Children are more vulnerable to adverse environmental exposures. The unique ways in which they interact with their environment and their dynamic developmental physiology mean that they generally receive a higher dose of toxicant for a given level of environmental exposure. In addition, children are frequently more likely to suffer adverse health outcomes from exposures. The developmental stage of the child during which the exposure occurs has a major influence on the consequences of the exposure. For example, exposures during organogenesis may result in permanent structural changes, whereas exposures once organogenesis is complete are more likely to result in functional consequences. The immune, respiratory, and central nervous systems are immature at birth and have a prolonged period of postnatal maturation. Thus, these organ systems are vulnerable to postnatal exposures.