Toxicity is generally measured by using standard bioassay procedures where test organisms are placed in a container with various dilutions of a toxic substance. Because the toxic effects of most pollutants are manifested rather quickly in the death of an animal, and death is an easily observable endpoint, lethality has been the criterion by which toxicity is measured. In standard bioassay studies, representative organisms (usually groups of ten fish) are subjected to varying concentrations of a toxicant. An evaluation of toxicity is then made from percent mortality over a given period of exposure; usually 24, 48, or 96 hours. The results are expressed in terms of LC50, or median lethal concentration.
The basic problem with standard bioassay procedures, however, is that the procedures are only concerned with measuring mortality. Because a toxic substance may have adverse effects on an organism without immediately causing death, it is dangerous to assume that certain sublethal levels of a toxicant may be considered safe. Concentrations of wastes that merely permit survival are meaningless since they may not permit proper functioning of the organism. Without the ability to function properly, survival is often temporary. Therefore, sublethal effects of pollutants may be as important in the long run as lethal doses in modifying populations and balances within an ecosystem.