Target Organ An organ in which a xenobiotic exerts a toxic effect. TD0 Toxic Dose 0%.  The estimated dose none of the population is expected to exhibit toxic effects. TD50 Toxic Dose 50%.  The estimated dose at which 50% of the population exhibit toxic effects. TD90 Toxic Dose 90%. The estimated dose at which 90% of the population exhibit toxic effects. Teratogen A compound that causes birth defects in a developing fetus. Teratogenesis The process by which a substance causes the abnormal development of tissues or organs in a developing fetus. Teratogenicity The development of birth defects as the result of exposure to a teratogenic toxicant. Therapeutic Index (TI) The ratio of the dose needed to produce the desired therapeutic response to the dose producing toxicity. Threshold Dose The dose at which a toxic effect is first encountered. Threshold Limit Value (TLV) A recommendation by the ACGIH for the highest level of exposure to a chemical that is safe. TLV see Threshold Limit Value Tolerance The ability to endure unusually large doses of a substance without ill effect.  Toxic effects are decreased with continued exposure to the substance. Total Dose The sum of all individual doses which may be received over a period of time. Toxicant An agent that produces adverse effects when absorbed into the body. Toxicokinetics The pharmacokinetics of a toxic chemical. Toxicologist A person who studies harmful effects of chemicals including the mechanisms by which the effects are produced and the probability that the effects will occur under specific exposure conditions. Toxicology The study of the harmful interactions of chemicals on living organisms and biological systems. Toxin A specific protein produced by certain plants, animals and microorganisms that is highly toxic to other organisms (snake venom). TSCA Toxic Substances Control Act.  It is federal law regulating chemicals in the environment.  It is administered by the Environmental Protection Agency. Tumor see Neoplasm