This guest post was written by Matt Shipman, a science writer and public information officer at North Carolina State University. He blogs at Communication Breakdown on SciLogs, and you can follow him on Twitter at @ShipLives.
For parents, summer is when we take our kids on hikes, get them to play in the yard or take them to the pool. All that time spent outdoors increases the likelihood that we’ll come into contact with mosquitoes, ticks and other pests. And bug bites aren’t just annoying; they can transmit diseases ranging from malaria to West Nile virus.
So to help keep our kids safe (and itch-free), we apply insect repellent. But a lot of parents want to know whether the repellents themselves are safe. And how do they work, anyway?
Which insect repellents actually work?
The U.S. Centers for Disease Control and Prevention (CDC) lists several mosquito repellents as being at least moderately effective at repelling mosquitoes, based on data provided by the Environmental Protection Agency (EPA). However, the CDC lists only two of those repellents as providing “longer-lasting” protection: DEET and picaridin. (And, it’s worth noting, the words “EPA approved” are no guarantee of effectiveness.)
Research has shown that neither is 100% effective, but they are pretty good at keeping blood-sucking arthropods at bay. In short, they are the best (and most common) active ingredients used in U.S. mosquito and tick repellents. (And don’t bother with electronic devices that say they repel insects. They don’t work. For that matter, don’t bother with citronella candles either – unless you want to sit directly in the plume of smoke they put out.)
How do they work?
How does DEET work? That is such a good question that scientists have been arguing about it more or less since the U.S. Army developed it in the 1940s. For a long time, many researchers thought DEET worked by preventing mosquitoes from smelling the chemicals that humans produce, essentially making people “invisible” to the bloodsuckers.
But a 2008 paper published in the Proceedings of the National Academy of Sciences reported that mosquitoes could smell DEET – and tried to avoid it. And in 2011, researchers reported in Nature that DEET confuses the way insects smell things – so that the blood-feeders don’t respond to odors that would normally trigger feeding behavior. And, of course, it may be some combination of all of those things.
Picaridin is much newer – though I couldn’t determine just how new: There are research references to the chemical at least as far back as 1999, and references to its use as an insect repellent dating to 2000, though it has only been available in the U.S. since 2005. And there appears to be even more uncertainty about how it works than there is with DEET. The best I could find was that “it seems to block mosquitoes from sensing their prey” (from the National Pesticide Information Center.)
Are these chemicals safe for my kids?
If your question is “Should I use insect repellent if my kids are going to be outside?” the answer is yes. The risks associated with bug bites are significant.
Mosquitoes and ticks are responsible for transmitting a host of diseases. Around 1,500 people are diagnosed with malaria every year in the U.S., nearly all cases in people who have returned from traveling abroad**. In 2012, 5,674 people contracted West Nile virus – of whom 286 died. And more than 19,000 people have contracted Lyme disease every year since 2004. And those are only a few of the diseases that can be transmitted by ticks and mosquitoes.
There are other risks as well, such as allergic reactions to mosquito bites, which are more common among infants and young children. And, of course, the skin infections (such as impetigo) that can result from kids scratching their itchy bug bites until they’re raw.
But are repellents totally harmless? No.
One thing to bear in mind, and I can’t stress this enough, is that you should follow the instructions on the insect repellent packaging when it comes to how much – and how often – you apply the repellent to yourself or your kids. I asked EPA for some input on this, and the agency sent me this statement: “Application guidelines and directions for safely using a particular insect repellent product (including how to apply the product and when to reapply, restrictions on how often to apply and directions for using on children) can be found on individual product labels, as they’re specific to that product’s specific formulation. Labels are the consumer’s guide to using that pesticide safely and effectively. Consumers are always advised to read and follow label directions in using any pesticide product, including insect repellents. A critical aspect of registering a pesticide product is the approval of the product label.”
EPA also has an entire fact sheet on how to use DEET-based products safely that is worth checking out. For example, the fact sheet notes that you should not apply insect repellent on or near the hands or mouth of young children. And you shouldn’t apply the repellent to areas where there are cuts, wounds or irritated skin. (And there’s more on pesticide product labels here.)
