Research from Institut de Recherche pour le Developpement indicates that the chemical deet may attack the nervous systems not only of the insects we're trying to kill but also of mammals. Deet is found in most mosquito repellents. It was developed by the U.S. Army in 1946 to repel mosquitoes.
"In experiments performed in cockroaches and rats, the researchers found that deet blocked the action of the neurological enzyme acetylcholinesterase. This is the same mechanism that causes the toxic effects of popular carbamate and organophosphate pesticides, as well as chemical weapons such as sarin and VX nerve gas. This may mean that deet repellents are actually insecticides and could damage the human nervous system."
The study posits that the reason deet repellents are so effective may be because they could be interfering with the insects' nervous systems. And the inference is that they may do the same to humans.
The researchers recommend that pregnant women and children under six should not use insect repellents containing deet.