INTRODUCTION: The main reason, causing failure of the crew, weapon and submarine military equipment and other hermetically sealed craft of military purpose, is fire. The study is a continuation of sets of works aimed at justifying the creation of hypoxic gaseous medium in sealed habitable objects that provide fire prevention.OBJECTIVE: To study possible long-term effects of extended (for 100 days) man’s stay in regulated hypoxic firesafe gaseous medium for confirming the possibility to perform professional activities by the staff of a sealed craft in such conditions.MATERIALS AND METHODS: For 12 months after finishing 100-day sealing the volunteers were evaluated for any disease, possible deviation of functional state and working capacity. The examined contingent: 5 men, aged 25– 32, 1 man, aged 53.RESULTS: No mental and somatic health disorders were found among all the volunteers within a year of observations. Furthermore, no volunteer experienced any decrease in the body’s resistance, quality of life and other deviations that might be a consequence of the sealing negative influence in regulated hypoxic gaseous medium. The results preliminary allow to use technologies of regulated normobaric hypoxic gaseous medium under well-developed regimes for increasing fire safety of submarines and other sealed habitable objects.