## 1 Introduction  
In modern cities, multi-unit residential apartment buildings, which usually exceed five stories, are commonly used as living spaces. In South Korea, as of 2022, the proportion of residents living in apartments exceeded 52.4% in terms of household composition, and apartments accounted for 64% of all housing types [1]. Consequently, fire damage in residential high-rise apartments has significantly increased [2]. When examining the causes of fatalities in multi-unit residential fires, 77.9% of the deaths were related to the inhalation of smoke and toxic gases [3]. To prevent toxic gases and smoke in such fires, the Korean NFSC501A, American NFPA 92, and British NFPA 101 advise fire prevention through the use of pressurization systems, and various related studies are ongoing [4].  
Various studies are being conducted to achieve fire control by installing air curtains as an alternative to pressurization systems. The following studies assessed the effectiveness of air curtains in the case of a fire. In bi-directional tunnels, it was found that air curtains set at a 30\({}^{\circ}\) angle have a smoke control effect, and high air-curtain speeds with angles above 10\({}^{\circ}\) showed areas where smoke was blocked, even at high temperatures during a fire [5,6]. In a computational fluid dynamics (CFD) model study, the sealing efficiency of horizontal air curtains, vertical air curtains, and bi-directional vertical air curtains was compared at a doorway connecting two rooms at different temperatures [7]. Furthermore, CFD simulations using double-jet air curtains to test the smoke suppression and exhaust efficiency involved experiments to determine the toxic gas blocking performance in three scenarios: without air curtains, with double-jet air curtains, and with conventional air curtains [8]. In studies examining the effects of vertical and horizontal air curtains in multi-story buildings, vertical air curtains showed a reduced performance in under-ventilated situations, but were effective in over-ventilated conditions for fire blocking. Horizontal air curtains were found to be effective at controlling toxic gases even in poorly ventilatedconditions. Furthermore, to enhance the air curtain performance, increasing the jet wind speed of the air curtain and providing indoor ventilation systems were proposed [9; 10].  
[PERSON] et al. designed operational conditions through investigating the impact of the spray direction of a composite air curtain on the extinguishing effect [11]. [PERSON], S.M.J. analyzed the conditions of an emergency ventilation system consisting of air curtains and exhaust fans to manage heat and toxicity in various fire scenarios [12]. [PERSON] et al. revealed that the minimum thickness for sealing effectiveness is 0.3 m and that the jet velocity should be at least 2 m/s, depending on the thickness and spray velocity of the air curtain [13].  
These studies demonstrate the effectiveness of air curtains at blocking toxic gases and smoke in fire situations. This study compared and analyzed the performance of air curtains installed in outdoor escape stairs, where separate pressurization equipment cannot be installed, to block smoke and toxic gases entering the stairs. Simulation results for cases installed in actual buildings were examined, and a hot smoke test was conducted to verify the effectiveness.