chemical concentration that is expected to kill 50% of a group of organisms. Median Effective Concentration (EC50) – The chemical concentration that is expected to have one or more specified effects in 50% of a group of organisms. Critical Body Residue (CBR) – An approach that routinely examines whole-body chemical concentrations of an exposed organism that is associated with an adverse biological response. Baseline toxicity – Refers to narcosis which is a depression in biological activity due to toxicants being present in the organism. Biomagnification – The process by which the concentration of a chemical in the tissues of an organism increases as it passes through several levels in the food web. Lowest Observed Effect Concentration (LOEC) – The lowest test concentration that has a statistically significant effect over a specified exposure time. No Observed Effect Concentration (NOEC) – The highest test concentration for which no effect is observed relative to a control over a specified exposure time. Maximum Acceptable Toxicant Concentration (MATC) – An estimated value that represents the highest “no-effect” concentration of a specific substance within the range including the NOEC and LOEC. Application Factor (AF) – An empirically derived “safe” concentration of a chemical. Biomonitoring – The consistent use of living organisms to analyze environmental changes over time. Effluent – Liquid, industrial discharge that usually contain varying chemical toxicants. Quantitative Structure-Activity Relationship (QSAR) – A method of modeling the relationship between biological activity and the structure of organic chemicals. Mode of Action – A set of common behavioral or physiological signs that represent a type of adverse response. Mechanism of Action – The detailed events that take place at the molecular level during an adverse biological response. KOW – The octanol-water partition coefficient which represents the ratio of the concentration of octanol to the concentration of chemical in the water. Bioconcentration Factor (BCF) – The ratio of the average chemical concentration in the tissues of the organism under steady-state conditions to the average chemical concentration measured in the water to which the organisms are exposed. All terms were derived from Rand. Significance in regulatory context In the United States, aquatic toxicology plays an important role in the NPDES wastewater permit program. While most wastewater dischargers typically conduct analytical chemistry testing for known pollutants, whole effluent toxicity tests have been standardized and are performed routinely as a tool for evaluating the potential harmful effects of other pollutants not