Shortcomings of the laboratory-derived median lethal concentration for predicting mortality in field populations: exposure duration and latent mortality.

Exposure duration and intensity (concentration or dose) determine lethal effects of toxicants. However, environmental regulators have focused on exposure intensity and have considered duration only peripherally. Conventional testing for toxicology tends to fix exposure time and to use the median lethal concentration (LC50) at that time to quantify mortality. Fixing the exposure duration and selecting the 50% mortality level for reasons of statistical and logistical convenience result in the loss of ecologically relevant information generated at all other times and ignore latent mortality that manifests after the exposure ends. In the present study, we used survival analysis, which is widely employed in other fields, to include both time and concentration as covariates and to quantify latent mortality. This was done with two contrasting toxicants, copper sulfate (CuSO4) and sodium pentachlorophenol (NaPCP). Amphipods (Hyalella azteca) were exposed to different toxicant concentrations, and the percentage mortalities were noted both during and after the exposure ended. For CuSO4 at the conventional 48-h LC50 concentrations, the predicted proportions dead after including latent mortality were 65 to 85%, not 50%. In contrast, only 5% or fewer additional animals died if the latent mortality was included for NaPCP. The data (including exposure time, concentration, and proportion dead at each time) for each toxicant were then successfully fit with survival models. The proportion of organisms dying at any combination of exposure concentration and time can be predicted from such models. Survival models including latent mortality produced predictions of lethal effects that were more meaningful in an ecological or field context than those from conventional LC50 methods.