Chapter 5 Real-Time Fire Monitoring for the Post-Ignition Fire State in a Building
5.4 Event Detection Model
5.4.1 Hazard 1: Smoke Toxicity
Smoke toxicity has been seen as the cause of death for building occupants on many occasions. One way that it is understood is in terms of the concentration and exposure time which leads to either incapacitation or death. Alaire [41] discusses, for example, oxygen depletion in terms of what was observed in blood samples from autopsies of fire victims. This concept of blood concentration is useful in quantifying the effects of exposure to harmful gases. For example, in the case of CO, four minutes of exposure results in about 40% carboxyhemoglobin (COHb) in the bloodstream leading to incapacitation due to the CO binding with the hemoglobin in the blood [40]. But for the purpose of real-time monitoring of smoke toxicity levels, it is necessary to translate this information into engineering parameters for tracking human exposure during the fire event: something that has been established through the use of FED in the past.
For the current study, toxicity levels were monitored using measurements of CO, CO2, HCN, and O2 (depletion of O2) based on limits that are linked to smoke inhalation incapacitation and death. According to the study by Alarie [41], if oxygen levels in the room of origin reach less
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than 7%, then this will become a primary cause of local incapacitation and death. However, such low oxygen levels usually accompany very hot smoke which would cause skin surface and systemic hyperthermia to become major factors as well. Low oxygen levelsplus extreme heat are fast-acting and principal factors leading to incapacitation and death [41]. Since burn threats due to such high-intensity heating represents another hazard altogether, they were handled in the following subsection.
Due to the nature of the hazard of smoke inhalation, checking thresholds instantaneously does not sufficiently capture the time of exposure to such harmful gases. Thus, the Fractional Effective Dose (FED) methods were implemented to assess each room in the building for smoke hazards [38], but in real time for this application. FED has been used in other applications as well, including smoke visualization [42], path safety evaluation [43], fire simulation software such as FDS [39], evacuation planning based on GIS [44], and in research related to testing materials for smoke toxicity [45]. In recent years, Xu et al. [42] used FED in their definition of smoke hazard as part of a virtual reality tool using smoke visualization and evacuation pathfinding. FED has been used in various applications of fire safety and even visualization, but is not normally found in the real-time monitoring space.
While these expressions for FED that follow were designed for unprotected human exposure based on testing with rodents in the laboratory setting [38], there is still value in using these calculations for warning firefighters as well, even though personal protective equipment in a typical firefighting scenario can be expected. Using these FED indicators in real-time, though, can help provide assistance in assessing the smoke toxicity dangers with early warning, which could also improve decision-making for rescue applications of trapped building occupants or simply for avoiding severely toxic regions altogether if full evacuation was assumed.
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The FED is typically used to determine the increasing threat due to exposure to toxic gases over time during a fire event. It is based on the concept of taking a summation of several consecutive, short, transient “exposure-doses” of harmful gases relative to known threatening levels of these gases. The basic equation for computing the FED is shown in Eq. (5.3) below:
interval Δti over which that concentration was measured. The denominator holds the limiting value of the total exposure-dose (Ct)max required to cause a reaction in 50% of the occupants. When the summation reaches a value of 1.0 at some point during the summation, the threshold has been crossed and the particular reaction is expected in 50% of the human population (where the reaction thresholds are typically incapacitation and death).The SFPE Handbook [40] as well as the FDS User Guide [39] provide us with the details of the empirical formulas that are used to assess smoke toxicity: specifically for irritants using the Fractional Lethal Dose (FLD) and asphyxiants using the FED. The primary equation for assessing smoke toxicity using FED was presented in the SFPE Handbook as follows:
2
The instantaneous FED at time ti can include multiple gases, as indicated in Eq. (5.4) above. In particular, carbon monoxide (CO) was one of the gases considered:
D t CO V
FEDCO (3.317105)[ ]1.036 (5.5) The concentration of carbon monoxide is measured in units of ppm and is given by [CO]. The V term represents a breathing volume rate measured in units of L/min and has the following associated values: 8.5 L/min for resting or sleeping, 25 L/min for light work (which is typically
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used and equivalent to walking to a fire exit), and 50 L/min which represents slow running or walking up a staircase [40]. The limitation is found in D, which represents the exposure-dose of COHb required for incapacitation (30% is default). Finally, the time step is defined as Δt = ti – ti-1
(in minutes), which is the difference between the last two time steps received by the event detection model in this case.
Additionally, the effects due to hydrogen cyanide (HCN) were also considered:
CN t
Instead, the concentrations of nitric oxide (NO) and nitrogen dioxide (NO2) must be subtracted first: [CN] = [HCN] – [NO2] – [NO] (all in units of ppm for this equation). Irritants such as NOx
gases, hydrochloric acid (HCl), hydrobromic acid (HBr), hydrogen flouride (HF), formaldehyde, and acrolein may also exist within buildings during real fire events and they are handled in the SFPE Handbook. However, in the current study, none of the NOx gases nor any other irritants were measured or used in the simulations. As a result, [NO2] = [NO] = 0 and consequently the value of FLDirr in Eq. (5.4) is also zero.
Another important contribution in Eq. (5.4) is the hyperventilation factor, which serves to increase the occupants’ potential consumption of toxic gases during extreme conditions experienced during the fire: oxygen depletion is handled in the final term of the FED equation given in Eq. (5.4):
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Equations (5.4-5.8) were used in the event detection model to compute the FED of the smoke over time in the simulation.
Relating computed values to the warning levels for fire monitoring in this component of the model, FED > 1.0 corresponds to warning Level 1. If the oxygen percentage decreases to 7%, the warning is incremented by one as well (i.e., to Level 2). This limit of 7% is not checked in the FED (explicitly), but it is associated with rapid deterioration and death in previous fire events [41]
and is thus included as another criteria for assessing the hazard level. Therefore the possible outcomes are warning levels in range of {0, 1, 2} as implemented here in the event detection model for smoke toxicity.