It is especially important to be able to identify and ultimately characterize the nature and extent of behavioral patterns with respect to age and stage of development, since infants, toddlers, and young children may exhibit great differences in exposure related to their mobility and physical development. There are several interconnected factors that may contribute to increased vulnerability for children, depending on the toxic substance under consideration and the age of the child. Children's tissues, organs, and biological systems are still developing, with several stages of rapid growth and development occurring from infancy to adolescence. This rapid development and immaturity of body organs and systems predisposes children to potentially more severe consequences within certain age ranges and windows of vulnerability. Differences in the absorption, metabolism, distribution, storage, and elimination of toxic chemicals that enter the body at different ages may also contribute to increased vulnerability and higher doses to target organs and tissues.
Physiological differences influence the amount of chemical that is absorbed into the body.
Children have a greater ratio of surface area to body weight than adults which may lead to increased dermal absorption. Comparisons of absorption through the respiratory and gastrointestinal tracts between children and adults are complex and could lead to either increased or decreased risk depending on the physicochemical properties of the toxic chemical. The rate at which a substance is distributed to various organs in the body may be influenced by developmental changes. For example, a child's greater volume of extracellular water may dilute substances in this compartment. Also, circulatory flow rates are generally higher in children and may impact the distribution of toxic chemicals, and reduced plasma binding may increase a child's susceptibility to toxic effects.