N,N-diethyl-m-toluamide, better known as DEET, is one of the world's most widely used insect repellents – and it is now turning up in rivers, lakes, groundwater and even drinking water around the globe, according to a new review by an international research team. The authors warn that while DEET helps protect millions of people from mosquito-borne diseases, its growing footprint in aquatic environments raises questions about long‑term ecological and health risks.
"We shouldn't wait for a crisis"
"DEET has been a public‑health success story for decades, but our analysis shows it is also becoming a quiet, global water contaminant," said lead author Yan Zhao of Dalian Maritime University. "We shouldn't wait for a crisis; it is time to treat DEET as an emerging pollutant that requires better monitoring and smarter management."
Co‑corresponding author Song Cui added that the goal is not to scare people away from using repellents, but to inform regulators and the public. "Vector‑borne diseases like dengue and malaria are still major threats, especially in tropical and subtropical regions, so repellents remain essential," Cui said. "The question is how to control DEET's environmental release while preserving its public‑health benefits."
Everyday repellent, global pollutant
DEET was originally developed for military use and is now a staple ingredient in commercial mosquito and tick repellents worldwide. Beyond personal care products, it is also used in agriculture as a feeding deterrent, in pharmaceuticals as a skin‑penetration enhancer, and in materials science as a plasticizer and specialty solvent, broadening its routes into the environment.
The review reports that the global insect repellent market was valued at about 6.27 billion US dollars in 2024 and is projected to nearly double by 2033, with DEET‑based products dominating many national markets. In some countries, such as Argentina, more than three‑quarters of registered repellents contain DEET, underscoring the chemical's central role in current mosquito control strategies.
How DEET reaches water
Most DEET entering the aquatic environment comes from everyday consumer use, the authors found. When people swim, shower, or wash DEET‑treated clothing, the compound is washed off into sewers or directly into surface waters, and a fraction absorbed through the skin is later excreted in urine.
Because conventional wastewater treatment plants only partially remove DEET – with reported removal efficiencies ranging from about 10% to 90% – significant amounts pass through into rivers, lakes and coastal waters. Additional inputs come from agricultural use, urban runoff and landfill leachate, where discarded consumer products break down and release DEET that can migrate into nearby groundwater and surface waters.
Where and how much is found
Monitoring data compiled in the paper show that DEET is now detected on every continent surveyed, in a wide range of aquatic systems. In surface waters, concentrations typically fall in the nanogram‑per‑liter to microgram‑per‑liter range, with higher levels in densely populated or tourism‑intensive areas and in rivers receiving large volumes of treated wastewater.
Landfill leachate stands out as the most polluted water matrix, with DEET concentrations frequently reaching microgram‑per‑liter and in some cases milligram‑per‑liter levels, far above those measured in rivers or lakes. Groundwater in highly impacted regions can also be heavily contaminated: in one Indian city, DEET was found in 96% of sampled wells, with average concentrations around 30 micrograms per liter and a maximum of 92 micrograms per liter.
Ecological and health concerns
Laboratory studies summarized in the review indicate that even at relatively low concentrations, DEET can harm sensitive aquatic organisms. Algae exposed to high levels show reduced cytochrome content and irreversible cellular damage, while fish and invertebrates exhibit impaired growth and disrupted nervous‑system function.
The authors also highlight emerging evidence that DEET can alter the composition and activity of aquatic microbial communities, including microbes involved in key processes such as nitrification. Although DEET is not considered strongly bioaccumulative, it has been detected in mussels from the Great Lakes and in bees and honey in Mexico, suggesting that trophic transfer and accumulation in some food webs are possible.
Moderate risk, big data gaps
To gauge overall environmental risk, the team used a "weighted average risk quotient" that combines information on exposure levels and species sensitivity across different water types. Using a conservative protective threshold for aquatic life, they estimated that DEET currently poses a moderate ecological risk globally, with the highest concern in landfill leachate, followed by groundwater and then surface waters.
However, the authors stress that this assessment is limited by major data gaps. Many low‑ and middle‑income countries in tropical and subtropical regions – where mosquito repellents are used most intensively – lack systematic monitoring data on DEET in rivers, groundwater or drinking water, making it difficult to evaluate local risks.
Can treatment plants keep up?
The review points to promising advances in water‑treatment technologies that can more effectively remove DEET. Advanced oxidation processes that generate highly reactive radicals, optimized ozonation systems, and hybrid treatment trains combining physical, chemical and biological steps have all shown improved performance compared with conventional treatment alone.
Biological approaches are also emerging, including specialized bacteria, fungi and enzyme‑rich waste materials capable of degrading DEET or transforming it into less persistent compounds. Yet the authors caution that some advanced treatments can produce unwanted by‑products, and that full mineralization of DEET and its transformation products must be verified to ensure net environmental benefit.
Call for monitoring and smarter regulation
The paper concludes that DEET should be treated as a priority emerging contaminant in water management and regulatory frameworks. The authors call for expanded, globally coordinated monitoring – especially in disease‑endemic regions – together with refined ecological risk assessments that reflect local species and long‑term, low‑dose exposure.
"Mosquito repellents are here to stay, but the way society manages their lifecycles can and should change," said co‑author Jingwei Wang. "By combining better monitoring, improved treatment technologies and risk‑based regulation, it is possible to protect both public health and freshwater ecosystems at the same time."
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Journal Reference: Zhao Y, Wang J, Jia Q, Ma Q, Jia HL, et al. 2025. The impact of N,N-diethyl-m-toluamide in aquatic environments: occurrence, fate, and ecological risk. Agricultural Ecology and Environment 1: e009
https://www.maxapress.com/article/doi/10.48130/aee-0025-0009
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