# Numerical Investigation on the Droplet Dispersion inside a Bus and the Infection Risk Prediction
## 4 Results and Discussion
### Passenger Infection Probability Prediction  
The superposition method was used to obtain the number of virus particles inhaled by the passenger after the journey based on the previous simulation of cough droplet dispersion. Then, risk of airborne infection was estimated by the Wells-Riley equation. The infection risk of every individual passenger after a 4-h trip is shown in Figure 11.  
Figure 11 show the infection probability of passengers in Case 1, Case 2, Case 3, Case 4, respectively. As described in Section 4.3, the relative location of the infected person and the vent significantly influenced the dispersion of the droplets. Passengers at the pathway of droplet dispersion had a probability of infection of more than 3%, while the risk of infection for other passengers was less than 1%. Accordingly, the distribution of airborne infection risk in a cabin was determined by the relative position of an infected person and the vent. Furthermore, the infection risk for the passenger closest to the infected person is the highest, up to 20%. It could be found that the risk of infection of passengers is affected by their distance from the infected person. Figure 10: The distribution of droplets released from the index infected person’s cough over time, (**a**) Case 4.1, (**b**) Case 4.2, (**c**) Case 4.3. Figure 11: _Cont._  
The distance between the infected person and the vent directly determines the time for the droplets to escape. Droplets exhaled by the infected person would escape slowly when they are far from the vent. Therefore, the infected person located far from the vent might increase the airborne infection risk for passengers, as shown in Figure 11d,i. While the coughed droplets produced by the infected person close to the vent would quickly escape from the cabin, as shown in Figure 11a,f, the other passengers' infection risk was relatively small. The chances of passengers becoming infected can be effectively reduced when placing the infected person near the vent. Figure 12a-c show the infection probability of passengers in Case 5, Case 6 and Case 7, respectively. It was found that passengers in rows 2-7 were at high risk of infection, and passengers near the infected person had the highest infection risk. Basically, passengers who were closer to the infected person had a higher risk of infection. In other words, the risk of infection for passengers decreased from the second to the seventh row. In addition, the risk of passenger infection was significantly reduced as the air supply rate increased. The average infection probability of passenger in Case 7 was half 50% lower than that of Case 5. In Case 6, the infection risk of passengers for 2D and 2E was higher than that in Case 5 and Case 7, which seems counterintuitive. It might be that the lateral airflow was stronger in Case 6 so that more droplets spread to the right side of the second row. In Case 7, the longitudinal airflow with a relatively high velocity in the aisle might carry the droplets rapidly to the rear of the cabin when the droplets were moving from the left side to the right side, and only a small proportion of droplets spread to the right side of the second row. The droplets' residence time in the breathing area was short due to the relatively high  
Figure 11: Infection risk of every individual passenger (**a**) Case 1.1, (**b**) Case 2.1, (**c**) Case 3.1, (**d**) Case 4.1, (**e**) Case 1.2, (**f**) Case 2.2, (**g**) Case 3.2, (**h**) Case 4.2, (**i**) Case 1.3, (**j**) Case 2.3, (**k**) Case 3.3, (**l**) Case 4.3 (the data of the infected person has been removed). longitudinal airflow. Therefore, the risk of infection for the passenger on the right side of the second row in Case 6 may be higher. It should be noted that passengers on a different side of the infected person had a relatively low risk of infection, it could be explained that only a few droplets can cross the aisle and diffuse from one side to the other side in the bus.