In the broadest sense, toxicology is the science of poisons and the harmful or noxious effects these substances have on living things. A toxicologist is an individual who is responsible for predicting the toxic or harmful nature of a substance by designing experiments that will supply the data necessary to assess the toxicity of materials. These data help toxicologists make predictions about the hazardous nature of materials tested and their potential impact on the environment and on human populations. Acute toxicity, subchronic, chronic, and reproductive tests are the principal experiments conducted in a toxicology laboratory.
Acute Toxicity Tests
In most acute (short-term) toxicity tests, a single dose of a test substance is given to an animal. One measure of acute toxicity is the lethal dose 50 (LD50), or the dose of a substance that kills 50 percent of the animals tested. Other acute studies are set up to assess the toxicity of products placed in contact with animal tissue to evaluate local tissue irritation. The Draize skin and eye assays are examples of such tests. Acute toxicity tests using animals have been criticized, especially if they are used to evaluate non-pharmaceuticals such as cosmetics and household products. A large amount of research is currently underway seeking alternatives to the use of the LD50 and Draize tests in animals. Until the non-animal alternative tests have proved reliable, however, it is likely that tests using animals will continue to be used as the definitive means of maintaining public safety.
Subchronic Toxicity Testing
Subchronic toxicity studies are 13 to 26 weeks in duration. The animals are dosed daily by the same route that the substance would normally be administered to humans. They are then observed for any toxicity, as well as changes in body weight or food consumption. At the end of the dosing regimen, the animals are euthanized and their tissues evaluated for evidence of toxicity.
Chronic toxicity studies assess the longer-term toxic as well as carcinogenic (cancer-causing) potential of various substances. The test animals are observed for the same parameters as those on subchronic testing, only the observation period is longer (up to two years) and checked for tumors. Postmortem analysis includes evaluation for toxicity as well as carcinogenicity (determined by the presence of tumors).
Reproductive tests are devised to detect changes in the reproductive cycle caused by the compound being tested and the toxic effects of the compound on fertility, organ development, and behavior. Teratology (the study of abnormal fetal development) studies generally involve the exposure of developing litters of rodents to various chemicals or drugs. Changes in normal fetal anatomy, litter size, or fetus weight may indicate that the test substance is a teratogen (a substance that damages the developing fetus).