release formula with a 20% DEET concentration in a polymer base. the goal: to improve the feel and smell of the formula.

For its "Ultra" line of repellents, Sawyer encapsulates DEET (at concentration of 30%) in lipposomes, a lotion ordinarily associated with the cosmetics industry. The advantage, according to Avery: One lipposome molecule can encapsulate 10 DEET molecules, whereas polymers work on a 1-to-1 ratio with DEET. "So we had 70% of the formula available to make the 30% DEET nice to wear," Avery says.

Avery also points out Sawyer's controlled-release formula reduces DEET absorption by 67%, as supported by a 2004 study conducted by the Couston Research Foundation at Sawyer's request. An added bonus: During multiday trip where repellent is reapplied several days in a row, lotion-based controlled-release formulas avoid the skin irritation that alcohol-based repellents may cause some users.

Q: What do we know about mosquitoes?

A: Here are some interesting facts provided by the nonprofit American Mosquito Control Association:

• There are about 2,700 species of mosquito. There are 176 species in the United States. Texas has the most species (85), West Virginia the fewest (26).
• The average mosquito weighs about 2.5 milligrams.
• Mosquitoes fly an estimated 1 to 1.5 miles per hour.
• Only female mosquitoes feed on blood (it helps their eggs mature).
• The average mosquito takes in about 5-millionths of a liter of blood during feeding.
• Mosquitoes find hosts by sight (they observe movement); by detecting infrared radiation emitted by warm bodies; and by chemical signals (mosquitoes are attracted to carbon dioxide and lactic acid, among other chemicals) at distances of 25 to 35 meters.
• Bigger people are often more attractive to mosquitoes because they are larger targets and they produce more mosquito attractants, namely carbon dioxide (or CO₂, released mainly from the breath, but also the skin) and lactic acid.
• Active or fidgety people also produce more CO₂ and lactic acid.
• In general, adults are more likely to be bitten than children, although adults may become less attractive to mosquitoes as they age.
• Women are usually more attractive to mosquitoes than men because of the difference in hormones produced by the sexes.
• Blondes tend to be more attractive to mosquitoes than brunettes.
• Smelly feet are attractive to mosquitoes — as is Limburger cheese.
• Dark clothing attracts mosquitoes.
• Movement increased mosquito biting up to 50% in some research tests.
• A full moon increased mosquito activity 500% in one study

Additional Resources

• Wild Backpacker - Insect Repellent Buying Guide
• Wild Backpacker - Insect Repellent Active Ingredients
• REI: Expert Advice - How to Choose Insect Repellent

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