Eighty years on, nobody is sure exactly why DEET repels mosquitoes
DEET was created in 1944 for American soldiers fighting in jungles, and it remains the oldest and one of the most effective ingredients in insect repellents. After some 8 billion human applications, scientists still debate its mechanism: whether it smells foul to insects, scrambles their sense of smell, or simply hides us from them.
The name comes from the initials of diethyl toluamide. Samuel Gertler of the US Department of Agriculture developed it for the Army after wartime jungle campaigns; it was first tried as a farm insecticide, went to soldiers in 1946 and reached civilians in 1957. Troops once carried bug juice, three parts DEET to one part ethanol, later replaced by a polymer blend that slows evaporation, registered by the Environmental Protection Agency in 1991.
Several explanations compete. The smell-and-avoid idea says insects find the odour unpleasant; the bewilderment idea says it confuses their odorant receptors; a third proposes it dampens the skin scents that attract them. It also tastes bitter to receptors on contact, including chemosensory hairs on mosquito legs. A 1996 study found it made carbamate insecticides more toxic to cockroaches, a sign that it affects insect nerves as well as noses.
It is a strong solvent. The same oil that deters ticks, fleas, chiggers and leeches can dissolve watch crystals, varnish, nail polish, rayon and spandex, and can soften hard plastics by lodging inside them. Protection lasts longer at higher strengths up to about 50%, beyond which it does not extend. A 2018 review found picaridin performed about equally well in the field.
Its safety has been examined more closely than any other repellent. A 2002 New England Journal of Medicine review counted fewer than 50 serious toxic cases documented since 1960, many involving heavy or whole-body use, and US, Canadian and European agencies have not classed it as a carcinogen. Health Canada nonetheless capped consumer products at 30% from the end of 2004. In rivers it breaks down well where oxygen is present, though traces have turned up in the Mississippi and it is mildly toxic to some fish and zooplankton.
Source: DEET
