Many botanical sprays tested were nearly worthless, but two new alternatives seem promising.
A: DEET is considered the king of mosquito repellents, though not everyone is keen to slather it on their skin. A study conducted in the late 1980s on Everglades National Park employees found that one-quarter of the subjects studied experienced negative health effects that they blamed on exposure to the chemical.
Duke University pharmacologist Mohamed Abou-Donia, after studying the effect of frequent and prolonged DEET exposure in rats, concluded that humans should stay away from products containing it. But other studies have shown that while a few people are sensitive to DEET applications, most are unaffected when DEET products are used sporadically and according to the instructions on the label.
The upside is that DEET is very effective. A 2002 study in the New England Journal of Medicine found that DEET-based repellents provided the longest-lasting protection against mosquitoes. A formulation containing 23.8 percent DEET completely protected study participants for more than 300 minutes, while a soybean-oil-based product only worked for 95 minutes. Several other botanical-based repellents lasted less than 20 minutes.