For years, researchers studying DEET—the chemical that’s widely used in insect repellants—have noted how in some cases it can harm brain cells, cause seizures and adversely affect the central nervous system. Now a group of scientists at Harvard University have found that the substance may jeopardize reproductive functions, too.

Mónica P. Colaiácovo, a professor of genetics at Harvard Medical School who was the lead author of a study published last week in the open-access journal iScience, said examining the potential harms of DEET-related products is especially significant given the effects of climate change on the spread of mosquito-borne diseases.

“What we know is that you’re now having cases of malaria and West Nile virus that are now here in the U.S.,” she said. “As climate changes, you’re going to have a shift or move and this is going to spread even further—throughout not only the U.S. but other parts of the world. And so the need for techniques, approaches, lotions, chemicals, things that we might need—whether it’s fabrics, etc., products that could allow us to be more protected and prevent the transmission of insect-borne diseases—becomes very important.”

The research focused on how DEET, or N,N-diethyl-meta-toluamide, affects the reproductive systems of a type of worm known as Caenorhabditis elegans, or C. elegans, which has long been used as a model organism to study human diseases.

Researchers focused specifically on how the chemicals affected meiosis, the division of cells which begins shortly after fertilization.

The study showed that exposure to different levels of DEET resulted in an increased rate of fatal mutations in embryonic cells, defects that occur during development and cell death. The higher the levels of the chemical, the more severe the reaction by the worm cells, the researchers found. 

For example, they noted increased mutations in embryonic cells when the worms were exposed to 10 micrometers of DEET for 24 hours. Those mutations doubled when the animals were exposed to 500 micrometers of the chemical over the same span.

“Our analysis suggests a link between DEET-induced alterations in the expression of genes,” the researchers wrote. “Comparison of the internal level detected in this C. elegans study with the levels detected in several human samples suggests that our findings may be relevant for mammalian reproductive health and requires further investigation.”

Colaiácovo said she hoped her research would raise awareness about the potential harms of DEET—which provides significant protection from mosquito bites that can infect people with such ailments as malaria and West Nile virus, and from tick bites that spread Lyme disease and Rocky Mountain spotted fever.

“Our study suggests that there are concerns in terms of how DEET could affect reproductive health,” she said. “However, this is still a very important line of defense against many of these insect-transmitted diseases. So I want to make sure that it’s clear that we’re not trying to say, ‘Don’t use these products.’ It’s just we need to be careful of how we use them. And people have to be informed about potential risks.”

Colaiácovo stressed that additional research was needed to determine precisely how DEET might affect reproduction in people.

“We need to understand more of what they’re doing, so more research is necessary,” she said. “One thing to remember is that when we did our studies, we were exposing these worms for 24 hours. And humans are not usually exposed continuously for 24 hours to something like DEET. The next generation studies that we’re going to be doing have to look at shorter windows of exposure and then define whether in these shorter windows we still see these defects. And then how does that compare and contrast with the levels that you find in human biological samples?”

DEET was developed by the U.S. Army as a pesticide just after the conclusion of World War II. By the end of the 1950s it had entered commercial use. 

Today, according to the National Institutes of Health, there are more than 230 products containing DEET registered with the Food and Drug Administration that are used by roughly 30 percent of Americans.

According to the Centers for Disease Control and Prevention, products containing DEET pose “very little risk” to human health. The agency also noted that after excessive use of the substances there have been occasional reports of a range of adverse health effects, including seizures, uncoordinated movements, agitation, aggressive behavior, low blood pressure and skin irritation.

Most experts say that the use of DEET, under the guidelines suggested by manufacturers, should not adversely affect the health of most people.

Joe Zagorski, a toxicologist for the Center of Research on Ingredient Safety at Michigan State University, said he has few concerns about the substance.

“The use of DEET—although some individuals might be apprehensive—is much, much, much, much, much less risky than if you get Lyme disease or Rocky Mountain spotted fever or malaria,” Zagorski said. “So I have no concerns about the safety of DEET when used as described.”

Colaiácovo said her research underscored the importance of striking a balance between the protection offered by DEET and being aware of its possible problems.

“We need barriers and protective measures such as what DEET offers,” she said. But we also want to make sure that whatever we’re using or we’re applying to our bodies is safe—not safe only to us, but also to, for example, in the case of pregnant women, the developing fetus.”

Laura I. Lascarez-Lagunas, one of the study’s co-authors, said that until more research is done that can definitively determine whether human reproduction is harmed by DEET, users of the products can take precautions if they have concerns.

“Apply the repellent to the clothes instead of the skin,” said Lascarez-Lagunas, a research associate in the genetics department at Harvard Medical School. “Follow all the procedures that are stated on the labels in terms of how long you should use the repellent.”

Another step, Colaiácovo said, could involve wearing long sleeves, and long pants, and other clothing to minimize how much skin is potentially exposed to mosquitoes.

“I know that can be uncomfortable, but if you have to be outdoors and in an area where you’re concerned about potential insect-borne diseases, you may want to be more covered up,” she said. “So carefully assess what you’re wearing, long shirt sleeves, etc., so that you’re more protected in that way.” 

She added: “What I really hope that our study does is it starts making us all think about alternative products, but that are effective at the same time, right?”