Reducing the airborne infection risk in tourist buses using medium-efficiency filters in air-conditioning systems

To reduce the airborne infection risk in tourist buses, medium-efficiency filters with a minimum- efficiency reporting value (MERV) of 11 or 13 are installed in the air-conditioning (AC) system inlets on the ceilings of three tourist buses, and the equivalent air-exchange rates (AERs) of artificial droplet nuclei and their behavior within buses are evaluated. Without filters, the equivalent AERs in the ventilation and internal air circulation modes are 12 and 1.5–5.2 /h, respectively, at the lowest air-flow, and 21 and 7.8–11 /h, respectively, at the highest air-flow. With filters installed and the AC system operating in the internal air circulation mode, the equivalent AERs are 10–15 and 27–42 /h at the lowest and highest airflows, respectively, demonstrating a droplet nuclei removal performance similar to or better than that of the ventilation mode. In the internal air circulation mode without filters, artificial droplet nuclei spread throughout the bus and decay slowly, even after emission ceases. Notably, with the source located in the center seat, the average in-vehicle concentration is higher when the AC system operates in the internal air circulation mode without filters than when it is turned off. Conversely, operating the AC system in the internal air circulation mode with a medium-efficiency filter considerably suppresses the spread of artificial droplet nuclei. Based on the advective spread of artificial droplet nuclei, the estimated average relative airborne infection risk in the bus is reduced to 52–79% in the ventilation mode and 25–50% in the internal air circulation mode with filters, normalized with that when the AC system is turned off.