Vehicle Indoor Air Quality Due to External Pollutant Ingress While Driving

Vehicle indoor air quality (VIAQ) remains poorly standardized despite its growing health relevance. This study developed and applied a real-road test protocol to quantify in-cabin exposure to particulate and gaseous pollutants under different heating, ventilation, and air-conditioning (HVAC) modes: outside air (OA), recirculation (RC), and automatic (Auto). Concentrations of PM2.5, particle number (PN), NO, and NO2 were simultaneously measured inside and outside passenger vehicles using validated instruments. In-cabin PM2.5 levels were lowest in RC, intermediate in Auto, and highest in OA, showing strong HVAC dependence. Particle number distributions were dominated by submicron particles (<1.0 μm). Under RC, NO gradually increased while NO2 decreased, likely due to NO–NO2 interconversion and activated-carbon filtration. Short-duration, reproducible on-road tests were conducted under standardized vehicle, occupant, and HVAC settings to minimize variability. Although external conditions could not be fully controlled, consistent routes and configurations ensured comparability. The findings highlight HVAC operation as the dominant factor governing short-term VIAQ and provide practical insight toward harmonized test procedures and design improvements for cabin air management.