An experimental study of the dust concentration in automobile passenger compartments was conducted to understand how driving conditions and a vehicle＇s ventilation system can affect cabin air quality. The results of this study are essential for designing appropriate cabin air filters and developing proper test procedures. The particle size distribution of atmospheric contaminants entering the vehicle＇s passenger compartment through air register and in the occupants＇ breathing zone was measured using an optical particle counter. The concentration and size distribution of aerosol in the cabin was found to be dependent on traffic conditions, road surface, environmental conditions (e.g., weather) and ventilation mode. In all cases over 90% of the particles were smaller than 1 mum. The vehicle＇s HVAC system (evaporator core) reduces the concentration of aerosol entering the car interior, especially larger particles. Commercial vehicle ventilation filters decrease particle concentrations in a driver＇s breathing zone for all particle sizes. However, the reduction in the submicron particle range is not sufficient to substantially improve air quality in the passenger cabin.