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Use of LC50 in aquatic regulatory toxicology-Disharmony in global harmonization of hazard classification of chemicals

In regulatory aquatic toxicology, acute toxicity studies with chemicals are conducted with a species of fish, crustacea, and or alga. The LC50/EC50 obtained from these studies is used for the hazard classification and labeling of the chemicals. The methods like probit or logit analysis and Litchfield and Wilcoxon method are prescribed in the OECD guidelines to determine the LC50. In the present study, LC50s were calculated using probit analysis, Litchfield & Wilcoxon method, and also using the method by Trevan (the inventor of median lethal dose) using three sets of concentration-mortality data of fish acute toxicity tests. The slopes of the concentration-mortality curves, fiducial limits (95% confidence interval) of LC50s, and ‘mode’ of the concentration-mortality curves were compared. Though the methods used in the study resulted in more or less similar LC50s, the LC10 and LC90, slopes and ‘mode’ differed considerably, indicating that LC50 does not reveal the exact toxicity profile of a chemical. The LC50 calculated using Finney’s probit analysis provides better information on the toxicity profile of a chemical than the LC50calculated by Litchfield & Wilcoxon method. While interpreting LC50, the mortality occurred below 16 % (eg.,LC10) and above 84 % (eg.,LC90), slope and ‘mode’ of the concentration-mortality curve may also be considered. It is worth having a relook at the current practice of hazard classification and labeling of the chemicals based only on LC50 in regulatory aquatic toxicology.