How do the oils work to repel insects?

Spice plants get their strong odors from their chemical
structure: They  consist of small molecules that evaporate easily. 
Their chemical structure also allows spice extracts to easily penetrate cell 
membranes in places like the brains of bugs.
By blocking or over-stimulating these molecules or their receptors, essential oils cause insects to become hyper or fly away.

People, pets and other animals don't have the same messenger molecules in their brains, making extracts that harm insects safe for us. Scientists are now eyeing these brain processes as targets for synthetic pesticides.

"Some of these essential oils have really outstanding repellency," said Joel Coats, an entomologist and toxicologist at Iowa State University in Ames. "If we  can understand how they work at a biochemical level, we may be able to use them  more readily, and that's a really important step right now."

For more info see:

http://news.discovery.com/human/spices-pesticides-bugs.html


THE DANGERS OF DEET:DEET has been linked  to neurological problems; according to the EPA, at least 18 different  cases of children suffering adverse neurological effects, as well as the  deaths of two adults, have been associated with DEET. Researchers at  Duke University Medical Center have found that DEET causes diffuse brain  cell death and behavioral changes in rats. DEET has been shown to have a negative impact on wildlife and water  sources in production and during use. DEET is toxic to birds and aquatic  life. DEET has been found in approximately 75 percent of U.S. water 
sources.
 

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