Other Routes of Excretion
Several minor routes of excretion exist, primarily via mother's milk, sweat, saliva, tears, and semen. Excretion into milk can be important since toxicants can be passed with milk to the nursing offspring. In addition, toxic substances may be passed from cow's milk to people. Toxic substances are excreted into milk by simple diffusion. Both basic substances and lipid-soluble compounds can be excreted into milk. Basic substances can be concentrated in milk since milk is more acidic (pH ~ 6.5) than blood plasma. Since milk contains 3-4% lipids, lipid-soluble xenobiotics can diffuse along with fats from plasma into the mammary gland and thus can be present in mother's milk. Substances that are chemically similar to calcium can also be excreted into milk along with calcium. Examples of substances that can be excreted in milk are DDT, polybrominated biphenyls, and lead (which follows calcium kinetics).
Excretion of xenobiotics in all other body secretions or tissues (including the saliva, sweat, tears, hair, and skin) are of only minor importance. Under conditions of great sweat production, excretion in sweat may reach a significant degree. Some metals, including cadmium, copper, iron, lead, nickel, and zinc, may be eliminated in sweat to some extent. Hair, in some cases, may be used as a biomonitoring tool for metals. Xenobiotics that passively diffuse into saliva may be swallowed and absorbed by the gastrointestinal system. The excretion of some substances into saliva is responsible for the unpleasant taste that sometimes occurs with time after exposure to a substance.