Linking in-vehicle ultrafine particle exposures to on-road concentrations

► Significant overall ultrafine particle (UFP) exposure occurs in vehicles. ► In-vehicle exposures depend on inside-to-outside UFP ratios (I/O). ► At outside air setting, I/O is determined mostly by fan strength. ► Under recirculation, I/O is determined by vehicle age and speed. ► Fleet-wide UFP exposure varies 10-fold.