and toxicity of pesticides in neonates and infants. Dermal absorption of a variety of chemicals is markedly increased under diapers and rubber pants. These materials retard the evaporation of volatile chemicals and enhance the hydration and temperature of the skin, thereby increasing penetration by water-soluble chemicals. Damage to the stratum corneum, as in diaper rash, circumvents this barrier layer. In addition, some skin surfaces such as the male scrotum and the face are more absorbent than skin in other areas of the body.
The ratio of surface area to body weight in newborns and infants is approximately 2.5-fold greater than that of adults. Thus, if the exposed area of skin and percutaneous absorption rate in a neonate and adult were equivalent, the neonate would receive almost three times the systemic dose, on a kilogram-of-body-weight basis (Wester and Maibach, 1982). Nevertheless, there are few data to indicate which types of chemicals may be more extensively absorbed through the skin of neonates and infants.
The alveolar epithelium is another potential portal of entry into the body for pesticides. Most xenobiotics are absorbed through the alveolar epithelium into the pulmonary (blood) circulation by simple passive diffusion, rather than specific active transport processes (Schanker, 1978). Therefore, changes in several parameters with age may be of consequence in pulmonary absorption of pesticides, including alveolar surface area, thickness of alveolar membranes, porosity and other properties of the membranes, pulmonary blood flow, and respiratory volume. The normal respiratory volume of the resting infant is approximately twice that of the resting adult, when expressed per unit of body weight. The structural development of the human lung is known to continue postnatally (Hislop and Reid, 1981; Langston, 1983). There is a marked increase in alveolar surface area for the first 18 to 24 months of life. Thereafter, pulmonary structures continue to increase in size, and alveolar surface area increases gradually throughout childhood (Thurlbeck, 1982). There is a progressive increase up to 18 years of age in collagenous elastic fiber bundles in the alveolar walls. Some of these factors may counteract others in terms of their influence on the pulmonary absorption of chemicals. For example, the effect of increased respiratory volume may be offset by the infant's smaller surface area for absorption. Unfortunately, little information is available on the pulmonary absorption and bioavailability of xenobiotic compounds in infants and children.
One group of investigators has studied the pulmonary absorption of nonvolatile drugs in neonatal and adult animals. Hemberger and Schanker (1978) injected measured doses of a series of drugs into the tracheas of neonatal (3 to 27 days of age) and adult rats. Systemic absorption was determined by assay of the quantity of drug remaining in the lungs