Insect repellents: Components and risks

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As warm weather arrives in the US, the number of insect bites and stings reported to poison centers increases dramatically. More than just an annoyance, insect bites can spread disease. The growing concern about the mosquito-borne Zika virus emphasizes this danger.

An insect repellent (IR) is defined as a chemical that makes the air within about 1.5 inch above human skin obnoxious enough to insects that they are less likely to land and bite. Current guidelines require IRs to provide at least a 50% repellency rate to make a reliable claim that they repel mosquitoes. IRs are divided into two chemical classes: synthetic chemicals such as DEET (N,N-diethyl-3-methylbenzamide), picaridin and IR3535; and plant-derived oils such as oil of lemon eucalyptus and oil of citronella.

Picaridin is a relatively new IR. Compared with DEET, its advantages include lack of chemical odor, non-greasy consistency, and non-damaging to clothing and plastics. Its vapor is noxious to mosquitoes, flies, chiggers, and ticks. Picaridin 20% spray has been shown to be comparable to DEET 20% for protection against Aedes, Anopheles, and Culex mosquito species. 

US Environmental Protection Agency (EPA) registration data indicate that picaridin at a concentration of 20% is effective against mosquitoes and ticks for 8-14 hours. The 10% concentration is effective for 3.5-8 hours. It is recommended by the Centers for Disease Control and Prevention (CDC) for use in malaria-endemic areas. Some studies have shown a shorter protection time for picaridin compared to DEET. Picaridin is not known to produce the same nervous system toxicity as DEET but has not been subjected to as much long-term testing. Human and animal studies of picaridin have not demonstrated skin, internal organ, or reproductive toxicity. Picaridin is not recommended for use in children under 2 years of age.

IR3535 (ethyl butylacetylaminopropionate) was first marketed in the US as a skin moisturizer but, when noted to be as effective as DEET against biting midges or "no-see-ums", it was adopted for use as an IR. IR3535 has also shown greater effectiveness against black fly and sand fly bites than DEET, with a longer duration of protection. IR3535 20% offers protection against Aedes and Culex mosquitoes for 7-10 hours but only 3.8 hours of protection against Anopheles in some studies. Therefore, IR3535 is not recommended for use in malaria-endemic areas. It is a serious eye irritant but otherwise has a good safety profile. It can melt some plastics and damage some fabrics.

IR3535 is found in some combination sunscreen/IR products. This is a problem because most sunscreens should be reapplied about every 2 hours. Following those instructions could overexpose the user to the IR. Animal studies have not demonstrated developmental toxicity, but there are no specific recommendations for use or avoidance of IR3535 in children or pregnant women.

Oil of lemon eucalyptus (OLE) is an extract of the leaves of the lemon eucalyptus tree, Corymbia citriodora. The leaf extract is refined to increase the concentration of a naturally occurring substance, para-menthane-3,8-diol, also known as PMD. Commonly available products have a concentration of 30% OLE and 20% PMD. Some studies have suggested that concentrations of 20-26% PMD may be as effective as 15-20% DEET against both mosquitoes and ticks. However, the EPA believes that PMD’s maximum protection time against mosquitoes and ticks is shorter than DEET’s. When used at concentrations of 30%, PMD offers complete protection for 4-6 hours against ticks and several Aedes, Anopheles, and Culex species of mosquitoes. It is recommended by the CDC for use in malaria-endemic areas but it requires more frequent application than DEET.

OLE with enhanced PMD concentration is considered the most effective botanical IR but it can cause allergic skin reactions. Synthetic PMD can be found in some repellent products at a concentration of 10%. These products provide only a few hours of protection. They have less risk of allergic skin reactions than natural PMD. The US Food and Drug Administration has recommended that PMD not be used in children younger than 3 years.

Pure oil of lemon eucalyptus, an essential oil not formulated for use as an IR, has not undergone testing for safety and effectiveness and is not registered by the EPA as an IR.

Citronella is a natural plant oil obtained from several species of Cymbopogon lemongrasses. Citronellol and geraniol are major components of oil of citronella. Citronella is available as a lotion, oil, solid wax, and as a component of candles and flame pots. Citronella has a short duration of action. A 4.2% concentration provides 1 hour of mosquito and tick protection. Citronella in candle form used indoors or citronella incense diffusers used outdoors do not reduce mosquito bites to a degree acceptable to the EPA.

Citronella is ineffective against Asian tiger mosquitoes (the Asian tiger mosquito is now found in more than 20 states). The pests have been found to be infected with LaCrosse encephalitis viruses and West Nile virus, which can cause encephalitis (inflammation of the brain). Citronella is not effective against flies, fleas, or ticks. Oil of citronella can be mildly irritating to the skin and eyes. It can also cause skin allergies for some people with prolonged or frequent exposure.

Botanical blend IRs have not been adequately tested and don’t repel aggressive species such as the Asian Tiger mosquito. Some that contain geranium oil and soybean oil mixtures might provide some insect repellency but frequent reapplication is necessary. These can cause allergic skin reactions.

Bug zappers are not effective and might actually attract mosquitoes while killing insects that are beneficial.

Clip-on IRs or fans are an inhalation concern especially for those with allergies to ragweed or chrysanthemums. They can contain insecticides such as metofluthrin and allethrin that are more toxic than IRs.

For other ways to help prevent mosquito and insect bites and stings, read the article on mosquitoes in The Poison Post^®.

If insect repellent is swallowed or gets in the eyes:

• If you suspect someone has swallowed an insect repellent, immediately use the webPOISONCONTROL^® online tool to get help, or call your poison center at 1-800-222-1222. Whether online or by phone, expert guidance is always free, confidential, and available 24 hours a day.

• When an IR gets in the eyes, irritation and redness are common. Serious eye injury is not likely. The eyes should be rinsed immediately. Remove contact lenses. Use lots of room temperature water. For children, pour water onto the bridge of the nose and let it gently run into the eyes. Encourage blinking. After rinsing, use the webPOISONCONTROL^® online tool to get help, or call your poison center at 1-800-222-1222.