How to Choose Insect Repellent

Bio

Bugs are annoying, potentially even hazardous. To keep them away, DEET is considered the most effective repellent ingredient, but other options (picaridin, IR 3535, citronella and others) are also available.

Based on our own survey of repellent-makers, product-testers, outdoor-loving colleagues and other bug-bedeviled souls, we offer the following advice (Opinions and Recommendations) followed by the research and rationale that led us to these views. We offer them to help guide you as you choose a repellent that is best for your circumstances.

Opinions and Recommendations

Comparison Chart of Active Ingredients (Conventional vs. Natural)

Q: What active ingredient stays effective the longest?

A: The champion is DEET—in a controlled-release formula (which typically involves a 20% to 34% concentration of the ingredient) or in high concentrations (30% to 100%). Controlled-release formulas have a slight edge in duration of effectiveness.

DEET (Diethyl Toluamide) is a chemical. To some people, the thought applying it their skin makes them uneasy. Yet DEET has been in use since 1957 and has been repeatedly tested for toxicity. The human body does absorb DEET, but is believed to eliminate it through urine within 24 hours. The Centers for Disease Control and Prevention (CDC) recommends the use of DEET and considers it safe when applied according to label instructions.

DEET is the only active repellent ingredient available in high concentrations (30% to 100%) in the U.S. Why? Only DEET's higher concentrations have been screened extensively for toxicity, a lengthy and expensive process. Such data is required by the Environmental Protection Agency (EPA), which regulates repellent ingredients in the U.S.

In 2008 an alternative to DEET, a 20% formula of the chemical picaridin (p-CARE-i-din) became available in the U.S. Previously it had been available only in a 7% solution. Picaridin's advantages over DEET: It does not damage gear (in particular, DEET negatively impacts plastics), usually works better against flies and, in the opinions of many people, smells better. DEET in controlled-release formulas and higher concentrations still outlasts it.

In lower concentrations, active ingredients (DEET included) provide shorter-term effectiveness. The duration of effectiveness will vary by active ingredient.

For example, in an often-cited study published in the New England Journal of Medicine in 2002, DEET at 6.7% was effective for 112 minutes before the first bite was registered. Citronella, the first widely used "natural" active ingredient (and one that is fading in popularity today), at 10% yielded a bite after 19.7 minutes. A 2004 study produced similar results.

What does that mean for you? Base your choice on the amount of time you plan to spend outside. If you are planning a vigorous all-day hike, it makes sense to choose long-duration repellent that contains a higher concentration of an active ingredient—you may have no need to reapply the repellent. If you are planning just a short walk or a casual backyard activity, you could choose a low-concentration repellent—usually effective for 1 to 4 hours.

Q: What if I'm planning an extended stay in an area that is dense with insects?

A: A good strategy is to give bugs a double-whammy:

1. Carry a long-duration repellent. Choose either a controlled-release formula (examples: Sawyer Controlled Release, REI Ultra 30, 3M Ultrathon) or repellents with high concentrations of an active ingredient, and the king of that category is 100% DEET (found in REI Jungle Juice 100, Repel 100, Ben's 100 Max). Both can provide 10-plus hours of protection from a single application. Controlled-release formulas, though, are the overall endurance champs and can potentially deliver up to 14 hours of protection—even up to 20 hours, according to one repellent-maker—against most Culex mosquito species, the primary carriers of West Nile Virus. Shop REI's selection of DEET-based insect repellents.

2. Wear bug-repelling clothing. Clothing and hats treated with permethrin (a safe-for-humans insecticide explained later in this article) dissuades insects from surreptitiously landing on out-of-sight body zones (shoulders, backs and calves, for instance) and attempting bites through clothing. Ex Officio makes a popular line of pretreated clothing known as Insect Shield®; do-it-yourselfers can treat their own clothing using permethrin products from Sawyer.

This 2-pronged approach can be a real sanity-saver when bugs are thick, even though mosquitoes will likely still hover annoyingly close on the fringes of the invisible force field your repellent-permethrin combo has erected. Need even more of a buffer zone? Add a headnet and body netting; both can be treated with permethrin.

