There is no one right answer to this question. The various concentrations of DEET are out there to address different needs. As a general rule, higher concentrations of DEET will offer longer-lasting protection, but this effect tends to level out at concentrations of DEET over 50%. Under most circumstances of casual use, 10% - 35% DEET will provide adequate protection. However, there are certain conditions in which using a higher concentration of DEET may be preferable. These circumstances include the following:

• Conditions in which there is a rapid loss of repellent to the skin due to washoff from rain, perspiration, or high ambient temperatures
• When traveling to an area where there is a very high density of biting insects (e.g., the Everglades or Alaska)
• When traveling to areas where insect bites can transmit serious diseases to humans (e.g., malaria, yellow fever, filariasis, viral encephalitus, etc.)

Although it is true that there have been rare reports of seizures and neurological side effects associated with DEET use, it is important to realize how rare these reports are. The U.S. EPA estimates that 200 million people use DEET repellent every year. After more than eight billion applications of DEET worldwide, there have been only 21 cases reported in the medical literature in which the use of DEET seemed to have been associated with the development of neurological toxicity. Six of these cases were a direct result of deliberate ingestion. Twelve of these 21 cases resolved completely, without any residual effects. When the EPA reviewed all available DEET human and animal neurotoxicity data in 1998, they concluded there was no evidence that DEET was a selective neurotoxin. Even if all the reported cases of neurological toxicity ascribed to DEET use were confirmed, the real-life risk of neurological side effects from DEET would be less than 1 in 100 million users.

Currently available "natural" or "plant-based" insect repellents cannot match the broad-spectrum efficacy and long-lasting action of DEET repellents. Most natural repellents contain citronella, which is a lemony-scented oil derived from two cultivated grasses. Very variable efficacy has been reported in scientific studies of citronella, depending on the product tested and the species of insect examined. In general, these studies show very short protection times, lasting just a few minutes to up to two hours.

Of the available plant-based repellents, Blocker™ (available in the United States since 1997) seems to be the most effective. This repellent contains soybean oil in a base of coconut and geranium oils and has been shown to work against certain species of mosquitoes for as long as eight hours.