DEET
N,N-diethyl-m-toluamide, abbreviated DEET, is an insect repellent chemical. It is intended to be applied to the skin or to clothing, and is primarily used to protect against insect bites.  In particular, DEET protects against tick bites (which transmit Lyme disease) and mosquito bites (which transmit dengue fever, West Nile virus, Eastern Equine Encephalitis (EEE), and malaria).
DEET is believed to work by blocking insect receptors (notably those which detect carbon dioxide and lactic acid) which are used to locate hosts.  DEET effectively "blinds" the insect's senses so that the biting/feeding instinct is not triggered by humans or animals which produce these chemicals.

# History
DEET was developed by the United States Army, following its experience of jungle warfare during World War II. It entered military use in 1946  and civilian use in 1957. Originally tested as a pesticide on farm fields,the US Government applied it for war time usage, particularly when in Vietnam and around that region of Asia.

# Chemistry
A slightly yellow liquid at room temperature, it can be prepared from m-methylbenzoic acid and diethylamine. This can be achieved by preparing the acid chloride and subsequently reacting that with the diethylamine. It can be distilled under vacuum: b.p. 111C at 1 mm Hg.
It is considered a mild irritant.

# Concentrations
DEET is often sold and used 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 (20%-34%) offered 3-6 hours of protection. Other research has corroborated the effectiveness of DEET.

# Effects on health
DEET is the most common active ingredient in insect repellents.
The American Academy of Pediatrics found no difference in safety for children, between products containing 10% and 30% DEET, when used as directed, but recommends that DEET not be used on infants less than two months old.
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. In rare cases, it may cause skin reactions.
In the DEET Reregistration Eligibility Decision (RED), the EPA 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," but observed that with 30% of the US population using DEET, the likely seizure rate is only about one per 100 million users.
A study which examined the risk factors for testicular cancer found evidence that use of insect repellents "mostly containing N,N-diethyl-m-toluamide (DEET)" were associated with an elevated risk of testicular cancer.
The Pesticide Information Project of Cooperative Extension Offices of Cornell University states 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". 

# Effects on materials
DEET is an effective solvent, and may dissolve (part of) some plastics, rayon, spandex, other synthetic fabrics, leather, and painted or varnished surfaces.

# Effects on the environment
Although few studies have been conducted to assess possible effects on the environment, DEET is a moderate chemical pesticide and may not be suitable for use in and around water sources. Though DEET is not expected to bioaccumulate, it has been found to have a slight toxicity for coldwater fish such as the rainbow trout and the tilapia , and it has also been shown to be toxic for some species of freshwater zooplankton. DEET has been detected in significant levels in waterbodies as a result of production and use, such as in the Mississippi River and its tributaries, where a 1991 study detected levels varying from 5 to 201 ng/L. 

# Natural alternatives
A test of various marketed insect repellents by an independent consumer organization found that synthetic repellents, including DEET, were more effective than repellents with natural active ingredients. All the synthetics gave almost 100% repellency for the first 2 hours, whereas the natural repellent products were most effective for the first 30-60 minutes and required reapplication to be effective over several hours.
Citronella oil has been used as an insect repellent for 60 years.  Its mosquito repellency qualities have been verified by research,  however, the repellency duration of DEET is much greater.
While most essential oil based repellents are not as effective as DEET, research also shows that some EO-based formulas can be comparable to DEET, and somewhat better.