Fire Safety Science Digital Archive

Test results from the NIST 2008 Smoke Alarm Sensitivity Study were used in a smoke alarm performance analysis to examine the effects of pre-movement time, reduced travel speeds through smoke, and smoke optical density limit on occupant survivability given different smoke alarm installations. Smoke alarm installations that meet the requirements in the current National Fire Alarm code NFPA 72 were considered. Alarm times from commercially-available photoelectric, ionization, and dual photoelectric/ionization alarms were used in the analysis to examine the effects of smoke alarm type on the predicted survivability for a range of fire and egress scenarios. Fire scenarios included both flaming and initially smoldering upholstered chair mock-ups. Egress scenarios considered occupants located in, or remote from the room of fire origin. Reduced travel speed through smoke was included in the analysis. Prior to occupant movement and as an occupant travels to the exit, the fractional effective dose from toxic gas and heat exposure were computed to determine survivability. The concept of relative effectiveness as performance metric for smoke alarms is introduced. The relative effectiveness is the fraction of occupants that successfully escape a given fire and egress scenario. It is computed by considering a frequency distribution for the pre-movement time and determining the cumulative fraction of occupants that successfully escape. Thus, the relative effectiveness of a smoke alarm type or installation requirement can be averaged over a large number of fire and egress scenarios. The pre-movement frequency distribution was modeled as a log-normal function. Experimental studies suggest that the median value of the distribution relates to characteristics of the population and a geometric standard deviation of 1.6 characterizes the width the distributions. The distribution median was varied to examine relative effectiveness skewed to more vulnerable populations (those slower to react). Travel speed was modeled as a function of smoke optical density which predicts reduced travel speed as thicker smoke is encountered. Model results showed photoelectric alarms had the lowest relative effectiveness values for flaming fires, while ionization alarms had the lowest relative effectiveness values for smoldering fires. These trends were expected based the results of previous studies. It was observed that there can be a steep increase in relative effectiveness, depending on the smoke alarm type and fire scenario, as the smoke optical density limit was increased from 0.25 m-1 to 0.50 m-1. However, the ranking of smoke alarms tend to remain the same. Given the magnitude of statistically significant mean values of relative effectiveness for all flaming and smoldering fires considered, the model results suggest that there is a benefit from a combination of alarm technologies, and that vulnerable populations who may require significantly more time to escape, regardless of the fire scenario, would benefit the most from dual alarms or side-by side photoelectric and ionization alarms.