two extremes of the life cycle, that is, young children and the aged, are similar in many ways due either to the immaturity or normal decline in functioning of major physiologic processes.

Although there are wide individual variations, elderly populations have progressively decreasing function of cardiac, renal, pulmonary, and immune system processes (Tarcher, 1992, p. 198). As a result of these changes—most of which have been documented in the study of drug therapies in the aged—elderly individuals may have impaired host defenses, impaired immune system function, and changes in their ability to detoxify chemicals. Changes in the stratum corneum of the skin can increase the percutaneous absorption of chemicals. Structural and functional changes that occur in the lung with advanced age, including loss of elasticity and impaired ciliary action, can result in more rapid absorption and decreased clearance of foreign substances in the lung. A decline in the metabolic clearance of certain drugs that require oxidative mechanisms for biotransformation has been noted in aged populations that may also result in a decreased ability to detoxify environmental toxins. Declines in blood flow to both liver and kidney, in part due to declining cardiac output estimated at 1 percent annually after the age of 30, may result in a decreased ability to detoxify and eliminate toxic substances from the body among aged populations. Immune system function is also impaired with aging, including a reduction in cell-mediated immunity and T lymphocytes. Finally, a change in body composition occurs with aging; there is a marked increase in adipose tissue mass with a decline in lean body mass. As a result of changes in body composition, water soluble drugs and chemicals have a smaller volume of distribution and greater serum levels, while lipid-soluble substances have an increased volume of distribution. This spectrum of physiologic changes in the aged may increase or decrease both their susceptibility to, and the magnitude of, adverse health outcomes associated with exposure to environmental hazards.

Children are also uniquely susceptible to environmental hazards. They have a higher basal metabolic rate than adults, which affects the absorption and metabolism of toxicants. Children also have a different breathing zone than adults; they are closer to the floor, where dust, dirt, and toxic heavy metals such as lead are deposited. The rapid growth and differentiation of cells in young children leaves them more susceptible to genetic alterations associated with many chemical exposures. An increased rate of cell proliferation can indirectly lead to carcinogenesis by increasing the likelihood that spontaneous mutation will occur or by decreasing the time available to repair DNA damage (NRC, 1993b). Moreover, the normal hand-to-mouth activity of toddlers increases the likelihood of exposure through ingestion of toxic substances. Because some



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