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It’s Mosquito Season: The 411 on Repellents

By Brenda Goodman, MA
WebMD Health News Reviewed by Brunilda Nazario, MD
May 27, 2016 -- Thwack. They’re baaaack. Yep, it’s mosquito season again.

It’s not just about itchy bumps, either -- mosquito bites can make you sick, especially if you’re traveling. Think Zika, chikungunya, West Nile, dengue, or even malaria or yellow fever if you’re going to some parts of Africa.

That means you need some insect repellent -- but surprisingly, lots of people don’t use it. An April 2016 survey done by market research firm TNS Global found that only about half of Americans (49%) follow the CDC’s recommendation to use a mosquito repellent.

Zika poses a particular danger to pregnant women, since it causes birth defects. As it creeps northward from South America and the Caribbean, health officials’ mission is to get the message out loud and clear: Mosquito repellents are safe, and you should use them.

And there are more choices than ever.

The product that’s right for you will depend on why you need it. Are you pregnant or traveling to an area with a lot of mosquito-borne disease? Are you fishing or camping and handling lots of gear? Are you applying it to a young child’s skin? All those things should factor into your buying decision.

We reached out to mosquito experts for advice on choosing and using these products. Here are their rules for making mosquito repellents work for you.

What’s the best active ingredient?

According to the CDC and EPA, there are four ingredients to look for: DEET, picaridin, oil of lemon eucalyptus or PMD, and IR3535.

DEET is the granddaddy of bug stoppers. It was developed by the U.S. military in 1946, and it’s the most widely used and studied active ingredient out there. It's long gotten a bad rap as a scary chemical, but experts say that reputation isn’t justified, even for pregnant women. Extremely high doses have, on rare occasions, caused nervous system problems like seizures, tremors, and slurred speech, though, so it’s still worth handling with care.

It's found in products like Off! Deep Woods spray, Sawyer’s Ultra 30 Insect Repellent lotion, and 3M’s Ultrathon Insect Repellent lotions and sprays.
While no repellent has been studied extensively in pregnancy, DEET at least has a little data backing its safety. A study of nearly 900 pregnant women in Thailand, which followed moms and babies for a full year after birth, found no harmful effects of DEET used on the skin during the second and third trimesters. Animal studies have also failed to find any ill effects of DEET to a fetus when applied at any stage of pregnancy.

No other active ingredient has been studied during pregnancy. For that reason, some doctors say DEET should be your go-to if you’re expecting. The EPA, on the other hand, puts no restrictions on the use of any repellents during pregnancy.

But DEET has its drawbacks, too. It can be heavy and feel oily on the skin. And it has a pungent odor. It also melts plastic, which is bad news for fishing and camping gear, synthetic fabrics like spandex, or even a nice pair of sunglasses. So, if a heightened sense of smell is keeping you from putting it on, or you don’t want it to ruin your gear or clothes, choose a different option.

Picaridin is a chemical cousin of piperine, a chemical made by black pepper plants. It's been widely used in Europe and Australia, but has only been available in the U.S. since 2005. It can be found in sprays, lotions, and wipes, including Sawyer’s Premium Picaridin Insect Repellent lotions and sprays, Avon’s Skin So Soft Bug Guard Plus Picaridin Pump, and Cutter Advanced Insect Repellent Fragrance Free Wipes. In recent tests, Consumer Reports gave two picaridin products top marks for repelling the species of mosquitoes that carries the Zika virus for 8 full hours.

The chemical PMD (for para-menthane-3,8-diol) is also sometimes labeled as oil of lemon eucalyptus. It’s a chemical copy of a pungent oil made by the lemon eucalyptus tree. It can be found in Cutter Lemon Eucalyptus spray and Repel Lemon Eucalyptus. In Consumer Reports’ tests, the Repel product, which contains 30% PMD, kept mosquitos away for 7 hours.
IR3535 has been used in Europe for 20 years, but was only registered in the U.S. in 1999. It’s a close chemical cousin to the amino acid B-alanine. Like DEET, it can harm plastics and synthetic fabrics. Manufacturers also warn that it can mar painted surfaces, including a recent manicure. No IR3535 products made Consumer Reports’ list of recommended repellents.

How strong should your repellent be?

Depends on how long you need it to last.

The first thing you’ll notice when you start checking labels is that insect repellents come in different strengths. You can find concentrations of DEET, for example, ranging from 4% up to 100%.

Do you really need a product that’s pure DEET? Experts say no, but you want to avoid the lowest concentrations, too. Here’s why.

After being sprayed or rubbed on the skin, the active ingredients in repellents begin to evaporate, creating a chemical cloud -- or "vapor barrier" -- that hovers around you and keeps mosquitoes at bay.

It takes a concentration of about 20% DEET to create a strong-enough barrier, especially in an area that has a lot of mosquito-borne diseases. Higher concentrations help the barrier last longer. But that protection maxes out around 50%. Any more than that exposes you to more chemical, but doesn’t provide more protection.

A product with 20% to 50% DEET should give you 6 to 13 hours of protection, depending on weather conditions.

