How does DEET work? Scientists aren’t exactly sure how DEET repels insects. But contrary to what many people may think, DEET is not an insecticide, Dr. Kennedy said. It doesn’t kill mosquitoes and is not poisonous to them. Instead, it simply deters them, likely with its noxious smell. “The whole point of smelling unpleasant is you’re not the only one that thinks that,” Dr. Kennedy said. “Insects do, too,” he said. That includes other bugs like ticks, fleas and biting flies. Another theory about how DEET works is that it masks certain human emissions, like carbon dioxide, which insects are normally attracted to, Dr. Kennedy said.

Is DEET safe? Millions of people in the United States use DEET-containing products every year, according to the Environmental Protection Agency. Yet between 2007 and 2012, there were only about 2,800 reported health incidents involving the chemical, the agency said in its most recent review. DEET has also been used for about 80 years. If it was toxic or caused cancer, “it would have showed up a long time ago,” said Jeffrey Bloomquist, an insect toxicologist at the University of Florida. Most incidents the E.P.A. noted were minor, involving skin, eye or respiratory irritation like rashes, watery eyes or wheezing.