I just did a literature search and found (as I did a couple of years ago when I last did the same search) that there is little to be worried about when using DEET. The best data comes from a review of >20,000 poison control center contacts regarding DEET exposure. Now remember, people usually only call the poison control center when they have had an abnormal exposure - abnormally large or via an unusual route. Even then, the rate of symptoms was quite low - 70% of these patients with abnormal exposure had no symptoms at all. Only 2 individuals died. Interestingly, it did not appear that the actual concentration of the DEET played a role in the rate of side effects.

After reviewing the 110 articles that seemed to be related in any way, I would conclude the following:

1. DEET is probably safer than the insect bites it prevents and certainly has a lower rate of side effects (I truly hate the side effects of mosquito bites). In fact, more people seem to die of water toxicity than from DEET exposure.
2. Use the time release versions of DEET - not because they are safer, but because they seems to work as well and for a longer period of time than the 100% DEET
3. Don't let your dog drink DEET. But even then, the one article I found on companion dog exposure indicated that even the worst canine exposures did not result in long-lasting or fatal symptoms.
4. There was no published research indicating that any of the other products on the market were safer than DEET. In fact, I was unable to find any studies on the safety profile of oil of lemon eucalyptus at all. The fact that it is "natural" does not convince me of it's safety.

Finally, I will mention that there was not a single, solitary, piece of medical evidence in the medical literature that documented a long-term side effect arising from moderate use of DEET that was generally consistant with the labeling. Even in kids or dogs.

I'm not sure of the exact composition of the Repel lemon eucalyptus repellent but eucalyptus oil itself has been used for centuries with no ill effects that I've heard of. In S. Amercia I've even seen people throw green eucalyptus leaves over hot coals for use as a cheap area repellent. However, as with all menthol/mint type oils, if its concentrated and too much is used, it can irritate the skin.

Unfortunately, there are large numbers of substances that have been used for centuries that turn out to be not-so-good for you. And the problem with "natural" products is that there is no legal requirement for such safety research to be done before the product is sold commercially. So, again, there is no medical research data that speaks to the safety of lemon eucalyptus whereas there is safety data on DEET (though not the greatest data ever, I will agree). Not saying lemon eucalyptus is unsafe -- simply saying there is no medical data one way or the other. It is totally possible that if someone actually researched the subject they would find that lemon eucalyptus is associated with skin cancer or some such. Or they might find that it is totally completely safe. Who knows?