Aquatic Toxicology Calculator

Aquatic toxicology testing exposes organisms to a substance at several concentrations and records what fraction die, then fits a dose-response curve to estimate benchmark concentrations like the LC50 -- the concentration lethal to half the test population. This calculator uses a simplified two-point version of that method: probit analysis converts each mortality percentage into a probit value (roughly, how many standard deviations from the mean the response sits), then interpolates linearly between the two points in log-concentration/probit space to find where the line crosses probit 5 (50% mortality). The interpolated LC50 is most sensitive to the mortality percentage recorded at your higher test concentration -- that point anchors the steeper, more information-dense end of a typical dose-response curve, so a change there swings the fitted line more than an equivalent change at the shallower low-concentration end -- except when the two mortality percentages sit too close together, where the interpolation line's slope becomes too shallow to support a reliable estimate. Two test points with nearly identical mortality readings genuinely cannot pin down a dose-response slope, so when your two mortality percentages differ by only a few points this calculator falls back to reporting your low test concentration itself rather than extrapolating through a near-zero slope -- enter dose-response data with mortality percentages that are meaningfully different from each other for a trustworthy estimate; a near-tied pair is a real limit of two-point interpolation, not an engine error.

Environmental concentration and the assessment factor never move the LC50, LC10, or LC90 estimates themselves -- they only enter downstream, when the LC50 is divided by the assessment factor to derive PNEC (Predicted No-Effect Concentration), and PNEC is then compared against environmental concentration to compute the risk quotient. A larger assessment factor produces a smaller, more conservative PNEC and therefore a larger risk quotient for the same environmental exposure. This two-point probit interpolation is a coarse approximation -- real regulatory LC50 determinations use multiple concentrations, replicate counts, and full probit or logistic regression with confidence intervals, so treat this calculator's output as an illustrative estimate, not a substitute for a validated toxicology study.