Q: How can I best defend myself against West Nile virus and Lyme disease?

A: Here are recommendations from the CDC:

West Nile virus (carried by mosquitoes): Use repellents containing DEET, picaridin, IR 3535 or oil of lemon eucalyptus and wear clothing treated with permethrin.

Lyme disease (carried by ticks): Use repellents containing higher concentrations of DEET (at least 20%) and wear clothing treated with permethrin.

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: Statistics suggest the answer is yes. In 2008 West Nile virus, carried by some mosquitoes, was detected in 45 states (the exceptions: Alaska, Hawaii, Maine, New Hampshire and Vermont), with 1,356 reported cases and 44 fatalities. That's an encouraging decrease from 4,261 cases and 174 fatalities reported in 2006. However, in 1999 only one state (New York) had reported a human case.

As for tick-transmitted Lyme disease, 27,444 cases were reported in the U.S. in 2007 (the most recent year with published statistics available at the time of this update). That's an increase from 23,305 cases reported in 2005.

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: 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 a heavier dosage of repellent controlled-release DEET or 100% DEET than you would at home. If using a lower-concentration version of DEET, reapply regularly.

Other mosquito-transmitted diseases include yellow fever (for which a vaccine is available; it is required for entry into some countries), dengue fever, Japanese encephalitis and chikungunya fever, making repellent (and bug-repelling clothing) an important travel accessory in some areas of the world. Check for airlines restrictions regarding aerosol containers. For info on your travel location, visit www.cdc.gov/travel.

Tip:In international locales, hang or lay permethrin-treated items around your room or sleeping quarters. Doing so many eliminate some pests overnight while you sleep. That beats leaving them packed within your luggage where they cannot benefit you.

Q: What are my choices of active ingredients?

Conventional Active Ingredients in Detail

DEET

(N,N-diethyl-3-methylbenzamide, or N,N-diethyl-m-toluamide.)

Pros:

• Globally regarded as the gold standard of repellents.
• Developed for military use in 1946; approved for public use in 1957.
• Regarded as the longest-lasting, most effective active ingredient in the insect-repellent arsenal; some formulas provide up to 14 hours of protection.
• Used safely by humans in small or moderate amounts for 5 decades; has been the subject of numerous studies since its introduction.
• According to the American Academy of Pediatrics, suitable for children at least 2 months old in concentrations of up to 30% when used according to directions on product labels.

Cons:

• DEET is a "plasticizer." It can have adverse affects on many synthetic fabrics (such as rayon or spandex), auto paint, watch crystals, varnished surfaces, golf-club grips, leather and plastic. It will not damage cotton, wool or nylon.
• A potentially objectionable odor. Interestingly, N,N-diethyl-m-toluamide in its pure form is odorless. When it comes in contact with impurities during production is when an odor is generated. Sawyer says it uses an extra refining process to reduce or rid its repellents (and those it manufactures for REI) of such impurities, though usually an odor is still detectable.
• Less effective against some flies. For problematic flies, consider repellents that use:
  • Picaridin
  • IR 3535
  • DEET combined with a fly-targeted ingredient such as R-326.
  • DEET combined with permethrin-treated clothing, which some experts regard as the supreme bug-fighting combo—DEET (widely regarded as the best mosquito deterrent) plus the fly-and mosquito-repelling punch of permethrin.

Picaridin

(KBR 3023.)

Pros:

• Developed in Europe and the U.S. in 1998; approved for use in U.S. in 2005.
• Effective against mosquitoes and ticks; more effective against flies and biting midges than DEET.
• Studies have shown the 20% formula can remain effective for up to 14 hours against mosquitoes, 8 hours for sand flies and biting midges.
• Causes no damage after contact with synthetic fabrics or painted/varnished surfaces.

Cons:

• Fragrance elicits varying responses; though many have no trouble tolerating its odor/fragrance, occasionally some wearers find it unappealing.

IR 3535

(3-[N-Butyl-N-acetyl]-aminopropionic acid, ethyl ester.)

