The article describes the methodology which makes it possible to evaluate probable irretrievable losses among personnel and population taking into account the structure and dynamics of the disaster factor, as well as the following data: the time of detection and warning notice about contamination with strong noxious toxic substances (SNTS), availability of individual and collective protection against chemical agents for troops and population, toxic dosage of SNTS, acceptable time of safety gas masking, estimated defeated time of gas mask due to lack of oxygen inside SNTS cloud, evaluation of territory contamination by different SNTS if several chemical cisterns were destroyed, etc.