A minimal disturbance is necessary to bring about thermal runaway in a system on the verge of criticality. The nature and size of this disturbance is governed by the unstable standing solitary wave whose existence was proved in a previous paper (Gray & Kordylewski, Proc. R. Soc. Lond. A 398, 281-288 (1985)). The effect of reactant consumption on the minimal disturbance necessary is important. A proportionally greater increase is necessary here than in the `classical' problem of thermal explosion with reactant consumption.