in selecting an appropriate animal model for developmental pharmacokinetic and toxicology studies, in interpreting the data, and in extrapolating the data to humans. Animal studies are presented here when data on humans are inadequate or when findings in animals elucidate ontogenetic mechanisms.
For infants and children, exposure to pesticides occurs primarily through ingestion, inhalation, and through the skin. The newborn may have previously encountered chemical agents in utero, but an in-depth examination of in-utero exposure is beyond the prescribed scope of this report. The major emphasis in this section is the absorption and disposition of ingested chemicals. Because children put all kinds of things into their mouths, they are at risk of ingesting pesticides from nonfood sources, including contaminated household objects, ornamental plants, sod, and paint. In certain situations, significant exposure may result from inhalation of pesticides or skin contact with contaminated surfaces (see Chapter 7). Dermal and inhalation exposures are also addressed because they may contribute to the total systemic dose and need to be considered when establishing prudent levels of dietary intake for infants and children.
The skin area of the infant per unit of body weight is double that of the adult, whereas the permeability of the infant's skin, except for those born prematurely, appears to be similar to that of the adult. These are important factors to remember when considering dermal absorption or penetration of xenobiotic compounds. The stratum corneum (the outer layer of the skin, which serves as the barrier to penetration by chemicals) is fully developed in the human newborn. Studies of the bacteria-inhibiting agent hexachlorophene in premature and full-term infants, the hormone testosterone in infant and adult monkeys (Wester et al., 1977), and alcohols in premature and full-term infants and human adults have shown no differences in penetration, but differences in absorption have been shown for fatty acids (Wester and Maibach, 1982).
There is little evidence to suggest that percutaneous absorption of chemicals varies greatly with age during the preadolescent period, since the overall thickness of the stratum corneum remains relatively constant throughout postnatal development (Rasmussen, 1979). There is a paucity of information, however, from well-controlled studies on percutaneous absorption of chemicals in this age group. McCormack et al. (1982) observed no difference in the rate of penetration of a series of alcohols through premature, full-term newborn, and adult skin specimens in vitro. They did find differences in penetration of a series of fatty acids, which the investigators attributed to differences in solubilization of the fatty