https://doi.org/10.1351/goldbook.14840
Concentration of a substance in air, water, sediment, or soil at which \(\pu{50\%}\) of the test organisms survive after a specified time of exposure. The \(\pu{TL_{50}}\) (equivalent to the \(\pu{TL_{m}}\)) is usually expressed as a time-dependent value (e.g., \(\pu{24\!\mbox{-}h}\) or \(\pu{96\!\mbox{-}h}\) \(\pu{TL_{50}}\); the estimated concentration at which \(\pu{50\%}\) of test organisms survive after \(\pu{24 h}\) or \(\pu{96 h}\) of exposure).
Note: Unlike lethal concentration and lethal dose, the term "tolerance limit" is applicable in designating the level of any measurable lethal condition (e.g., extremes in \(\rm{pH}\), temperature, dissolved oxygen). \(\pu{TL_{m}}\) and \(\pu{TL_{50}}\) have been replaced by median lethal concentration (\(\pu{LC_{50}}\)) and median effective concentration (\(\pu{EC_{50}}\)).