DEET is N, N diethyl-m-toluamide, a broad spectrum insect repellent developed by the U.S. government during the 1940s. Deet is by far the most commonly used insect repellent in the world.
The British National Formulary (used by all British Doctors) says "DEET in lotions, sprays or roll-on formulations is safe and effective when applied to the skin but the protective effect only lasts for a few hours..."

• Aerosol and pump-spray products are preferred for treating clothing (only on natural fibers) as well as for applying to skin. They provide an even application.
• Liquid, cream, lotion, and stick products offer more precise skin applications.
• Lower-concentration DEET products (5 to 15 percent) are appropriate for most situations where exposure to pests is minimal or for shorter exposure periods.
• Products with higher concentrations of DEET can give increased protection in highly infested areas, for people more prone to insect bites, and against difficult to repel species such as black flies. These products also provide longer protection (from 6 to 8 hours).
• Generally, the higher the DEET concentration, the longer the protection

• Only apply insect repellents to exposed skin taking care to avoid the eyes and mouth.
• Read the entire product label before using the repellent.
• Do not apply on wounds or scratches.
• Apply to clothing (natural fibers only) for added protection as directed on the product label.
• Avoid over-saturation -- it's not necessary for adequate protection.
• Do not apply to the hands of small children, since they frequently put their hands into their mouths.
• Do not allow children to apply the product themselves.
• Cleanse the skin with soap and water after returning indoors.
• Keep out of the reach of children, because like many chemicals, DEET can be toxic if ingested.
• A very small percentage of children and adults may be sensitive to chemicals such as DEET. If there is a suspected reaction to an insect repellent, wash the area and seek medical attention.