We show how reformulating a first-order response theory with respect to both toxicant concentration and exposure time, in terms of relative instead of absolute increments, leads to a generalized version of Haber's law and to a combined sigmoid model for concentration and time response, derived on analytical rather than empirical grounds. More general presuppositions of Haber's law (without assuming sigmoid response) are briefly discussed. The relative intensities of time response and concentration response are measured by a single dimensionless parameter, characterizing each quadruple {toxicant, route, species, adverse effect}. Cross-species and cross-toxicant variability of this parameter is discussed and illustrated by new findings on algae and Daphniae exposed to selected inorganic and organic pollutants.