Hazard Structure and Repair Kinetics: A 2x2 Classification that Resolves the Stochastic-Death / Individual-Tolerance Dilemma

A 2x2 GUTS framework shows that, for time-variable exposures in Gammarus pulex, a peak-based hazard with saturating or proportional repair better matches survival carry-over ratios in 14 of 15 cases.

• Why it matters: Stochastic-death and individual-tolerance versions of GUTS disagree on no-effect concentrations under variable exposure, and there is no standard test to distinguish them.
• What they did: The authors compare cumulative vs peak-based hazard accumulation and proportional vs saturating repair, using calibration fits across fourteen compounds and a carry-over test with two identical pulses separated by complete recovery.
• The result: Cumulative hazard is excluded when the second-to-first survival ratio exceeds one (14 of 15 ratios at or above one; 7 significantly), while peak-based hazard matches the ratio’s maximum (5 of 6 compounds), and for diazinon in Daphnia magna measured damage favors peak-based hazard.

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