EPA also said in its statement that “DEET and picaridin are approved for use on children with no age restriction. Based on the available toxicological data, both chemicals have low acute toxicity, and the agency believes that the normal use of these chemicals, when used according to the label directions, do not present a health concern to the general U.S. population, including children. Products containing either repellent have directions for use on how to apply the products to children. The labels specify the reapplication interval. The reapplication interval is not based on age. For instance, a 20% picaridin product should not be applied more than twice per day to adults or children. As with all pesticide products, labeling for both chemicals indicate that children should not handle the product. Only adults should make applications to children.”
But there are some worrisome studies out there on DEET and picaridin – until you dig a bit deeper.
A 2001 study published in Human & Experimental Toxicology reported extremely rare instances of DEET exposure “causing” encephalopathy (brain disease or swelling), seizures and – in some cases – death in children. The authors of that study noted that “surprisingly, [mortality] did not correlate with the concentration of the DEET liquid used, the duration of skin exposure, or the pattern of use,” a finding which casts some doubt on the certainty that DEET caused all the incidents. And the cases were extremely rare – three deaths out of these 18 children occurred – and these were the only cases recorded worldwide. Five of the cases involved oral DEET exposure though all three deaths* followed skin exposure.
In fact, in a more recent review of DEET in early June in Parasites and Vectors, the cases in the 2001 paper were among those discussed when the authors noted that “the role of DEET in either the illness or deaths was and remains purely speculative.” Despite the use of the term “DEET-induced encephalopathy,” they wrote, “no link to dose or mechanistic pathway has been demonstrated between the use of DEET and the occurrence of encephalopathy.” In other words, it’s never been shown how DEET might have physiologically caused those incidents, and there is not enough evidence to say it did.
This 2014 review – which is open access, so anyone can read the full paper without a paywall – the researchers noted, “Animal testing, observational studies and intervention trials have found no evidence of severe adverse events associated with recommended DEET use,” and “The safety surveillance from extensive humans use reveals no association with severe adverse events.” Their main takeaway comes down to a risk-benefit analysis: “The theoretical risks associated with wearing an insect repellent should be weighed against the reduction or prevention of the risk of fatal or debilitating diseases including malaria, dengue, yellow fever and filariasis.”
Millions of people use DEET each year, and the risk of seizures like those cited in the 2001 paper, if even caused by DEET, is very, very low. EPA stated in a 1998 review of DEET that “the observed incidence of recognized seizures is about one per 100 million users” – equivalent to three people in the entire U.S. each year if every resident applied repellent with DEET. That’s literally several thousand times lower than the risk of contracting a disease via mosquito or tick bites, and those diseases are awful. Malaria and West Nile Virus are often lethal. Even when they don’t kill you, they can cause incredible suffering – including fever, seizures and coma.
A 2003 review paper of research into DEET toxicity notes that Canada’s Pest Management Regulatory Agency found that children age 12 or younger should not use products containing more than 10% DEET. But the paper also reports that the EPA “concluded that there was no evidence that would lead the agency to conclude that DEET is uniquely toxic to infants and/or children.”
The CDC has even urged women who are pregnant or breastfeeding to use repellents containing DEET, in order to limit the risk of transmitting West Nile virus. And a 2001 paper in the American Journal of Tropical Medicine and Hygiene found no adverse effects among women who were exposed to DEET during pregnancy, though there was some evidence of DEET in the cord blood of some study participants.
Finally, a 2008 paper in Regulatory Toxicology and Pharmacology assessed the risks associated with both DEET and picaridin. Here’s my interpretation of the findings:
- Do NOT use products with 40% DEET on children under the age of 12
- For chronic use – something you’d apply daily or several times a week – don’t use products containing 25% DEET on children under 12
- For chronic use of picaridin, don’t use products with more than 15% picaridin on children 17 years and younger.
- For acute or infrequent use, products containing 15% picaridin (or less) posed no significant risk for any age group.
Like the authors of the 2014 paper, the authors of this 2008 paper also stressed that potential risks of use need to be balanced against the health risks associated with insect bites.
