diffusion from the stomach and intestine. Active transport mechanisms exist for some other substances. Absorption in the respiratory tract takes place primarily in the lung. Both water-soluble and lipid-soluble agents can diffuse through the thin lining of the lung. The rate-limiting factor is the solubility of the substance in the blood. The skin has several cell layers and is relatively impenetrable to polar substances. It is thus a strong barrier to most toxicants. Those toxicants that