In-car occupants' exposure to airborne fine particles under different ventilation settings: Practical implications

Passenger cars/taxis represent the commonly used mode of transport in cities worldwide. This study offers insights into in-car occupants' exposure to fine particles (PM2.5) in the city-state of Singapore. Real-time assessment of personal exposure to PM2.5 was carried out simultaneously among four seating positions (two seats on the front side and two seats on the rear side) in a petrol-driven passenger car under different ventilation settings. Results show that the windows-open setting exposes car commuters to higher PM2.5 concentrations, while the windows-closed configuration with the operation of the air-conditioning system and internal/external air circulation leads to relatively lower PM2.5 exposure. To gain insights into airflow patterns inside the car, we carried out Computational Fluid Dynamics (CFD) simulations, which reveal that the internal mixing of air inside the car cabin is restricted on its backside during the windows-closed ventilation setting, leading to higher exposure of passengers seated in rear seats than those sitting in front seats (about two times). In contrast, with the open ventilation, the PM2.5 concentration is almost identical among the four seating positions, indicating that the car cabin is a well-mixed microenvironment in this ventilation setting. This is the first study in which PM2.5 concentrations in multiple seating positions were simultaneously measured and their spatial distributions were simulated under different ventilation settings. Based on the study outcomes, we provide recommendations to reduce commuters' exposure to traffic-related airborne particles inside passenger cars.