The chemical DEET which is found in many insect repellent sprays has been shown to be toxic and can cause fits in children, the current advice is that it should not be used by pregnant women. In research studies DEET blocked the enzyme cholinesterase, which is essential for sending messages between the muscles and the brain which can cause muscle spasms and in severe cases death. Other side effects of DEET are excessive salivation and eye watering.
DEET, is also known as diethyl toluamide, and is used in a many repellent sprays. Initially it was thought that the DEET simply altered the insects sense of smell to prevent them from detected the smell of the human. However on further study it was found that it acts on the enzyme which is present in both insects and mammals. Therefore the idea that it simply modifies insect behaviour is false as it directly inhibits enzyme activity in both mammal and insect nerves, research also shows that DEET interacts and strengthens the toxicity of carbamates, a class of insecticides also known to block acetylcholinesterase.
This research raises questions with regards to the safety of insect repellents, however at present it is mainly based on theory and tests on animals if you are at all concerned speak to a pharmacist or a doctor. Alternatively try alternatives from your health food shop