The U.S. Centers for Disease Control and Prevention says repellents with DEET should not cause health problems. DPH says DEET should not be used on infants under two months old. On older children, concentrations of 30% or less are acceptable. Products that contain more than 30-50% DEET do not provide better protection, but they do last longer. A 20% concentration provides about four hours of protection. DEET should be applied to clothes and exposed skin.