DEET is the active ingredient in many insect repellent products. DEET repels bothersome insects such as ticks, flies and mosquitoes. Approxiamtely 1/3 of the US population is expected to use DEET each year.

The danger in using products containing DEET is that it has possible damaging effects on brain cells. After frequent and long-term exposure, studies performed on rats show that neurons can die in regions of the brain that control muscle movement, learning, memory and concentration.

To learn more, go to:
http://www.epa.gov/opp00001/factsheets/chemicals/deet.htm
http://www.quantumhealth.com/news/dangers_of_DEET.html