The environmental protection agency and the CDC both state officially that DEET is not harmful when used as directed. However, this advice is based on the idea that DEET is not absorbed through the skin. This is a common myth in the medical and pharmaceutical industries -- that cosmetically-applied lotions somehow stay outside the body and don't interact with the blood stream and internal organs of the body. In fact, as any good medical researcher knows, nearly all chemicals that are placed on the skin, especially in liquid form, are eventually absorbed and enter the bloodstream. DEET is known to cause neurological damage, and once it enters the bloodstream, it makes its way to the nervous system, where it is known to cause seizures and even deaths. It can be especially harmful to children, which is why its use should be strictly limited with children.
Even the EPA says that DEET should not be frequently used -- in other words, they're saying it's okay to poison yourself just a little bit, but not too much. The American Academy of Pediatrics recommends no more than one application of DEET per day for children. Once again, this is a position that says it's okay to poison your children just a little bit. The Duke University Medical Center has concluded through laboratory rat studies that long-term use of DEET kills brain neurons.
People who are interested in protecting their health, and who don't want to give themselves cancer or liver damage from products containing toxic chemicals like DEET, typically want to know what kind of products they can use as a replacement for commercial, brand-name insect repellents. One of the best products is simply soybean oil. Soybean oil insect repellants may often contain cloves, cinnamon oil or other extracts containing rather strong oils from the natural plant kingdom. These repellants do work, but certainly not as effectively as DEET.
There's no question that DEET is a highly effective insect repellant, and that's probably because its neurotoxicity extends to insects as well as human beings. If you coat your body with something that kills nerve cells, and kills anything around it, it will also cause insects and other animals to retreat. But it doesn't mean that these items should be put on your skin. If you want to avoid mosquitoes, stick with natural products and avoid products containing toxic chemicals such as DEET.