Toxicity is complex with many influencing factors; dosage is the
most important. Xenobiotics, or chemicals foreign to the body,
cause many types of toxicity by a variety of mechanisms. Some
chemicals are themselves toxic and are sometimes referred to as
"parent" compounds. Others must be metabolized (chemically
changed within the body) before they cause toxicity. Many
xenobiotics distribute in the body and often affect only
specific target organs. Others, however, can damage any cell or
tissue that they contact. The target organs that are affected
may vary depending on dosage and route of exposure. For example,
the target for a chemical after acute exposure may be the
nervous system, but after chronic exposure the liver. Toxicity
can result from adverse cellular, biochemical, or macromolecular
changes. Examples are: - cell replacement, such as fibrosis -
damage to an enzyme system - disruption of protein synthesis -
production of reactive chemicals in cells - DNA damage Some
xenobiotics may also act indirectl