The smallest newborns can be less than half the weight of the largest newborns and this difference can continue until approximately 6 months of age. Guidelines should prevent potential harm from mercury exposure in this most vulnerable population, as pointed out by Kathryn R. Mahaffey, PhD, from the EPA. "If we take the weights of these children, and assume that they are receiving thimerosal-containing vaccines for each of the recommended vaccines by age, the following figure shows the dose of mercury in micrograms per kilogram that they would receive on the day vaccine is administered."