Repellents with 30% PMD or oil of lemon eucalyptus keep mosquitoes away for 4 to 6 hours.

Products with at least 20% picaridin last for about 6 hours.

And repellents with at least 20% IR3535 work for 7 to 10 hours, but they don’t work as well against the species of mosquito that carries malaria. So, pick another kind of active ingredient if you’re going to an area where that’s a problem.

Time-released lotions use a lower concentration of active ingredient, but they can protect you longer.

What about natural or homemade repellents?

When fighting mosquitoes, “natural” is not the way to go. In recent tests, Consumer Reports found that four repellents made with essential oils of citronella, clove, lemongrass, or rosemary failed to keep the bloodsuckers at bay for even an hour. (PMD, or oil of lemon eucalyptus, is not considered “natural,” since it’s a chemical copy of the oil.)
Other home remedies to be wary of include Listerine, which may give you minty fresh breath but doesn’t block mosquitoes if you rub it on your skin, and garlic pills or B vitamins, since there’s no evidence they offer any protection.

Spray vs. lotion

Sprays work faster, says Joseph Conlon, a technical advisor for the American Mosquito Control Association.

“Lotions will take approximately 20 minutes to allow the concentration of repellent layer above the skin to exert its effect,” he says.

But lotions can have other advantages. Some have time-released ingredients so that a lower concentration of active ingredient can offer longer protection, which reduces a person’s chemical exposure, too.

Guarding against the sun and bugs at the same time?

A few studies have shown that using a bug repellent with a sunscreen can make both products less effective. But sometimes you need both. So, what do you do?

The CDC suggests starting with sunscreen, then putting mosquito repellent over it. Keep in mind that you may need to reapply the sunscreen more often than you normally would and that your bug spray may wear off more quickly than expected.

The CDC says it’s not a good idea to use products that combine a sunscreen with a repellent, since sunscreens need to be reapplied more often than repellents. Using a combination product frequently could result in higher-than-needed doses of repellent chemicals.

Do I really need a new bottle?

Maybe not. But before you fish out the rusty can you keep stashed with the camping gear, keep in mind that most products are optimally effective for 3 years after you buy them, according to S.C. Johnson, the company that makes OFF. And that’s only if they haven’t been exposed to extreme heat or cold. Most products will continue to work after that, but they may not last as long.

Is it safe to put insect repellents on kids?

Yes. Products with DEET can be used on babies older than 8 weeks of age. Picaridin is considered safe for kids over the age of 2, and products with PMD/oil of lemon eucalyptus are considered safe for kids older than 3. There’s no safety data on the use of IR3535 for kids.
Be sure to read the directions before applying. One survey found as many of a third of parents apply insect repellents incorrectly. A few “don’ts”:

Don’t apply to kids’ hands, since they often put them in their mouths.
Don’t apply before kids get dressed. Insect repellents should only be used on exposed skin and clothing. It doesn’t do any good to use it under clothing.
Don’t forget to wash off at the end of the day with soap and water. It’s not a good idea to sleep with repellents on, since this increases the absorption of the chemicals.
Why didn’t my repellent work?

According to experts, the No. 1 reason repellents don’t work is because they were put on incorrectly, so read the directions and apply thoroughly.

“Most repellents have pretty clear instructions on the label about how to apply them, but people never, ever read the label,” says Jonathan Day, PhD, a professor at the University of Florida’s Medical Entomology Laboratory in Vero Beach, FL.

If you did everything correctly and you still came home covered in bites, there might be another explanation.

There’s emerging science to suggest that mosquitoes can become resistant to DEET and perhaps other insect repellents, too.

James Logan, PhD, is the director of ArcTec (Arthropod Control Product Test Centre) at the London School of Hygiene and Tropical Medicine in the U.K.

He applied DEET to a human arm and then let a cage full of mosquitoes approach to feed on it. Most mosquitoes stayed away, but 13% of the females weren’t repelled by the DEET. He captured and bred those mosquitoes and then repeated the experiment with their daughters. In the next generation, about half of the mosquitoes weren’t repelled by DEET.

In another set of experiments, he unleashed the bugs to a human arm covered in DEET. Then he waited 3 hours and exposed those same mosquitoes to a DEET-covered arm again. On the second try, more than twice as many mosquitoes landed and fed as they had the first time, suggesting that they had somehow gotten used to it.
“They can become resistant through genetics and behaviorally as well,” Logan says.

He’s quick to point out that his experiments were done in a carefully controlled lab, and not in a house or backyard. Researchers don’t know if mosquitoes in the wild are becoming resistant to DEET.

“What we don’t know is whether this is occurring in the field. Nobody has ever looked at it,” he says. He’s working to answer the question now.

It’s possible, he says, that people might need to use two different kinds of repellents at the same time to boost their effectiveness.

“The point is that repellents should be used. They should be recommended. There should be no scare mongering about the fact that DEET may fail. That is not necessarily the case. As scientists, we need to be monitoring the situation and staying one step ahead of the game, and not making assumptions that it will be OK. Mosquitoes have a very good way of being one step ahead of us and finding a way 'round the tools we have to control them.”

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