In short, you should probably still use insect repellent. There are a ton of insect repellents on the market, some of which target kids. If the repellents don’t contain any of the ingredients listed as effective by the CDC, you may want to avoid them. And don’t trust that a repellent marketed for kids is necessarily safer. Try to avoid products that contain 25% DEET or more, particularly for younger kids, and to avoid products that use 15% picaridin or more. If you’re still worried about what to use, talk to your pediatrician.
Note: Special thanks to Andrew Maynard, NSF International Chair of Environmental Health Sciences at the University of Michigan School of Public Health and director of the University of Michigan’s Risk Science Center, for helping me understand some of the technical language used in the risk assessment literature. He shares credit for everything I got right, but any mistakes are mine alone. Additional reporting regarding the 2014 paper – and its impact on interpretation of the 2001 case series study – came from Tara Haelle (who is therefore responsible for those mistakes).
*Among the three deaths, one was a 5-year-old in South Africa who had had 10% DEET repellent applied every night for three months; one was a 6-year-old in Canada who had 15% DEET repellent applied 10 times but who also had either Reye syndrome or a congenital health deficiency; and the last was a 1-year-old in Uruguay who had 10% DEET repellent applied daily for three weeks. Again, causation was not definitely established as the link between DEET exposure and death.
**The post has been corrected to note that the malaria cases in the US are typically travelers returning from overseas with the disease, not US residents contracting the disease from within the US.
“Field Evaluation of Repellent Formulations Containing DEET and Picaridin Against Mosquitoes in Northern Territory, Australia,” S. P. Frances, D.G.E. Waterson, N.W. Beebe and R.D. Cooper, Journal of Medical Entomology, Vol. 41, No. 3 (May 2004), pp. 414-417. DOI: 10.1603/0022-2585-41.3.414
“Mosquitoes smell and avoid the insect repellant DEET,” Zainulabeuddin Syed and Walter S. Leal, Proceedings of the National Academy of Sciences, Vol. 105, No. 36 (Sept. 9, 2008), pp. 13598-13603. DOI: 10.1073/pnas.0805312105
“A natural polymorphism alters odour and DEET sensitivity in an insect odorant receptor,” Maurizio Pellegrino, Nicole Steinbach, Marcus C. Stensmyr, Bill S. Hansson and Leslie B. Vosshall, Nature, 478 (Oct. 27, 2011), pp. 511-514. DOI: 10.1038/nature10438
“Toxic encephalopathy associated with use of DEET insect repellents: a case analysis of its toxicity in children,” G. Briassoulis, M. Narlioglou and T. Hatzis, Human & Experimental Toxicology, Vol. 20, No. 1 (Jan. 2001), pp. 8-14. DOI: 10.1191/096032701676731093
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“Safety of the insect repellent N, N-Diethyl-M-Toluamide (DEET) in pregnancy,” Rose McGready, et al., American Journal of Tropical Medicine and Hygiene, Vol. 65, No. 4 (2001), pp. 285-289.
“Risk assessments for the insect repellents DEET and picaridin,” Frank B. Antwi, Leslie M. Shama and Robert K.D. Peterson, Regulatory Toxicology and Pharmacology, Vol. 51, Issue 1 (June 2008), pp. 31-36. DOI: 10.1016/j.yrtph.2008.03.002
“Evidence for natural desensitization to mosquito salivary allergens: mosquito saliva specific IgE and IgG levels in children,” Zhikang Peng, Man Ki Ho, Caihe Li and F. Estelle R. Simons, Annals of Asthma, Allergy and Immunology, Vol. 93, Issue 6 (Dec. 2004), pp. 553-556. DOI: 10.1016/S1081-1206(10)61262-8
“Assessment of methods used to determine the safety of the topical insect repellent N,N-diethyl-m-toluamide (DEET),” Vanessa Chen-Hussey, Ron Behrens and James G Logan, Parasites & Vectors, Vol. 7, Issue 1 (June 2014). DOI: 10.1186/1756-3305-7-173