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?