If youve spent much time outdoors, chances are youve had to fight with mosquitoes intent on making you a meal. Mosquitoes are more than just a nuisance. They transmit diseases like encephalitis, malaria and West Nile virus to hundreds of millions of people every year [source: CDC]. Add in the threat of catching Lyme disease from ticks, and its easy to understand the importance of insect repellent.

Different insect repellents contain different active ingredients, some natural and some artificial, but all of them work in a similar way. Insects, particularly mosquitoes, are drawn to the carbon dioxide people release when they breathe, as well as certain odors from the skin. Insect repellent has the same effect on insects as bad cologne or perfume does on people avoidance. Interestingly enough, insect repellent doesnt actually harm insects, making it fundamentally different from pesticides.

So repellents dont harm insects, but can they harm you? For the most part, the answer is "no." The most popular repellents are registered with the U.S. Environmental Protection Agency (EPA), which tests their safety and effectiveness. The list of registered repellents is a short one, including the synthetic repellent DEET, as well as the natural repellents picaridin and oil of lemon eucalyptus, though you can find other repellents like citronella and IR3535 on the shelves.

While some of these repellents have undergone more testing than others, they all prove safe for the vast majority of people when used according to their safety guidelines. Yet insect repellents, by their very nature, work by spending hours in contact with the skin. Can these chemicals that insects abhor possibly be OK for your skin? Read on to find out.

REPELLENTS THREE TIMES AS GOOD?

Researchers from the University of Florida recently set out to find an insect repellent even more effective than the ones currently on the market. After isolating 34 of the most promising formulations, the researchers found some bold participants to test the repellents in mosquito-filled cages. Their research paid off; several of the repellents fended off mosquito bites three times longer than DEET, the current champ in bite protection [source:Highfield].

If something as seemingly benign as sunlight can harm your skin, its not hard to imagine that insect repellents might do the same. In fact, insect repellents can cause skin rashes and irritation. For instance, one study found nearly half of the people using a highly concentrated formula of DEET on the sensitive inner elbow reported adverse effects [source: ASTDR]. 

The good news is that if one repellent reacts badly with your skin, you probably can find another one that doesnt. Lets look at some of the most popular insect repellents to learn what theyre made of and how they affect your skin.

DEET. By far the most popular repellent sold in the United States, DEET was used by more than 200 million people worldwide in 2008 [source: ASTDR]. Its perhaps the most effective repellent on the market, particularly when used in high concentrations. Unfortunately, frequent use of repellents containing a high percentage of DEET can cause a number of adverse reactions, including blisters, rashes and even scarring. Even so, negative reactions to DEET are rare considering the number of people who use the repellent, and washing your skin after returning indoors can help further reduce irritation from exposure.

IR3535. IR3535 is a synthetic repellent available in concentrations of up to 20 percent. The repellent has proven to be nontoxic, causing no skin irritation to speak of. While IR3535 has a great safety record, its effectiveness cant match DEETs. According to one study, IR3535 was found to be only one-eighth to one-hundredth as effective as a similarly concentrated DEET repellent [source: Fradin].

Picaridin. Derived from pepper, picaridin is widely available in Europe and Australia, but is only available in lower concentrations in the United States, barring further study. The repellent appears to be completely safe for the skin, though it can still cause irritation if it touches the eyes. Best of all, one study found picaridin to last 70 percent as long as DEET, making it a highly effective alternative [source: MSDH].

Oil of lemon eucalyptus. Derived from a particular species of eucalyptus tree, oil of lemon eucalyptus is one of only three repellents registered with the U.S. Environmental Protection Agency. Research has shown oil of lemon eucalyptus, often referred to as PMD, to be the most effective of all plant-based repellents. Studies have shown PMD to cause eye irritation but little else, though individuals have reported occasional skin irritation after using a PMD-based repellent.