Thermal environments faced by firefighters
Any uncontrolled fire hazard starts with the ignition of at least one combustible substance. This substance is usually organic hydrocarbon or cellulose that contains carbon (C), hydrogen (H), and/or oxygen (O) [58,59]. The general combustion reactions of this substance are presented in Eq. (2.1) (for hydrocarbon) and Eq. (2.2) (for cellulose) [60,61]. According to Eqs. (2.1), (2.2), the fire chemistry of a combustible substance generates a visible luminous flame. This luminous flame creates a thermal environment of a certain intensity in the surrounding of fire hazards [62-65]. Subsequently, the flame propagates from one combustible substance to another combustible, and/or semicombustible and/or noncombustible substance during the early stage of the fire hazard. This increases the intensity of the thermal environment with respect to time. This stage is referred to as Stage 1 or the “growth period.” Eventually, all the combustible substances present in the vicinity of the fire hazard may come in contact with the flame and could burn; if this occurs, the intensity rapidly rises before a flashover condition is reached. This situation causes the full development of fire, and is called Stage 2. In this fully developed fire period, the intensity of the thermal environment becomes almost consistent. Thereafter, the flame starts to diminish as the amount of combustible substances reduces over time; eventually, the intensity of the thermal environment quickly decreases (Stage 3).
Generally, the intensity is quantitatively represented by the temperature that measures the average kinetic energy of the molecules of various hot combustible, semicombustible, or noncombustible substances present in the thermal environment. In this context, many researchers suggested that the intensity can also be quantitatively represented by heat flux that defines the rate of thermal energy transfer through any given space, gaseous, liquid, or solid . As the intensity of the thermal environment changes in all three stages of any fire hazard, it reflects that both temperature (°C) and heat flux (kW/m2) can vary with respect to time (s) [62-65]. In such variable temperature and heat flux conditions, firefighters have to perform various job activities (eg, extinguishing the fire, rescuing fire victims and property) while navigating various intensified thermal environments [67,68].