When used properly, the only one at risk from DEET is the mosquito trying to bite you. In this article we’ll take a deeper look into DEET insect repellent.
What is DEET?
N,N-Diethyl-meta-toluamide, more widely known as DEET, was developed in 1946 by the U.S. Army and has since become one of the most commonly used active ingredients in insect repellents. A 2002 study in the New England Journal of Medicine found that “non-DEET repellents do not provide protection for durations similar to those of DEET-based repellents and cannot be relied on to provide prolonged protection in environments where mosquito-borne diseases are a substantial threat.” 
How does DEET work?
Historically it was believed that DEET worked by preventing the mosquito from smelling human sweat and breath, meaning a mosquito may land on you but it won’t bite. 
Recent evidence with Anopheles gambiae mosquitos suggests that DEET doesn’t stop the mosquito from smelling, but rather masks the smell of human sweat and breath. This means mosquitos aren’t attracted to you. 
In contrast, another study with Culex quinquefasciatus mosquitos has shown that they intensely dislike the small of the chemical, meaning it works as a true ‘repellent’.
Overall, DEET has been shown to have different effects on different types of mosquito. However, on all types of mosquito, it is an effective repellent. DEET does not kill mosquitos.
The percentage of DEET that a product contains affects the length of time it acts as a repellent, not how well it works. For example, a product with 7% DEET will normally last about 90 minutes, while a 30% DEET preparation will last up to 10 hours.
Is DEET harmful to your health?
DEET can act as an irritant and may cause a skin rash in rare cases. The same 2002 study in the New England Journal of Medicine states “DEET has a remarkable safety profile after 40 years of use and nearly 8 billion human applications. When applied with common sense, DEET-based repellents can be expected to provide a safe as well as long-lasting repellent effect. Despite the substantial attention paid by the lay press every year to the safety of DEET, this repellent has been subjected to more scientific and toxicological scrutiny than any other repellent substance." 
In a Pesticide Information Profile by Cornell University in 1997, it was found that "Everglades National Park employees having extensive DEET exposure were more likely to have insomnia, mood disturbances and impaired cognitive function than were lesser exposed co-workers"  However, these effects are normally only found following frequent use of preparations with a high concentration of 50-70% DEET.
Jeffrey Bloomquist, a professor of insecticide toxicology in Florida says it’s safe. “There are no significant health risks when using DEET repellents in the general population. There could be bad reactions if misused – like if you drink it or breathe it in. But it’s safe when used properly,” he says.