Whether you're holidaying at home or going abroad this summer, the chances are, you'll be stocking up on plenty of insect repellent to keep those biting bugs at bay.
The active ingredient in most insect repellents is N,N-diethyl-meta-tolua-mide, or DEET, which has been fending off midges and mosquitoes for more than half a century. But, although DEET works, there are concerns over its use, especially in children. Studies show that it can cause a range of health effects-from allergic reactions to permanent neurological damage.
DEET is a pesticide intended to be directly applied to the skin to repel-rather than kill-insects. It is found in varying concentrations in numerous products, including aerosols, pump sprays, lotions, towelettes, roll-ons and wristbands. It's used by an estimated 200 million people around the world.
Yet, despite its long and widespread use, precisely how DEET works is still not clear. One of the more simple theories is that bugs dislike the smell, so they avoid it (Proc Natl Acad Sci U S A, 2008; 105: 13598-603).
However, what is unpleasant to bugs may be hazardous to humans. It is clear that DEET is absorbed through the skin and passes into the blood. According to one report, after six hours, 9-56 per cent of the dose will appear in the body's circulation (CMAJ, 2003; 169: 209-12).
For this reason, it's not surprising that the topical application of DEET has been linked to a variety of systemic adverse effects-and children appear to be especially vulnerable. In 2001, a review by researchers from the Aghia Sophia Children's Hospital in Athens, Greece, identified 17 reports of
DEET-induced encephalopathy (brain dysfunction) in children. The most common symptom was seizures, seen in 72 per cent of cases, and these
were most likely to happen when DEET solutions were applied directly to the skin (rather than ingested).
More worrying, even short-term exposure to low levels of DEET could cause serious reactions. In an 18-month-old boy, for instance, breathing problems and seizures developed after only a brief exposure to a product that was just 17.6-per-cent DEET. The report concluded that "repellents containing DEET are not safe when applied to children's skin" (Hum Exp Toxicol, 2001; 20: 8-14).
In adults, besides causing allergic-type skin reactions such as redness, itching, burning, blistering and, sometimes, scarring (Arch Dermatol, 1982; 118: 582-3; CMAJ, 2003; 169: 209-12), the topical application of DEET has also been linked to manic psychosis, cardiovascular toxicity, reproductive effects and even death (J Am Mosq Control Assoc, 1998; 14: 12-27; Int J Toxicol, 2002; 21: 341-52). It's even been implicated, along with other chemicals, in the disabling neurological syndrome seen in Gulf War veterans (JAMA, 1997; 277: 231-7).
Just how toxic the chemical is can be seen from case reports of DEET ingestion. One describes five cases in which hypotension (abnormally low blood pressure), seizures and comas developed within an hour of swallowing large amounts of DEET. Although three of the patients recovered, two of them died (JAMA, 1987; 258: 1509-11).
As would be expected, much of the evidence against DEET comes from individual case reports rather than large-scale studies. However, there is compelling data from animal studies to support the fact that DEET can cause serious systemic effects.
Mohamed Abou-Donia and his colleagues at Duke University Medical Center in Durham, NC, have done numerous studies in rats, and found that frequent and prolonged applica-tions of DEET cause neurons to die in those parts of the brain that control muscle movement, learning, memory and concentration (Exp Neurol, 2001; 172: 153-71). What's more, the rats treated with the equivalent of the average human dose of DEET (40 mg/kg body weight) performed far worse than control rats when faced with tasks requiring muscle control, strength and coordination (J Toxicol Environ Health A, 2001; 62: 523-41). Although the results may not apply to humans, such effects are consistent with the reported symp-toms seen in military personnel after DEET use in the Persian Gulf War.
As Abou-Donia has stated, "If used sparingly, infrequently and by itself, DEET may not have negative effects. . . But frequent and heavy use . . . especially in combination with other chemicals or medications, could cause brain deficits in vulnerable popu-lations" (www.dukehealth.org/HealthLibrary/ News/5500).
Indeed, a number of chemicals-including other pesticides such as malathion and permethrin-have been found to work in combination with DEET to produce even greater adverse effects than DEET alone (J Toxicol Environ Health A, 2004; 67: 331-56).
Of particular concern is the sun-screen chemical oxybenzone, which is commonly combined with DEET to make an all-in-one sun-protection and insect-repellent product. Research shows that oxybenzone is a pene-tration enhancer-in other words, it helps other chemicals to penetrate the skin. When mixed with DEET, oxy-benzone dramatically increases the amount of DEET absorbed by the skin into the blood, thereby further increasing the risk of adverse effects (Toxicol Appl Pharmacol, 2007; 223: 187-94).
Despite these concerns over DEET, some researchers still insist that side-effects are rare and that, when used properly, DEET is perfectly safe. Nevertheless, DEET is not the only way to keep bugs at bay. There are a number of much less risky agents now being used to fend off mosquitoes and other biting bugs.
One of these is picaridin, also known as 'icaridin', which is used in Autan Repel and other products. This relatively new insect repellent appears to be comparable in effect, but less irritating than, DEET (J Drugs Dermatol, 2004; 3: 59-60). It also appears to have a relatively good safety profile (see www.epa.gov/opprd001/factsheets/picaridin.pdf). However, like DEET, picaridin is a completely synthetic organic compound.