Pros:

• Used in U.S. since 1999 marketed in Europe in early 1980s (where it is known as Merck 3535) and in Asia in the 1990s.
• Effective against mosquitoes, deer ticks and biting flies; potentially may also repel lice, bees and wasps.
• Registered as a biopesticide with EPA, meaning it can be viewed as a natural product, though it is a synthetic substance structurally based upon a naturally occurring amino acid (B-Alanine).
• Exhibits lower toxicity than other active ingredients.

Cons:

• Some researchers consider its duration of effectiveness as unpredictable.
• May damage plastics, but it falls far short of the corrosive potential of DEET; technically, it is considered a "virtual non-plasticizer," but the EPA will not allow it to be marketed with the phrase "Does not harm gear." Plastics should show little or no impact if IR 3535 is wiped off shortly after contact.
• Limited availability in U.S. At REI in 2009, it is available only in sunscreen/repellent combos. It is expected to become more widely available in 2010.

Mechanical Deterrents

(Products such as smoldering mosquito sticks, coils, bug zappers and wrist bands.)

Pros:

• Skin remains free of lotions and sprays.

Cons:

• Level of protection is irregular, unpredictable.

Natural Active Ingredients in Detail

Herb-based formulas

(Ingredients range from soybean oil to geranium oil to crushed chrysanthemums.)

Pros:

• Effective against mosquitoes and ticks.
• Often a less objectionable odor compared to DEET.
• One executive at a company that makes only conventional repellents regards geranium oil to be the most effective natural repellent "by far." An example of a repellent based on geranium oil: BugBand Geraniol Pump Spray.

Cons:

• Duration of effectiveness is unpredictable.

Oil of Lemon Eucalyptus

(Natural oil is extracted lemon eucalyptus leaves and twigs, then synthesized for use as a repellent; it is also known as PMD, with the chemical name para-Menthane-3,8-diol.)

Pros:

• Used in the U.S. since 2000.
• Effective against mosquitoes, ticks and some flies.
• Causes no damage after contact with synthetic fabrics or material surfaces.

Cons:

• A strong, distinctive odor.
• Label instructions advise applying it to skin no more than twice a day. may irritate sensitive skin. Accordingly, it is not recommended for repeated use during a prolonged excursion such as a multiday hike.
• Not for use on children under age 3.
• Duration of effectiveness is fair (up to 4, sometimes 6 hours).

Citronella

(Natural oil is derived from dried cultivated grasses.)

Pros:

• Used in the U.S. since 1948.
• Marginally effective against mosquitoes, ticks and some flies.

Cons:

• A potentially objectionable odor.
• May irritate some sensitive skin.
• A faint risk to stain some fabrics.
• Duration of effectiveness is unpredictable, though some products claim, perhaps optimistically, up to 6 hours of protection.

Permethrin: A Repellent for Fabrics

Permethrin

(A chemical insecticide; applied to clothing either by soaking or spraying.)

Pros:

• Used in the U.S. since 1979.
• Effective against ticks, mosquitoes, mites and up to 50 other insects.
• An application can last up to 12 washings (particularly if the treated item is hand-washed (and line-dried) or machine-washed and dried on the gentle cycle); Insect Shield® which uses a specialized heating process to melt waxy permethrin molecules into the fibers of its clothing, says its apparel can accept up to 70 washings. (An option to extend permethrin's effectiveness for anyone self-treating their own clothing: Apply permethrin a little heavier than directed in product instructions.)
• Won't stain fabrics; almost never irritates skin.
• Does not interfere with the wicking capabilities of technical fabrics.
• Considered to be biodegradable; shown to not accumulate in the environment.
• In some circumstances may preclude the use of topical repellents, especially if ticks are the main concern.
• Repels insects; as an actual insecticide, can kill insects that contact the treated clothing and bed-netting.

Cons:

• Avoid flushing permethrin in its liquid form into a sewer system or water supply. Once dry, however, there is no risk when washing a permethrin-treated garment.
• Keep permethrin in its liquid form away from cats. Cats are fine around permethrin-treated fabrics once permethrin has dried. Dogs are fine around permethrin wet or dry. Permethrin, in fact, is registered for direct application on dogs and is believed to remain effective for 2 weeks. Another option: Treat pet beds with permethrin to combat ticks.

