When it comes to keeping away creatures that creep and crawl, the U.S. doesn’t mess around. More than 198 million Americans used insect repellent last year, a figure that is expected to top 200 million this year. Much of that is to ward off ticks, which are technically arachnids and carry more than a dozen diseases.
But the repellents themselves can present concerns of their own with their pungent smells, occasional irritation, and labels full of mystifying chemicals most people have never heard of.
With 2023 turning into one of the worst tick seasons in recent memory, plenty of people are looking for ways to repel the pests without quite so much commercial chemistry. The question is, do natural alternatives to chemical repellents even exist, and if they do, how effective are they? Here’s what to know about tick repellants.
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The most common ingredient in insect repellants is N,N-diethyl-meta-toluamide, better known as DEET. It was developed in the 1940s by the U.S. military and the Department of Agriculture. Scientists know it is effective at keeping pests away, but they haven’t yet figured out whether it’s because there is something in the odor of the chemical that repels the bugs or something that masks the attractive smell of humans and causes insects like mosquitoes or arachnids like ticks simply to fly or crawl past us.
“It was a long time ago that this chemical was developed, and we still argue about how it works,” says Aaron Gross, associate professor of toxicology and physiology at Virginia Tech.
Though it’s generally considered to be safe when used as directed—and the single most effective way to repel ticks—experts disagree on just how safe it is. “All chemical compounds pose some risk to the user,” says Gross. “But most of the concerns with DEET involve when it is used inappropriately”—in this case, he says, that means when it gets anywhere near the eyes, or is applied more than once a day.
Joel Coats, professor of entomology and toxicology at Iowa State University, is equally positive about the safety record of DEET. “Although literally billions of people have used [DEET] for a long time, I can count on one hand the number of serious health issues that have resulted,” he says. But there are some small caveats. “It can cause some minor issues, mostly skin irritation. Also, some people don’t like the smell or feel. And sometimes the chemicals can interact with clothing, melting certain synthetic fabrics.”
A chemical powerful enough to melt fabric could give some consumers pause, and both the U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) caution people about its risks. The CDC warns that it can cause rashes, blisters, and skin and mucus membrane irritation, and even seizures, agitation and aggressive behavior. The WHO lists it as a Class III pesticide, which is defined as “slightly hazardous.” Repellents that are meant for application to the skin may range in DEET concentration from 4% to 100%, but according to the CDC, concentrations over 50% provide no added protection from ticks.
In the world of repellants, it pays to read the labels carefully. Those that don’t contain DEET may contain the repellent picaridin.
A synthetic compound developed in the 1980s, picaridin is designed to resemble peperine, a class of chemicals found in plants that produce black pepper. But picaridin, Coats says, is a “DEET analog,” designed to work the same way as DEET. Picaridin may thus carry similar risks. The Environmental Protection Agency (EPA) classifies picaridin as a Category III toxin, which means mildly toxic to the skin and eyes and body as a whole if swallowed.
IR3535 is another DEET analog. It was developed in the 1980s and is thought to repel insects due to its odor—but, like DEET, it may also cause negative reactions in some people. One study in the journal PeerJ found IR3535 to be less toxic than DEET, but to still pose a risk of eye irritation.
Another effective way to keep away ticks is permethrin, which shouldn’t be used like a typical insect repellent but should instead only be sprayed on outerwear. Permethrin-based sprays are often confused with repellents, but they are in fact insecticides—toxic to insects and unsafe for humans, too. “Permethrin does have repellent activity,” says Coats. “But is not labeled for use on human skin—only on things like clothing, screens, and tents. If it contacts skin, there can be some irritant effect.” In addition, the EPA considers permethrin a “weak carcinogen,” meaning that the risk of cancer from exposure is low, but not non-existent.
Are there safer, just-as-effective alternatives?
The short answer: Not really.
Biopesticides are substances that are extracted naturally from plants but may be genetically altered to repel or kill bugs. All biopesticides are closer to nature than DEET and its chemical cousins and don’t carry the same risks—but none work as well, and none are truly “natural” in the sense that they may be lab-adulterated. “For ticks, our research has shown undecanone”—a biopesticide derived from the tomato plant—“to be the most effective.” Coats says.
Much closer to what most people think of as “natural” repellents are essential oils distilled from plants. Among the most popular essential oils used by chemical-averse people are clove, patchouli, peppermint, geranium, eucalyptus, lavender, cedarwood, lemon grass oil, amyris oil, and oil of citronella. The good news: essential oils can work—kind of. The bad news: they don’t keep you safe for all that long.
Unlike repellents that mask the human scent, essential oils’ effect comes from the fact that pests simply don’t like the way they smell. “There are studies that show that essential oils can work at specific receptors within the antenna, or at specific receptors within the nervous system,” says Gross. “If you’re measuring the electrical activity along those nerves and you blow an essential oil on there, you’ll get an electrical response. If it were masking the [human] smell, you wouldn’t expect to see an electrical response.”
Not every brand of essential oil is the same quality, and for people who try them, Gross recommends careful shopping. Some manufacturers of peppermint oil, for example, take the menthol out and sell it to toothpaste companies. “These essential oils will vary not only by the plant they’re distilled from and the region they’re grown, but by the manufacturer,” says Gross.
Another problem with essential oils is that not all of them work equally well. A person’s particular odor may affect how the oil smells. Among all of the essential oils, Coats has found that the ones that generally work best against ticks are lemongrass oil, amyris oil, oil of citronella, and peppermint. But all essential oils, no matter how well they work, have a common drawback: they are made of small, highly volatile molecules that don’t last long on the skin.
“The relatively fast evaporation limits their utility,” says Coats. “That requires frequent reapplication.”
Essential oils, like other repellents, are not entirely risk-free. Studies in 2007 and 2018 showed that compounds found in lavender and tea tree oil could act as endocrine disruptors, interfering with the body’s hormonal system. In the small 2007 case study, pre-teen boys who used tea tree and lavender oils developed oversized breasts—a condition that disappeared when they stopped using the oils.
Ultimately, using any tick repellent involves a balancing of risks, and Coats believes that when it comes to the chemical repellents, the choice is a relatively straightforward one. “The repellents on the market have all been judged safe for humans to use, when the directions are followed,” he says. “Ticks can transmit numerous nasty diseases, such that the health risks from the disease possibilities far outweigh the risks from using any of the repellents.”
Whatever repellent you use, you can limit your odds of a tick bite by taking certain precautions: avoid tall grass and shrubs, where ticks like to lurk; walk in the center of trails to avoid vegetation along the edges; wear long sleeves and pants when in the woods; tuck your pants legs into your socks; and wear light-colored clothing to make it easier to see ticks.
Write to Jeffrey Kluger at firstname.lastname@example.org.