DEET
N,N-Diethyl-meta-toluamide, also called DEET or diethyltoluamide, is the most common active ingredient in insect repellents. It is a slightly yellow oil intended to be applied to the skin or to clothing and provides protection against mosquitoes, ticks, fleas, chiggers, leeches and many biting insects.
History
DEET was developed in 1944 by Samuel Gertler of the United States Department of Agriculture for use by the United States Army, following its experience of jungle warfare during World War II. It was originally tested as a pesticide on farm fields, and entered military use in 1946 and civilian use in 1957. It was used in Vietnam and Southeast Asia.In its original form known as "bug juice", the application solution for DEET was composed of 75% DEET and ethanol. Later, a new version of the repellent was developed by U.S. Army and the USDA. This incarnation consisted of DEET and polymers that extended the release of the DEET and reduced its evaporation rate. This extended-release application was registered by the EPA in 1991.
Preparation
A slightly yellow liquid at room temperature, it can be prepared by converting m-toluic acid to the corresponding acyl chloride using thionyl chloride, and then allowing that product to react with diethylamine:Mechanism and effectiveness
DEET was historically believed to work by blocking insect olfactory receptors for 1-octen-3-ol, a volatile substance that is contained in human sweat and breath. The prevailing theory was that DEET effectively "blinds" or "confuses" the insect's senses so that the biting/feeding instinct is not triggered by the chemicals present in the sweat and breath of humans or other animals. DEET does not appear to affect the insect's ability to smell carbon dioxide, as had been suspected earlier.Recent evidence with Anopheles gambiae mosquitoes suggests DEET does not directly inhibit olfactory receptors, but instead reduces the volatility of the odorants to which it is mixed. By reducing odor volatility, DEET functions to "mask" the ability of volatile odorants on the skin to activate olfactory neurons and attract mosquitoes. A study with adult Anopheles gambiae mosquitoes found no activation of olfactory receptor neurons by DEET. Behavioral tests with Anopheles gambiae mosquitoes indicated that DEET placed 5 cm from a resting mosquito did not repel the mosquito. These data suggest an olfactory function of DEET is to reduce attraction.
In contrast, evidence with Culex quinquefasciatus mosquitoes shows that DEET may serve as a true repellent in that Culex mosquitoes intensely dislike the smell of the chemical. A type of olfactory receptor neuron in special antennal sensilla of mosquitoes that is activated by DEET, as well as other known insect repellents such as eucalyptol, linalool, and thujone, has been identified. Moreover, in a behavioral test, DEET had a strong repellent activity in the absence of body odor attractants such as 1-octen-3-ol, lactic acid, or carbon dioxide. Both female and male mosquitoes showed the same response.
The evidence across mosquito species suggests that anthropophilic mosquitoes may respond differently to DEET odors.
A 2011 structural study revealed that DEET binds to Anopheles gambiae odorant binding protein 1 with high shape complementarity, suggesting that AgamOBP1 is a molecular target of DEET and perhaps other repellents. Currently, different studies have focused on the development of new bioinspired repellents against mosquitos using as a start point the DEET structure complexed with AgamOBP1
A 2013 study suggests that mosquitoes can at least temporarily overcome or adapt to the repellent effect of DEET after an initial exposure, representing a non-genetic behavioral change. This observation, if verified, has significant implications for how repellent effectiveness should be assessed.
A 2019 study indicated that neurons on the tarsi of Aedes aegypti mosquitoes respond to DEET, and this response repels mosquitoes upon contact. This data indicates DEET functions as a contact repellent.
Concentrations
Insect repellents containing DEET are the most effective. The concentration of DEET in products may range from less than 10 percent to nearly 100 percent. Concentrations of 10 to 30 percent are recommended for infants and children. DEET should not be used on children under 2 months of age.DEET is often sold and used in spray or lotion in concentrations up to 100%. Consumer Reports found a direct correlation between DEET concentration and hours of protection against insect bites. 100% DEET was found to offer up to 12 hours of protection while several lower concentration DEET formulations offered 3–6 hours of protection. Other research has corroborated the effectiveness of DEET. The Centers for Disease Control and Prevention recommends 30–50% DEET to prevent the spread of pathogens carried by insects.
Effects on health
As a precaution, manufacturers advise that DEET products should not be used under clothing or on damaged skin, and that preparations be washed off after they are no longer needed or between applications. DEET can act as an irritant; in rare cases, it may cause severe epidermal reactions. Other symptoms that can occur are breathing difficulty, burning eyes, or headaches.The authors of a 2002 study published in The New England Journal of Medicine wrote, "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. Thanks to 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."
When DEET is used in combination with insecticides for cockroaches it can strengthen the toxicity of carbamate, an acetylcholinesterase inhibitor. These 1996 findings indicate that DEET has neurological effects on insects in addition to known olfactory effects, and that its toxicity is strengthened in combination with other insecticides.
In the DEET Reregistration Eligibility Decision in 1998, the United States Environmental Protection Agency reported 14 to 46 cases of potential DEET-associated seizures, including 4 deaths. The EPA states: "... it does appear that some cases are likely related to DEET toxicity," which may underreport the risk as physicians may fail to check for history of DEET use or fail to report cases of seizure subsequent to DEET use.
In 1997, the Pesticide Information Project of Cooperative Extension Offices of Cornell University stated 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".
When used as directed, products containing between 10% and 30% DEET have been found by the American Academy of Pediatrics to be safe to use on children, as well as adults, but the Academy recommends that DEET not be used on infants less than two months old.
Citing human health reasons, Health Canada barred the sale of insect repellents for human use that contained more than 30% DEET in a 2002 re-evaluation. The agency recommended that DEET-based products be used on children between the ages of 2 and 12 only if the concentration of DEET is 10% or less and that repellents be applied no more than 3 times a day, children under 2 should not receive more than 1 application of repellent in a day and DEET-based products of any concentration should not be used on infants under 6 months. Some experts recommend against applying DEET and sunscreen simultaneously since that would increase DEET penetration; Xiaochen Gu, a professor at the University of Manitoba’s faculty of Pharmacy who led a study about mosquitos, advises that DEET should be applied 30 or more minutes later.