Itchy bumps are a real bummer, and they're especially no fun on our little ones. Add the annoyance of scratching to the potential of scary mosquito-borne illnesses such as West Nile, Lyme Disease, Malaria, and Zika, and you get many parents searching for ways to keep bugs off of their families. The most effective way to repel insects is with bug spray containing the notorious ingredient DEET. But will spraying baby (or yourself) with DEET cause harm? Short answer: DEET has not been shown to be harmful to adults, children, or babies as long as usage recommendations are followed (provided below). In fact, a recent review on insect repellent states that "DEET has an excellent safety profile and remarkable protection against mosquitoes and various other arthropods" (Nguyen et a., 2018).
This answer may come as a surprise to many people, as DEET has acquired a bad rap. This started in the 1980's with reports correlating DEET-containing insect repellent exposure with seizures and encephalopathy (damage to the brain) in children (Roland et al., 1985; Edwards and Johnson, 1987). However, DEET's bad reputation based on these reports is unwarranted. Studies in the intervening decades have shown that DEET is safe when used as recommended (i.e. on skin, not ingested), and that it's the most effective repellent against insects.
Let's dive into the safety data. The reports of DEET inducing brain dysfunction are certainly alarming; how do we know that they aren't valid? First off, we have to pay attention to correlation vs. causation: are two events actually happening together because one thing causes the other? In humans, two large studies investigating over 29,000 DEET exposure cases reported to poison control revealed that "DEET presents little to no risk when applied according to product labels" (Swale and Bloomquist, 2019). In fact, as "seizure disorders occur in 3%–5% of children and an estimated 23%–29% of children in this continent are exposed to DEET", the few reported seizure cases overlapping with DEET exposure likely happened by chance (Koren et al., 2003). Even for skin applications of 75% DEET (a level that exceeds recommendations), blood concentration only reaches 0.0005 mmol/L; it requires ingestion of a similar amount of DEET to reach toxic blood concentration levels (1 mmol/L; Koren et al., 2003). In rats, lethal dose studies "place DEET in EPA acute toxicity category 3, which is the same category as NaCl table salt" (Swale DR, Bloomquist, 2019).
What about DEET exposure during pregnancy? The available data in both animals and humans are again reassuring. A study in rats and rabbits revealed no adverse effects on offspring when DEET was ingested, except for the highest dose of 325 mg/kg per day which by far exceeds recommended levels. This dosage led to maternal toxic effects and low birth weights of offspring, but even at this extreme dose “there was no evidence of fetal toxic effects or malformations in the offspring of exposed animals” (Koren et al., 2003). In humans, a randomized study of 900 pregnant women demonstrated that even with daily topical application “no adverse neurological, gastrointestinal or dermatological effects were observed in the women exposed to DEET, and no adverse effects on survival or growth and development at birth and at 1 year of age were detected in the babies whose mothers used DEET” (Koren et al., 2003).
While we aren't entirely sure how DEET works, the main theory is that it disrupts a mosquito's ability to locate humans by blocking antennae receptors. DEET's effectiveness is directly related to the concentration applied, with higher levels effective for longer time periods. The max recommended concentration of DEET is 30%, which provides 6 hours of protection. Other commercially available repellents contain 15%, 10%, or 5% DEET, which protects for 5, 3, or 2 hours, respectively (Koren et al., 2003).
The recommendations for DEET application by age are as follows (based on Sorge, 2009 and AAP guidelines):