The chemical DEET is a plasticizer -- it's actually capable of deteriorating plastic -- and the idea that people were spraying such a chemical on the bare skin of their children understandably caused some to shudder.
A 2002 study of 897 pregnant women in Thailand produced results that some found unsettling. While DEET has been shown in most people to be almost entirely metabolized by the liver before it's excreted in the urine, the study found traces of the chemical in the cord blood of 8 percent of the women [sources: Cecchine, Koren]. This proved that DEET could cross the placental barrier. This was enough for many pregnant women to opt against using it.
Children using DEET-based sprays also became a cause for concern when reports emerged of at least 10 kids suffering seizures after using DEET-based repellents. When applied on the skin, DEET concentrations in the blood are relatively low and can be processed by the body as it's absorbed. When it's ingested orally, however, blood concentrations are hundreds of times higher. Seizures and death have occurred in this increased concentration, generally as a result of bradycardia -- a slowed pulse.
As public concern of DEET increased, a spate of studies examined human exposure to the chemical. Children were thought to be at an especially high risk for adverse effects from DEET exposure. A study by the American Association of Poison Control Centers compared the results among adults and children in nearly 21,000 cases of DEET exposure. The study found that accidental DEET exposure produced fewer cases of adverse effects or deaths among children than it did in adults [source: Koren]. Other studies pointed out that seizures are common in the pediatric population, and that the association with DEET exposure could actually be coincidental.
For nearly 50 years, DEET was sold to consumers in concentrations of 75 percent. In the early 21st century, researchers learned that DEET's effectiveness at preventing mosquito bites tapers off after a concentration of 50 percent. A concentration of 30-percent DEET became the typical concentration for consumer insect repellents. In short order, the Centers for Disease Control (CDC), the Environmental Protection Agency (EPA), the American Academy of Pediatrics (AAP) and other groups issued opinions that DEET-based insect repellents were safe for kids over 2 months of age -- as long as label directions are followed [source: AAP].
The concern that was fostered by early reports of adverse effects from DEET exposure still looms large in the public's mind, however.