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This document is only one of many tools available for use in fire safety engineering. It is intended to be used in conjunction with models for analysis of the initiation and development of fire, fire spread, smoke formation and movement, chemical species generation, transport and decay, and people movement, as well as fire detection and suppression. This document is to be used only within this context. This document is intended to address the consequences of human exposure to the heat, obscuration induced by smoke and proposed a simplified mass-loss-based toxicity model. The time-dependent thermal environment of a fire and aerosol concentrations are determined by the rate of fire growth and the ventilation pattern. Once these are determined, the methodology presented in this document can be used for the estimation of the time at which individuals can no longer accomplish their own escape or reach a place of safe refuge. This guidance can also be applied to estimation of the time limit for rescuing people who are immobile due to injury, medical condition, etc. This document establishes procedures to evaluate the life threat components of fire hazard analysis in terms of the status of exposed human subjects at discrete time intervals. It makes possible the estimation of a tenability endpoint, at which time it is estimated that occupants are no longer able to take effective action to accomplish their own escape. It enables estimation of a tenability endpoint for each of the fire effluent components, with the most important endpoint being the earliest to occur. Although occupants’ inability to accomplish their own escape is consistent with the definition of incapacitation (see ISO 13943), this term is not used in this document due to its potentially broad interpretation to include many effects, including collapse and unconsciousness, that are not addressed. This document focuses specifically on taking effective action to accomplish escape as may be influenced by both physiological and behavioural responses resulting from exposure to fire’s life threat components. The life threat components addressed in this standard include heat and visual obscuration due to smoke together with a simplified mass-loss-based model for toxicity assessment of combustibles.