The Frank-Kamenetskii theory of the self-heating of a reactive slab is extended to include volumetric heat losses. The method of non-dimensionalising the heat loss highlights the important role of the Frank-Kamenetskii parameter in the heat dissipation term. Two types of heat loss are considered: firstly that without any dependence on temperature and secondly that with a global (and linear) temperature dependence. It is shown that in either case there is a critical non-dimensional heaf-loss rate beyond which thermal runaway will not occur. This value corresponds to when the actual heat loss is a certain small fraction of the volumetric heat generation. The fraction depends, through the Frank-Kamenetskii parameter, on the constants governing the chemical reaction— in particular the activation energy and heat of reaction.