When it comes to protection from mosquitoes, opinions are abuzz. Burn citronella candles. Wear repellant bracelets. Douse yourself with Avon's Skin-So-Soft. Eat garlic. Take vitamin B1 supplements. Use concentrated DEET. Use dilute DEET. People are confused. Needlessly. There are many questions science cannot readily answer, but the question of what is the most effective mosquito repellant is not one of them.
That's because it doesn't take rocket science to design an appropriate study. You don't need sophisticated equipment and you don't have to extrapolate from rat studies. All you need are some human volunteers who are willing to stick their bare arms into a cage of hungry female mosquitoes. And that is exactly what researchers had fifteen volunteers do at the University of Florida in a study reported in the New England Journal of Medicine. And now we know what works and what does not. This study was very carefully done. Temperature, humidity, density of the mosquito population and state of hunger of the insects were all controlled. Sixteen popular products were purchased and tested repeatedly with the time until first bite being accurately measured. Lets' start with what doesn't work. You can forget about any of the "repellant" wristbands. They kept mosquitoes away for the stunning time of about twenty seconds. Avon's "Skin-So-Soft" may make your skin feel soft but will only keep the bugs away for about twenty minutes. After that your skin will get pretty bumpy from all the bites. Other citronella preparations fared even worse. So unless you are willing to walk around constantly spraying yourself, forget the citronella products.
So what works? N,N-diethyl-m-toluamide or "DEET." Some people worry about DEET because they've heard reports of toxic reactions and are suspicious of the substance because it is a synthetic chemical. Whether a substance is synthetic or not has nothing to do with its safety profile. Indeed DEET has a remarkable safety record. It is estimated that since its introduction around 1960, about 8 billion doses have been applied to humans. And this has resulted in fewer than 50 serious toxic effects of which more than 75% resolved with any further problems. In the handful of cases where there were residual effects, there was heavy, frequent application over the whole body. The New England Journal study has now shown that such applications are not necessary. A DEET concentration of 24% provided five hours of protection. Even a 5% concentration, which can be used on children, kept the mosquitoes away for an hour and a half. DEET should not be used under clothing, on an open cut or with sunscreen. So there it is. Look for a product that has between 25 and 30% DEET for adults and apply it every four hours or so. For young children use the 5% stuff.
If you really want something "natural," although I don't know why that should be appealing, "Bite Blocker" made with soy oil offers about ninety minutes of protection as do products made with eucalyptus oil. And if you think that citronella has a pleasant smell, you're not alone. Mosquitoes like it too. A recent study by Consumers Union corroborated these results. “Off Deep Woods” with 98% DEET kept mosquitoes and ticks away for over ten hours. Products with 20-34% DEET worked for at least five hours. The best non-DEET product was Repel Plant Based Lemon Eucalyptus which contains p-menthane-3,8-diol. It worked for about three and a half hours. Products with picaridin did not work well, neither did a shirt treated with permethrin.