A more natural product is essential oils. Citronella, derived from lemon-grass (Cymbopogon nardus), is a common ingredient in natural or herbal mosquito repellents. Studies show that it's surprisingly effective, particularly against mosquitoes (Entomol Res, 2005; 35: 117-20). However, citronella appears to provide a shorter protection time than DEET. Its repellency is probably greatest in the first 40 minutes after application, although frequent reapplications should make up for the short-term effect (Ann Intern Med, 1998; 128: 931-40).
Eucalyptus oil (from Eucalyptus citriodora and E. globulus) is another natural mosquito repellent that works for up to nearly eight hours. Research shows that some components of eucalyptus oil are as effective as DEET (J Am Mosq Control Assoc, 1996; 12: 243-6). One study found that a eucalyptus-based repellent was 99.5-per-cent effective at protecting against the Scottish biting midge, Culicoides impunctatus, even eight to 10 hours after application, while DEET was 97-per-cent effective (J Am Mosq Control Assoc, 1996; 12: 329-30).
Other insect-repelling essential oils include neem (Azadirachta indica), turmeric (Curcuma longa), hairy basil (Ocimum americanum) and kaffir lime (Citrus hystrix) (Indian J Malariol, 1995; 32: 47-53; J Vector Ecol, 2001; 26: 76-82). However, essential oils can cause allergic reactions in some people, so always do a patch test before using essential oil-based repellents, and avoid their heavy or prolonged use. Eucalyptus oil, in particular, should be used with caution, as overdosing has been known to trigger seizures (J Neurol, 1999; 246: 667-70).
For sensitive skin, soybean oil-based repellents may be a good alternative.
A 2-per-cent soybean oil solution (Bite Blocker) was able to provide protection from mosquito bites for an average of seven hours (J Med Entomol, 2004; 41: 726-30).
Besides using insect repellents, there are several other things you can do to reduce your chances of being bitten. Where mosquitoes are a problem, wear long-sleeved shirts and trousers, and keep your ankles and feet covered. Also, it may help to avoid wearing dark-coloured clothing, and using floral-fragranced perfumes, soaps, lotions and haircare products, as these may attract these biting insects (Ann Intern Med, 1998; 128: 931-40).
Finally, mosquito nets are useful, although the mesh needs to be extremely fine or the bugs will still get through. When using a net over a bed, it must reach the floor or be tucked under the mattress to be effective. You can also be bitten through the mesh if you sleep up against it.
These preventative measures-combined with the use of DEET-
free formulas-are a safer way to keep bugs from biting this summer.
The problem with permethrin
An active ingredient to watch out for is permethrin, which is used in clothing, shoes, bed nets and camping gear to both kill and repel insects.
Permethrin belongs to a group of chemicals called 'pyrethroids', which are known to have neurotoxic effects (Toxicology, 2002; 171: 3-59). When ingested, permethrin can cause dizziness, headache, blurred vision, coma, convulsions and even death (Toxicol Rev, 2005; 24: 93-106). Exposure through the skin, however, is thought to be responsible-at least in part-for the various illnesses seen in Gulf War veterans (Toxicol Lett, 2002; 135: 61-71).
Yet another concern is that permethrin may be carcinogenic. In animal studies, permethrin has been found to cause lung and liver tumours in mice, prompting the US Environmental Protection Agency to classify permethrin as 'likely to be carcinogenic to humans' when taken orally (www.epa.gov/oppsrrd1/REDs/factsheets/permethrin_fs.htm#health).
What's more, a recent US occupational study, which looked at nearly 50,000 pesticide applicators in Iowa and North Carolina, found elevated and statistically significant risks for multiple myeloma (bone marrow cancer) in people with the highest exposures to permethrin (Environ Health Perspect, 2009; 117: 581-6). This suggests that it doesn't have to be ingested to cause cancer.
Safer bug sprays
- Buzz Away (lb7.55 for 6 oz): a water-based solution infused with citronella, cedarwood, eucalyptus, lemon grass and peppermint; available at www.quantumhealth.com and www.21stcenturyhealth.co.uk, as well as from chemists and healthfood shops
- Neal's Yard Remedies Citronella Formula (lb7.40 for 100 mL): contains citronella, pine, lavender, cypress, eucalyptus, orange, thyme, marjoram, bay and clove essential oils, with added witch hazel; from Neal's Yard stores or online at www.nealsyardremedies.com
- Bioforce NeemCare Herbal Insect Repellent (lb6.80 for 100 mL): uses neem oil, bergamot, rosemary and eucalyptus in a water/alcohol/oil base; available at www.bioforceshop.co.uk or healthfood stores and chemists
- Mosi-guard Natural (lb5.99 for 100 mL): a popular spray made from a blend of refined eucalyptus oil (the label lists no ingredients other than this); available at www.safariquip.co.uk, chemists and healthfood shops
- Bite Blocker (lb8.85 for 133 mL): a water-based solution containing soybean, geranium and coconut oils, along with vanillin; widely available in the US and UK, it can be ordered at www.magellans.co.uk.
Vol. 20 04 July 2009