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18 the fire where conditions are changing rapidly. Near the fire the less tolerable than a longer exposure to a lower temperature radiation is created by the fire itself, as well as the hot smoke. or heat flux. A methodology based on additive FEDs, similar Farther from the fire it is only the smoke temperature that to that used with toxic gases, can be applied. Providing that creates a dangerous condition. To make evacuation possible, the temperature in the fire is stable or increasing, the total the radiation level must be under the limit that causes severe fractional effective dose of heat acquired during an exposure pain on bare skin for an exposure time of several minutes: can be calculated using Eq. 4. the threshold value is roughly 2 to 2.5 kW/m2 (635 to 800 Btu/ hr/ft2). Firefighters can normally withstand a radiation level 1 1 t2 FED = + t t Irad t Iconv (4) of 5 kW/m2 (1600 Btu/hr/ft2) for at least seven minutes because t1 of protective clothing. Their operation time is a function of a self-contained breathing apparatus and is typically not longer Note 1: In areas within occupancy where the radiant flux than 30 min. For a firefighter to withstand a stay of 20 min, to the skin is under 2.5 kW/m2 (800 Btu/hr/ft2) the first the radiation level cannot exceed 2 kW/m2 (20). term in Eq. 4 is to be set at zero. Note 2: The uncertainty associated with the use of Eq. 4 The amount of time to incapacitation, when exposed to con- would depend on the uncertainties associated with the vective heat from air containing less than 10% water vapor by use of the three previous equations. volume can be made by using either Eq. 2 or Eq. 3. As with toxic gases, an exposed occupant can be consid- The time at which the FED accumulated sum exceeds an ered to accumulate a dose of convected heat over a period of incapacitating threshold value of 0.3 represents the time avail- time. The FED of convective heat accumulated per minute is able for escape for the chosen radiant and convective heat expo- the reciprocal of tIconv. sures. Consider an example with the following characteristics: Convective heat accumulated per minute depends on the 1. Evacuees are lightly clothed. extent to which an exposed occupant is clothed and the nature 2. There is zero radiant heat flux. of the clothing. For fully clothed subjects, Eq. 2 is suggested: 3. The time to FED is reduced by 25% to allow for uncer- tainties in Eqs. 2 and 3. t Iconv = ( 4.1 108 ) T -3.61 (2) 4. The exposure temperature is constant. 5. The FED is not to exceed 0.3. where: Eqs. 3 and 4 can be manipulated to provide the following tIconv = time (minutes); and equation: T = temperature (C). texp = (1.125 10 7 ) T -3.4 (5) For unclothed or lightly clothed subjects, it might be more appropriate to use Eq. 3: where: t Iconv = ( 5.0 10 7 ) T -3.4 (3) texp = time of exposure to reach a FED of 0.3 (minutes). where: This gives the results shown in Table 2. tIconv = time (minutes); and T = temperature (C). AIR CARBON MONOXIDE CONTENT Eqs. 2 and 3 are empirical and can be used for humans. It is Air CO tenable environment content is as follows: estimated that the uncertainty associated with these equations is 25%. Maximum of 2,000 ppm for a few seconds. Averaging 1,150 ppm or less for the first 6 min of the Thermal tolerance data for unprotected human skin suggest exposure. a limit of about 120C (248F) for convective heat. Within Averaging 450 ppm or less for the first 15 min of the minutes of exposure above this temperature there will be exposure. an onset of considerable pain and the production of burns. Averaging 225 ppm or less for the first 30 min of the Depending on the length of exposure, convective heat below exposure. this temperature can also cause hyperthermia. Averaging 50 ppm or less for the remainder of the exposure. The body of an exposed individual can be regarded as acquiring a "dose" of heat over a period of time. Generally, a These values need to be adjusted for altitudes above 984 m short exposure to a high radiant heat flux or temperature is (3,000 ft).