Now that summer is upon us, we're all in search of the perfect insect repellent—one that will keep those pesky mosquitoes away without causing unwanted side effects. With the incidence of mosquito-borne diseases on the rise, this is more than just a matter of avoiding itchy bug bites. Insect-repellent products made with the chemical DEET have been around for 40 years, but due to concerns about the safety of this chemical, people are often looking for other options. A study published in the New England Journal of Medicine shows that products with DEET are the most effective at preventing mosquito bites, and that other products offer little protection. The researchers also offer reassurance about the safety of DEET.

About the study

Scientists from Chapel Hill Dermatology in North Carolina and the University of Florida purchased 16 insect repellent products and enlisted 15 volunteers to test them. Six products contained DEET; including one controlled-release product and two wristbands infused with the chemical. Several products containing citronella, one containing soybean oil, and one containing the chemical IR3535 rounded out the sample of products studied.

The effectiveness of each repellent was measured by having volunteers insert their repellent-treated arms into a cage with a fixed number of unfed mosquitoes. Volunteers kept their arms in the cages for one minute at a time at specific intervals until the first bug bite occurred. The duration of exposure to the bugs and the time until the first bite occurred were measured and used to calculate the "complete protection time." Each product was tested on each volunteer three times for a total of 720 individual tests.

The researchers compared the complete protection times of the 16 products.

The findings

Of the 16 products tested, those containing DEET provided the longest-lasting protection from insect bites. In addition, the greater the concentration of DEET, the longer protection lasted. Specifically, OFF! Deep Woods (23.8% DEET) protected arms for the longest time—an average of 5 hours. And OFF! Skintastic for Kids (4.75% DEET) kept arms bite-free for an average of 88 minutes. One non-DEET product, Bite Blocker for Kids, which contains soybean oil (2%), protected for 95 minutes.

Wristband repellents, including those with DEET, were ineffective; they provided only 0.3 minutes of protection. The Avon product Skin-So-Soft, which is widely believed to be a more natural means of repelling insects didn't fare very well in these studies; four of its products were tested and found to have complete protection times ranging from 3 minutes to 23 minutes.

Although these results are interesting, there are limitations to this study. For example, only one type of mosquito was used in the cages, so it is unknown how effective these products will be against other types of insects. In addition to the species of the biting insect, several factors affect the efficacy of insect repellents. These include the age and sex of the person being bitten, and the temperature, humidity, and wind speed of the environment. Because most of these factors were tightly controlled for in this experiment, which was done in a laboratory, the results may not be consistent in the great outdoors.

How does this affect you?

Which repellent should you reach for when heading out for a hike? The findings of this study suggest that one containing DEET appears to be the best choice. Those with the highest concentrations offer the longest-lasing protection, but those with lower concentrations work well, too, they just need to be reapplied more often.

If you are concerned about the safety of DEET, know that it has been in use for 40 years with very few cases of toxic side effects. Most reported cases of toxic effects were due to gross overuse of the product. In 1998 the Environmental Protection Agency stated that "normal use of DEET does not present a health concern to the general U.S. population." That said, still use care when applying a product with DEET, particularly for children where it is probably best to use the lowest effective concentration. Read the directions on the package and keep the repellent out of your eyes. Also, DEET can be washed off by sweat and rain, and its effectiveness drops quickly as the temperature rises. You may need to apply it more than once.