Sex Differences in the Physiological Response to the Modern Fire Environment

Abstract

Due to recent changes in building design and materials, modern structural fires tend towards a ‘ventilation-limited’ fire environment, resulting in globally low levels of oxygen (O2) and increased amounts of carbon monoxide (CO) and smoke. It is unknown how the dynamic hypoxic and hypercapnic environment as a result of the fire impacts an occupant’s physiological and cognitive ability to evacuate. Moreover, it is unknown if physiological sex differences in males and females may further impact egress abilities. This work explored the effects of this dynamic hypoxic and hypercapnic environment on the ability to egress by exposing males and females to sub-incapacitating levels of hypoxic and hypercapnic gases measured in large-scale ventilation-limited fires. Thirty participants (n=15 females) completed three testing days. Day 1 was a familiarization day, intended for participants to familiarize themselves with the experimental protocol of the egress scenario in ambient conditions. Day 2 consisted of five trials of the simulated evacuation scenario wherein real-time changes to O2 and CO2 were administered with CO saturations (%COHb) of 4% and 7%. Day 3 consisted of four trials of the simulated egress scenario wherein changes to O2 and CO2 were carried out in tandem and in isolation. The egress scenario consisted of the following 12.5 minute sequence: 1 min seated pre-test baseline, 5 mins seated, 3.5 mins walking, 4 mins walking carrying a 20lb weight and 2.5 mins seated post-test baseline. This scenario was intended to simulate a realistic evacuation of an occupant from a residential structure, however did not include the more dangerous fire elements (increased temperature, smoke and particulate matter). End-tidal gases, ventilation, heart rate and oxygen saturation were measured continuously. Response (decision, answer and reaction) time was assessed by participants answering a prompt every 15s throughout each trial. The prompts consisted of a directional EXIT sign in which participants had to select the arrow corresponding to the direction of the EXIT sign, as well as a modified Stroop Colour Test. Results demonstrate that females elicited a greater percent change within a condition compared to males for heart rate (p=0.04), tidal volume (p=0.03) and fraction of hemoglobin bound to oxygen (p=0.02). Physiological changes in responses for all participants were significantly greater during the hypercapnia egressing trials compared to hypoxia and control trials (Ventilation: 72±20 L/min, 34±8.9 L/min, 33±6.4 L/min, p< 0.05; tidal volume: 2.3±0.6L, 1.5±0.4, 1.4±0.3, p <0.05). Physiological responses during 4% COHb and 7% COHb trials did not differ other than the fraction of hemoglobin bound to oxygen during pre and post baselines (p<0.05). Overall, the presence of CO2 resulted in the greatest physiological response and coincided with a decrement in ability to complete the egress protocol. Smaller females appeared to be more affected by fire conditions during egress than males. These results indicate that the modern ventilation-limited fire environment results in physiological responses that could negatively impact an occupant’s ability to effectively evacuate.