UF Researchers Caution Against Using Sunscreen-DEET Combinations

September 14, 2004

GAINESVILLE, Fla. — Products combining sunscreen and the bug repellent DEET can be convenient, but University of Florida researchers are suggesting manufacturers take a closer look at the formulations after new research in mice found the mixtures greatly boost absorption of the chemical through the skin.

“DEET has an unbelievably superb safety record; it’s been used on millions of people and it’s critical that people don’t get the wrong message,” said Dr. Edward A. Ross, an associate professor of medicine and the lead author of a paper published recently in Drug Metabolism and Disposition. “People should continue using DEET because of the very real risks of mosquito and insect-borne illnesses, such as West Nile encephalitis, but use it in the lowest effective concentration, especially when you use it in combination with other topical lotions or in children.”

Compared with adults, children may absorb more DEET because their skin surface area is proportionally larger relative to their weight, Ross said. DEET, the synthetic chemical N, N-diethyl-m-toluamide, was developed in the 1950s for use by military personnel. Today it is the active ingredient in many consumer products, with concentrations ranging from 7 percent to 100 percent.

“A lot of people think if a little bit of something is good, a lot is even better,” Ross said. “We don’t think this is true for DEET. So the message is not to go for the higher percentage, especially when you’re using these other compounds. A little bit is better, not the other way around.”

The medical literature contains anecdotal accounts of frequent DEET exposure or accidental overdose leading to skin reactions, eye irritation and, rarely, confusion and seizures. These studies, however, do not establish a cause-and-effect relationship between DEET use and these medical problems. In addition, unlike the adverse effects associated with industrial accidents leading to tremendous overexposure, the complications physicians are most concerned about in children are subtle, mild, temporary and reversible behavioral effects, such as headache or confusion, Ross said.

Nevertheless, the American Academy of Pediatrics advises that DEET should not be used in a product that combines the repellent with a sunscreen, as sunscreens must be repeatedly applied because they wash off, and repeated applications may “increase the potential toxic effects of DEET.” DEET, however, is not water-soluble and can last up to eight hours.

The academy recommends selecting the lowest concentration of DEET possible and says DEET should not be applied more than once a day. In addition, the group does not recommend using DEET on children under 2 months and suggests applying it to older children’s clothing, not their skin.

UF researchers said their findings call attention to the potential “unappreciated toxicity” of mixtures of chemicals traditionally studied individually. In the study, they used newer instrumentation that can detect multiple drugs passing through the skin simultaneously. They applied DEET alone and commercial sunscreen products containing DEET to skin samples taken from hairless mice and measured the chemical’s penetration. In the presence of the sunscreen, DEET passed through the animals’ skin three times faster, rapidly boosting its concentration, Ross said. The researchers also used “microemulsion” products designed to slow absorption but found they were only minimally effective at best.

“The products we tested were not the magic answer to the problem,” Ross said. “Overall, this is a case where a little bit is good and less is better, not more. The technology is so simple and pharmaceutical companies already test skin absorption. This is a call for companies to test these common topical compounds together instead of individually. These can be complex interactions.”

The Washington, D.C.-based DEET Education Program discourages the use of sunscreen and DEET-based repellents simultaneously as individual products, saying the practice “significantly reduces the effectiveness of each product.” The program is part of the Consumer Specialty Products Association, which represents the interests of many companies that manufacture DEET or formulate DEET-based products.

“That is why specially formulated combination products containing sunscreen and DEET were developed within the past decade,” the program stated on its Web site, pointing out that the federal government has approved such products. Program officials maintain a database developed in partnership with the Environmental Protection Agency to record and study adverse events reported in connection with consumer use.

“Regulatory officials and independent experts agree that the small number of serious events reported after billions of product uses by individuals could not be definitively tied to DEET,” according to the program’s Web site. “None were associated with the simultaneous use of DEET and sunscreens or with the specially formulated combination sunscreen products.”

Canadian officials discourage the use of sunscreen-DEET combination products because of the “conflicting and confusing requirements for use of these products,” said Milton Tenenbein, a member of an advisory panel to the Canadian government’s Pest Management Regulatory Agency, which regulates DEET.

“We recommend sunscreens be applied frequently to provide protection from the sun, whereas DEET should be put on infrequently because it lasts a long time, and liberal use would increase the risk of adverse effects,” said Tenenbein, a professor of pediatrics and pharmacology at the University of Manitoba in Winnipeg, Canada. “So if you’ve got a mixture product where the strategy for one is to use it frequently and for the other is to use sparingly, how are you supposed to advise consumers regarding its correct use?”

Tenenbein noted DEET has a “tremendous safety record” and is the only truly effective mosquito repellent available for purchase, adding that “in general, everything is poisonous; it just depends on the dose. If you take too much prescription medication, you can be poisoned. If you apply too much of a consumer product to your skin, you can have an adverse effect. Whenever one is using drugs or chemicals one should use the lowest amount possible to produce the desired effect, because if you use more than you need you increase the risk of an adverse effect.”

Ross conceded the effects of combination products might differ in human skin. Still, UF researchers urge prudence.

“Definitely use DEET sparingly,” said UF toxicologist Ian Tebbett, one of the paper’s co-authors. “It’s a necessary precaution you have to take to avoid being bitten by mosquitoes that potentially carry disease. But treat it with care.”