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AbstractIn-vehicle exposures to particulate matter (PM) and black carbon (BC) can be a significant part of personal total exposure. The purpose of this study was to identify determinants of the in-vehicle pollutant level. This study measured in-vehicle exposure to PM number concentrations using an aerosol spectrometer and BC concentration by an aethalometer. The measurement was repeated 3 times a day for 13 consecutive days in May 2007 while a vehicle was driven on a 1-hr predetermined route in Lexington, KY. The vehicle’s windows were closed, its fan was operated at the lowest setting, and air conditioning and heating were off. The average speed of all runs was 37 ± 5.8 km/hr. Arithmetic mean PM1(particle sizes ±1 μm in aerodynamic diameter) number and BC concentrations were 286 #/cm3(morning, 302 #/cm3; afternoon, 270 #/cm3; evening, 287 #/cm3) and 1.9 μg/m3(morning, 2.5 μg/m3; afternoon, 1.6 μg/m3; evening, 1.5 μg/m3), respectively. Average PM and BC concentrations increased during weekdays and decreased during weekends, and PM1number and BC concentrations were highest in the morning. Particle size distribution for PM1did not differ with time of day, but the concentration of PM1-10(particle sizes between 1 and 10 μm in aerodynamic diameter) was higher in the evening and lower in the morning. Peak PM1number concentrations were observed when the test vehicle approached busy crossroads. In-vehicle PM and BC concentrations were associated with time of the day and location, whereas the in-vehicle 1-hr PM number concentrations were closely associated with distant ambient PM2.5(particle sizes ±2.5 μm in aerodynamic diameter) concentrations measured at a stationary monitoring site. The findings suggested that in-vehicle exposure is associated with surrounding traffic.