Insect repellents: To DEET or not to DEET

OTCs Today

Mary Warner

Summer is much more enjoyable without flies, mosquitos, gnats, or other insects in our outdoor spaces. What’s also important is protecting ourselves against diseases caused by mosquitos and ticks such as the Zika virus and Lyme disease.

When we think of insect repellents, we usually think about those applied to the skin, but clip-on products, lanterns, and candles may also be useful. Active ingredients include N,N-diethyl-m-toluamide (DEET); picaridin; ethyl butylacetylaminopropionate (IR3535); lemon eucalyptus oil; p-menthane-3,8-diol (PMD); citronella oil; and catnip oil.

Most of these products contain pesticides, and it’s important to use them safely.

Pesticide products

Insect repellents that contain pesticides are regulated by the U.S. Environmental Protection Agency (EPA), and in general they are considered more effective than botanical “natural” insect repellents. CDC recommends that consumers use only those repellent products registered by EPA, as registration indicates that EPA has determined that the product is both efficacious and safe for human use when applied according to the instructions on the label. Although DEET is considered to be the most effective of the pesticide insect repellents, alternatives are available.

• DEET, in concentrations ranging from 5% to 100%, is the most common active ingredient in insect repellents applied to the skin. It is designed to repel insects rather than kill them. According to CDC, insect repellents that contain DEET offer the best protection against mosquito bites. The concentration of DEET in a product affects how long the product will be effective; thus, products with lower concentrations may need to be reapplied. However, concentrations over 50% provide no added protection compared to lower concentrations. DEET can be toxic if swallowed and should not be put on wounds or broken skin.
• Picardin is a synthetic pesticide resembling the natural compound piperine, which is found in the black pepper plant. It is recommended by WHO for protection against mosquitos and has been widely used as an insect repellent in Europe and Australia, but it has only been available in the U.S. since 2005. Although skin irritation is possible from products containing picardin, it is considered nontoxic if inhaled and is readily broken down in the body.
• IR3535 is an ethyl ester structurally related to b-alanine, which occurs naturally. Toxicity tests show that IR3535 is not harmful when ingested, inhaled, or used on skin, although eye irritation could occur if it enters a person’s eyes.
• Lemon eucalyptus is an extract of a eucalyptus tree native to Australia. The active ingredient, PMD, results from refinement of the oil. Many products combine oil of lemon eucalyptus and PMD. According to EPA, oil of lemon eucalyptus/PMD is not recommended when the risk of West Nile virus is high or against sand flies or “no-see-ums”—a particularly annoying biting insect. CDC does not recommend using “pure” oil of lemon eucalyptus (essential oil not formulated) as a repellent because it has not undergone testing for safety and efficacy, and it is not registered with EPA as an insect repellent.

Botanical products

Botanical repellents, which often use the word “natural” on product labeling, include lemongrass, citronella, cedar, peppermint, geranium oil, soybean oil, and rosemary. These repellents are often found in products not applied to the skin such as candles, wearable clip-on products, and sprays.

These products don’t require EPA registration because they have been determined to pose minimal risk to human health in the levels found in insect repellent products.

However, testing has shown that they are generally less effective in repelling mosquitos and other flying insects than pesticide-containing products.

Safety considerations and efficacy

Skin-applied insect repellents should be used only when outdoors, and skin should be washed with soap and water after coming indoors. DEET, picardin, and IR3535 may be used on adults, children, and infants older than 6 months; CDC advises against using lemon eucalyptus oil or PMD on children younger than 3 years. DEET, picardin, IR3535, and lemon eucalyptus oil/PMD are all considered safe for use by pregnant or breastfeeding patients. Other products on the market, such as devices that rely on sound waves, are not registered by EPA; therefore, no safety data are available.

It’s important to ensure that patients understand that insect repellents are only useful in preventing bites from mosquitos, gnats, and ticks, and that they are not effective in repelling stinging insects such as bees or wasps. Selection of an insect repellent should be based on product ingredients, concentration, preferred formulation, anticipated type and length of exposure, and patient-specific considerations.

According to CDC, published data indicate that repellent efficacy and duration of protection vary considerably among products and among mosquito and tick species. In general, higher concentrations of active ingredients provide longer duration of protection regardless of the active ingredient. Products with < 10% active ingredient may offer only limited protection—often one to two hours—while products that offer sustained-release or controlled-release (microencapsulated) formulations, even with lower active ingredient concentrations, may provide a longer protection window of time. CDC recommends using products with ≥ 20% DEET on exposed skin to reduce biting by insects that may spread disease. ■