# Risk Assessment of Hydrogen Cyanide for Available Safe Egress Time in Fire Simulation
## 1 Introduction  
To mitigate the increased fire hazards in high-rise and complex buildings, many countries have adopted and implemented performance-based designs (PBDs) [1]. PBDs involve complex considerations of various factors that can influence fire hazard assessment, posing significant challenges. Therefore, evaluations are conducted through the comparison of available safe egress time (ASET) and required safe egress time (RSET) based on a simple timeline technique [2]. ASET is determined using the predicted results from fire simulations conducted for spaces and fire scenarios with relatively high fire hazards, in accordance with the objectives of PBDs. Specifically, ASET is calculated based on the earliest time to reach the threshold values for various parameters outlined in life safety codes. Recent PBD assessment results in the Republic of Korea indicate that 94% of the ASET calculations are determined by visibility [3]. However, validation and verification (V&V) results of fire simulations have shown that there is significant uncertainty in predicting visibility through smoke concentration, with an overestimation factor of approximately 2.6 [4]. This suggests that the reliability of ASET determined by visibility may be significantly compromised. Furthermore, annual statistical data compiled by the National Fire Protection Association (NFPA) and the United States Fire Administration (USFA) indicate that inhalation of smoke and toxic gases is the primary cause of deaths and injuries in fires [5; 6]. Therefore, there is a notable discrepancy between the finding that visibility is the main parameter determining ASET and the fact that the direct cause of fire-related casualties is largely due to the inhalation of toxic gases. In a fire environment, toxic gases that can be generated include carbon monoxide (CO) and carbon dioxide (CO\({}_{2}\)), in addition to hydrogen cyanide (HCN), nitrogen oxides (NOx), ammonia (NH\({}_{3}\)), hydrogen chloride (HCl), and formaldehyde (CH\({}_{2}\)O) [7]. Among these, except for CO, CO\({}_{2}\), and HCN, the remaining toxic gases are inherently lethal but require a longer time to exert toxic effects on humans, making them less likely to be the primary cause of death. In contrast, HCN is approximately 25 times more toxic than CO, which is a major cause of death in fires [8]. Furthermore, HCN has a faster binding rate to hemoglobin than CO, reducing the amount of oxygen supplied to body tissues in a short time, thereby incapacitating the body [9; 10]. HCN is primarily generated during combustion reactions of polymers and can be produced from combustibles readily encountered in daily life, such as furniture, clothing, and building materials [11]. Additionally, the yield of HCN is proportional to temperature, meaning that as the fire grows, the yield increases, posing an even greater hazard [12]. Table 1 summarizes the parameters and threshold values considered in life safety codes for various countries [13; 14; 15; 16; 17; 18]. Common parameters include gas temperature, visibility, and the concentrations of O\({}_{2}\), CO\({}_{2}\), and CO. However, it can be noted that HCN concentration is only considered in specific countries' regulations. Even when HCN concentration is taken into account, studies assessing the risk of HCN using fire simulation for performance-based design (PBD) are scarce. In the Republic of Korea, HCN concentration was considered during the initial phase of PBD implementation (2010-2015), but it has been excluded since 2015. This exclusion is due to the difficulty in determining the cause of death from toxic gases in fires, as well as the complexity and high cost of measurement, resulting in limited information on the chemical mechanism of HCN generation in fires. Consequently, the absence of input parameters for predicting HCN concentration in fire simulations led to its exclusion.