The authors of this paper propose an approximate method of calculating one of the commonest measures of fire and explosion risk encountered in safety engineering practice: the minimum ignition energy of a layer of gaseous suspension of metal particles. This determination is also associated with the estimation of the critical mass particle concentration, above which, for a given value of the igniting pulse, the particle system ignites. It should be noted that it is correct to speak of the existence of three types of ignition concentration limits, associated with the autoignition of the system, the propagation of a flame, and focal ignition. Whereas in the first two cases the concentration limits are associated with the relation between the heat supplied by the exothermic oxidation reaction and the reaction's external heat losses, in the latter case the concentration limit also depends on the intensity of the igniting pulse.