Tip: Plan ahead. Apply permethrin to fabric well in advance of a trip and allow it to fully dry before wearing or using it. And, as suggested earlier, hang permethrin-treated items in living quarters in bug-dense international areas. It can serve as a sort of nonelectric bug-zapper.

Why not just wear repellent on skin underneath clothing? Doing so reduces the effectiveness of repellent, disrupting the bug-disorienting vapor barrier repellents create on skin (explained later in this article). The preferred tactic is to wear permethrin-treated clothing.

In areas where insects are especially bothersome, consider treating tents, even backpacks, with permethrin. Since those items are infrequently laundered, permethrin's duration of effectiveness can be quite good.

Consider treating strollers, even baby blankets, with permethrin. Its presence may make topical repellents unnecessary on small children.

Insect Repellent FAQs

Q: How safe is DEET?

A: Much evidence exists:

• Lab tests have shown that skin absorbs DEET, but is eliminated from the body through urine (the highest concentrations being expelled several hours after application).
• Human and animal studies indicate that DEET does not accumulate in the body.
• DEET-related illnesses have been reported in medical literature, but typically because DEET was used inappropriately, excessively and/or over a prolonged time.
• It is estimated that each year humans experience millions of applications of DEET with the only recurring complaint involving its odor.
• Some medical professionals wonder about chemical exposure and a child's brain development. One study using rats indicated DEET produced a neurological effect; several other studies showed no neurological link.
• No repellent, DEET-based or otherwise, should be applied close to the eyes or on broken skin.

Note: DEET and all active ingredients described in this article are registered with the EPA. They have been evaluated for safety and, in most cases, efficacy when used according to label instructions. This is a key in appropriate use of insect repellents: Follow label instructions.

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%. As noted earlier, 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: 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 to skin will diminish the effectiveness 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. Preferably, us a lotion-based repellent; it is speculated that the alcohol used in repellents that offer, for example, 20% DEET or 20% picaridin, negates the effectiveness of sunscreen. A few sunscreen/repellent combos exist.

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.
• In one study, a full moon increased mosquito activity 500%.

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: 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 (which manufactures REI-brand repellents for REI) 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: Should I use lotions or sprays?

A: Sprays may allow active ingredients to evaporate faster than lotions, reducing the duration of their effectiveness. Sprays, though, can be useful when spraying repellents on clothing (if insects are biting you through your shirt, for example). Remember, however, that DEET (but not picaridin) can damage most synthetic fabrics. DEET does not damage cotton, wool or nylon.

Alcohol is used to "cut" DEET, picaridin and IR 3535 in lower-concentration repellents (such as repellents marketed with a 20% concentration of an active ingredient). Alcohol usually causes skin pores to open wider, possibly allowing the body to absorb more of the repellent's active ingredient. (Alcohol may irritate some sensitive skin types as well). This may not appeal to those who use a conventional (chemical-based) repellent. Water-based sprays, which are far less common, do not have the same effect.

These factors, Avery says, makes a stronger case for 100% DEET for anyone using liquid repellents. "One-third of a dosage of 100% DEET is the same as a normal dose of 30% DEET without the negatives of alcohol," he says.

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 (example: REI Ultra 30; estimated duration: up to 11 hours), 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.

Summary

• When choosing a repellent, consider 1) the primary active ingredient (it could be DEET, picaridin, a natural product or something similar), and 2) the concentration of that ingredient. Also take note of any claim made about what insects are targeted.
• Active ingredients in higher concentrations or controlled-release formulas last longer; low-concentration repellents are suitable for shorter-term activities such as a backyard get-together.
• DEET, used since 1957, is viewed as the most effective, longest-lasting ingredient available; other options (including natural ones) are also effective, but for shorter durations.
• Studies have shown no evidence that occasionally absorbing small amounts of DEET has harmful effects on humans.
• A combination of permethrin-treated clothing and topical repellents offers your most comprehensive package of defense against insects.
• Children older than 2 months can safely use repellents with active ingredients in concentrations of up to 30%.
• Despite diligent bug-repelling efforts, in heavily infested areas some bites still may occur. Skin irritation can be reduced with the use of treatment products such as Adventure Medical After Bite.

Back to Top