# Study on the Impact of Ventilation Methods on Droplet Nuclei Transmission in Subway Carriages
## 1 Introduction  
The global outbreak of viral pandemics has posed unprecedented challenges to public transportation, particularly in densely populated subway carriages. This makes these environments prone to being hotspots for the transmission of infectious diseases [1]. Furthermore, aerosolized droplets can remain suspended in the air for extended periods and are considered one of the primary transmission routes for viruses [2]. This characteristic makes controlling the spread of viruses in underground rail networks more complex and challenging. Studies have found that droplets smaller than 30 \(\upmu\)m are minimally affected by gravity and inertia and their transport is mainly governed by the airflow patterns within the room [3]. Therefore, effective ventilation is key to controlling the airborne transmission of respiratory infections, and it is crucial to explore the infection risks within subway systems. Droplets carrying viruses or bacteria are responsible for the majority of respiratory infections [4, 5]. Once susceptible individuals inhale aerosols containing pathogens, respiratory disease transmission events may occur. Viral transmission events are more likely to occur in densely populated indoor spaces, such as classrooms [6], fever clinics [7], and airplane cabins [8]. The particle size of cough droplets ranges from 0.1 to 500 \(\upmu\)m [9], with approximately 97% of droplets having a particle size of less than 1 \(\upmu\)m [10]. Each cough produces 947-2085 particles [11], with a concentration of 24-23,600 (per liter) [12], and a maximum flow rate of 5-22 m/s [13], with females coughing at a flow rate of 10.6-13.07 m/s [14], and males at 15.2-15.3 m/s [15]. High-velocity airflow rubbing against the respiratory mucosa during coughing may result in exhaled coughing at high velocities, which may cause the mucous membrane to rub against the mucous membrane, with a total of 13.07 m/s in women and 15.2-15.3 m/s in men. The friction between the high-velocity airflow and the mucous membranes of the respiratory tract during coughing may cause the temperature of the exhaled gas to be slightly higher than that of the body temperature (about 34-35 \({}^{\circ}\)C). The particle size of cough droplets ranges from 0.01 to 100 \(\upmu\)m [9], with about 87% of the droplets having a particle size of less than 1 \(\upmu\)m [16], and the maximum flow rate is 1.3-1.4 m/s, with a flow rate of 1.4 m/s for breathing through the nose and a flow rate of 1.3 m/s for breathing through the mouth [17]; the temperature of the inhaled gas is close to the body temperature (32-34 \({}^{\circ}\)C), and the temperature distribution is relatively uniform. The likelihood of transmitting respiratory diseases is significantly greater indoors than outdoors. Previous research has explored the transmission of aerosol droplets in various indoor environments, for example, in the case of operating rooms [18] and long-distance buses [19]. Studies have shown that droplets smaller than 30 \(\upmu\)m are minimally influenced by gravity and inertia; their transport is primarily influenced by the indoor flow field [3; 15]. Therefore, efficient ventilation reduces the airborne transmission of respiratory infections and effective ventilation systems help prevent such spread. Currently, most subway carriages use MV systems with fixed air-supply parameters. However, in relatively enclosed and densely populated subway carriages, traditional MV systems are not ideal for providing a safe and healthy environment. [PERSON] [20] conducted research on high-speed train carriages, considering the effects of factors such as temperature and humidity. The results indicate that factors such as ventilation rate, relative humidity, droplet sources, exhaust, physical barriers, and temperature have a significant impact on droplet evaporation and transmission. [PERSON] [21] studied displacement ventilation (DV) systems, demonstrating their effectiveness in improving ventilation performance in enclosed spaces while also reducing the risk of pollutant transmission.