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. N,N-Diethyl-meta-toluamide, abbreviated DEET, is the most common active ingredient in insect repellents.
DEET is an effective solvent and may dissolve (part of) some plastics, rayon, spandex, other synthetic fabrics, leather, and painted or varnished surfaces.
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 skin reactions.
A study that 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.
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 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.
More details on DEET - http://en.wikipedia.org/wiki/DEET