Insect Repellents: Which Ones Work? Which Don't?

We are in the thick of mosquito season, and I’m sure we all have our favorite bug-killer products, from zappers to plug-ins. But what really works? I found a nice new resource from the good folks at the doctor-oriented Prescriber’s Letter; they have a free detail-document discussing what really works against skeeters. They endorse the 2 major chemicals that are approved by the U.S. Environmental Protection Agency: DEET and Picaridin. Here’s a long snippet:

Products containing DEET (OFF!, Cutter, etc) are some of the best for repelling mosquitoes and ticks. These products can contain different concentrations of DEET. Higher concentrations last longer, but they don’t work better. DEET 30% lasts about six hours, and DEET 10% lasts about three hours. Pick a product with up to 30% DEET for adults and kids over two months.

Some product labels won’t actually say DEET, so look under the ingredients section of the label for its chemical name, N,N-diethyl-m-toluamide or N,N-diethyl-3-methylbenzamide.

Picaridin (Cutter Advanced, Natrapel, etc) is a good alternative to DEET for repelling mosquitoes and ticks. It works as well as similar concentrations of DEET and doesn’t irritate skin or damage plastic or clothing. As with DEET, products with higher concentrations of picaridin last longer than those with lower concentrations. Choose a product with up to 20% picaridin for adults and 5% to 10% picaridin for kids over six months.

Oil of lemon eucalyptus (Repel Lemon Eucalyptus, etc), also known as p-menthane-3,8-diol or PMD, is a good choice if you want a “natural” or plant-based repellent for mosquitoes and ticks. Its effect lasts for up to six hours, but it hasn’t been proven safe for kids younger than three years.

Other plant-based products, such as those that with soybean oil (BiteBlocker, etc), last for around four hours for mosquitoes and around two hours for ticks. Products with citronella oil (Buzz Away, etc) generally work for even less time.

You can read the full document here: Detail-Document: Prescriber’s Letter. You can read more reviews from WebMD; and the excellent Consumer Search has a nice review of which brands to buy.

Safe For Children? Yes, It Is

DEET has always had a stigma for many, especially as the smell is strong, but also due to the relatively common skin irritation issue. However, no major review has found any major problems. The EPA reviewed DEET’s status in 1998 and still found no major toxicity issues with normal use. In 2007 they again addressed this:

Using DEET on children

DEET is approved for use on children with no age restriction. There is no restriction on the percentage of DEET in the product for use on children, since data do not show any difference in effects between young animals and adult animals in tests done for product registration. There also are no data showing incidents that would lead EPA to believe there is a need to restrict the use of DEET. Consumers are always advised to read and follow label directions in using any pesticide product, including insect repellents.

The American Academy of Pediatrics has a position paper approving the use of DEET in children over 2 months:

Insect repellents containing DEET (N,N-diethyl-m-toluamide, also known as N,N-diethyl-3-methylbenzamide) with a concentration of 10% appear to be as safe as products with a concentration of 30% when used according to the directions on the product labels. DEET is not recommended for use on children under 2 months of age.

DEET-containing products are the most effective mosquito repellents available. DEET also is effective as a repellent against a variety of other insects, including ticks. It should be used when there is a need to prevent insect-borne disease. The concentration of DEET in products may range from less than 10% to over 30%. The efficacy of DEET plateaus at a concentration of 30%, the maximum concentration currently recommended for infants and children. The major difference in the efficacy of products relates to their duration of action. Products with concentrations around 10% are effective for periods of approximately two hours. As the concentration of DEET increases, the duration of activity increases; for example, a concentration of about 24% has been shown to provide an average of 5 hours of protection.

I also found a terrific review from 2009 which lucidly describes the ever-present worry about DEET. The gist of the article is that picaridin may be more tolerable to most people than DEET — although it may not be as effective as DEET for some mosquitos. Here’s one good quote:

For decades, DEET has faced public scrutiny regarding the health risks it poses to consumers. However, several studies indicate that DEET has a remarkable safety profile. For instance, a lab study conducted by Antwi et al. found that there were “no significant toxicological risks from typical usage of [DEET and picaridin]”25. Furthermore, a safety re-assessment conducted by the Environmental Protection Agency in 1998 deemed that “the normal use of DEET does not present a health concern to the general population”26. Much of the controversy associated with DEET comes from its safety record concerning children. The Department of Health and Human Services, Agency for Toxic Substances and Disease Registry (ASTDR), states that seventeen cases of significant toxicity from DEET exposure have been reported during the period between 1961 and 2002. Fourteen of these cases occurred in children under the age of eight—the most frequently reported symptoms were lethargy, headaches, tremors, involuntary movements, seizures, and convulsions27. Although these cases should in no way be deemphasized, they are likely attributed to inappropriate usage such as repellent ingestion27. Addtionally, the number of reported cases is miniscule for a forty-year span.

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