Abstract
In response to the need for effective infection risk mitigation in public transportation during infectious disease outbreaks and reduction of the exposure to particles in the atmospheric environment should be implemented, this study conducted experiments and field surveys to evaluate the effectiveness of medium-performance filters and their long-term performance degradation, particularly in reducing exposure to airborne particles. In laboratory experiments, three types of electrostatic filters were tested to assess changes in pressure drop and particle removal efficiency during extended indoor particle loading. Results showed a significant decline in removal efficiency from the early stages of use, primarily due to the loss of electrostatic charge. The removal efficiency, which was 79%, 48%, and 69% for filter A, B, and C at the start, decreased to 43%, 6.7%, and 21% after 73 days, respectively. In high-concentration particle loading tests, it was confirmed that larger particles were mainly captured through physical clogging, while electrostatic charge played a key role in collecting smaller particles. After discharging of the filter using isopropyl alcohol, the efficiency for small particles dropped markedly. Furthermore, in performance evaluations of filters installed in buses in actual operation, some vehicles maintained equivalent air change hour of approximately 20 /h even after two years of filter use, while others showed a decline to 10 /h after only six months of use. The performance changes of filters in buses showed a similar trend to those observed in indoor tests. These findings provide important insights into the aging behavior of medium-performance filters and inform design and maintenance strategies for vehicle HVAC systems.
