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17 CHAPTER THREE TENABLE ENVIRONMENT--LITERATURE REVIEW To understand and interpret the objectives of fire regulations ume do not occur in the absence of burns to the skin (the face); it is necessary to have basic knowledge in the physics of fire, therefore, tenability limits with regard to skin burns normally tenable limits for escaping civilians and firemen, and damage are lower than for burns to the respiratory tract. However, ther- criteria for tunnel construction and equipment. mal burns to the respiratory tract can occur upon inhalation of air with a temperature above 60C (140F) that is saturated Fire produces high temperatures, heat radiation, a low con- with water vapor. centration of oxygen, low visibility, and different lethal toxic and/or corrosive gases. All of these physical phenomena, The tenability limit for the exposure of skin to radiant heat some of which can be calculated with some accuracy, can be is approximately 2.5 kW/m2 (800 Btu/hr/ft2). Below this inci- dangerous to people, construction, equipment, and vehicles. dent heat flux level exposure can be tolerated for 30 min or longer without significantly affecting the time available for The tenable environment is an environment that supports escape. Above this threshold value the time to burn skin result- human life for a specific period of time. The goal of fire ing from radiant heat decreases rapidly according to Eq. 1. life safety systems is to provide a tenable environment for evacuation. t Irad = 4 q -1.36 (1) The current technology is capable of analyzing and evalu- where: ating each of the unique conditions of each path to provide proper ventilation for pre-identified emergency conditions. The tIrad = time to burning of skin resulting from radiant heat same ventilating devices may or may not serve both normal (minutes); and operating conditions and pre-identified emergency require- q = radiant heat flux (kW/m2 or Btu/hr/ft2). ments. The goals of the ventilation system, in addition to addressing fire and smoke emergencies, are to assist in the As with toxic gases, an exposed individual can be consid- containment and purging of hazardous gases and aerosols, ered to have accumulated a dose of radiant heat over a set such as those that could result from a chemical or biological period of time. The fraction equivalent dose (FED) of radiant release. Some information, especially on heat effects, was taken heat accumulated per minute is the reciprocal of tIrad. from the annex material of NFPA 502 and is summarized here. Radiation is created by temperature. The level of radia- tion depends on the temperature and the emissivity of the HEAT EFFECTS smoke. When the temperature within the smoke layer is not constant integration is necessary to calculate the radiation Exposure to heat can threaten life in three basic ways (NFPA level. The radiation is produced by the fire itself and by the 502 Standard for Road Tunnels, Bridges, and Other Limited hot smoke layer. Access Highways): Radiant heat tends to be directional, producing localized 1. Hyperthermia, heating of particular areas of skin even though the air tempera- 2. Body surface burns, and ture in contact with other parts of the body might be rela- 3. Respiratory tract burns. tively low. Skin temperature depends on the balance between the rate of heat applied to the skin surface and the removal of The following are used in the modeling of life threat heat subcutaneously by the blood. Thus, there is a threshold owing to heat exposure in fires: radiant flux below which significant heating of the skin is prevented but above which rapid heating occurs. The threshold of burning of the skin, and The exposure at which hyperthermia is sufficient to Based on the preceding information, it is estimated that the cause mental deterioration and thereby threaten survival. uncertainty associated with the use of Eq. 1 is 25%. More- over, an irradiance of 2.5 kW/m2 (800 Btu/hr/ft2) would cor- Note that thermal burns to the respiratory tract from the respond to a source surface temperature of approximately inhalation of air containing less than 10% water vapor by vol- 200C (392F), which most likely would be exceeded near