tion process. There are pronounced interspecies differences in the thickness of the stratum corneum, dermal blood flow rate, and other determinants of absorption (Mattie et al., 1994; Monteiro-Riviere et al., 1990). Human skin usually is less permeable than rodent skin to many chemicals (Poet, 2000).
Information on the absorption of pesticides and other chemicals from the lungs is often quite limited. Pulmonary exposure is not a major concern for many compounds that have low vapor pressures. Some pesticides, such as soil and grain fumigants, however, are relatively volatile and may be inhaled in high concentrations. Inhaled fumigants such as ethylene dibromide, trichloropropane, and dibromochloropropane are well absorbed and can exert toxic and/or carcinogenic effects in mice and rats. These lipophilic compounds readily diffuse across the respiratory epithelium of the alveoli into the profuse capillary supply of the pulmonary circulation. Systemic absorption of volatile organic chemicals (VOCs) (e.g., trichloroethylene) is often greater in rodents than in humans subjected to equivalent inhalation exposures (Fisher, 2000). The interspecies difference can be attributed to rodents’ higher respiratory rates, cardiac outputs, and blood-air partition coefficients—three major determinants of pulmonary absorption of VOCs (Bruckner and Warren, 2001).
The gastrointestinal (GI) tract is the major portal of entry of most pesticide contaminants of food and water. The rapidity and extent of systemic absorption depends on the physical and chemical properties of the compound, as well as conditions within the GI tract. Some of the more important endogenous factors include gastric emptying and intestinal motility; gut flora; acid and enzyme secretory activities; cellular transport systems; blood supply; and mucosal structure and surface area. The small intestine has the greatest surface area and is frequently the optimal absorption site. Systemic absorption of different classes of pesticides varies widely. As a rule, lipid soluble, unionized forms are relatively well absorbed throughout the GI tract. The molecular weight of highly lipophilic compounds such as pyrethroid insecticides (Anadon et al., 1996) can increase to a point that mucosal penetration diminishes. Ingested arsenic, copper, cadmium, and other metals are poorly absorbed by adults. Experiments by Kostial et al. (1978) reveal substantially lower blood levels of lead, mercury, cadmium, and manganese in adult rats than in sucklings given comparable oral doses of the metals. Morphological and functional immaturities of intestinal epithelial cells account for the greater penetrability of the gut of neonatal animals and humans.
Once a chemical has entered the bloodstream, it is distributed throughout the body. The initial phase of systemic distribution is governed largely by tissues’ rate of blood perfusion and by the rate at which the compound exits the bloodstream (Rozman and Klaassen, 2001). Cer-