Quantification of within-vehicle exposure to NO_x and particles: Variation with outside air quality, route choice and ventilation options

Vehicle use, as driver or passenger, is a key transport mode and important microenvironment for personal exposure to air pollutants. Here, the air pollution exposure of vehicle occupants, relative to external, ambient levels was explored under different vehicle ventilation settings and driving routes in an urban area. Four vehicles were driven on a consistent route encompassing three contrasting road types, measuring simultaneous within-vehicle and ambient levels of particulate matter (PM₁₀, PM_2.5, PM₁), ultrafine particles number (UFP), lung surface deposited area (LSDA), nitric oxide (NO) and nitrogen dioxide (NO₂). For the majority of ventilation settings available, the inside/outside (I/O_VEH) ratio for PM₁₀, PM_2.5, PM₁, UFP, LSDA and NO₂ concentrations was below 1, with the exception of NO, for which the ratio was independent of ventilation settings, within uncertainty. The lowest within-vehicle exposure for particles and gases was observed under the conditions of windows closed, recirculation and air conditioning on. Vehicle occupants are exposed to and inhale more air pollution when traveling on urban roads, followed by ring-roads and sub-urban roads. However, through reduced within-vehicle concentrations and reduced physical activity and hence breathing rate, they inhale less air pollution than people cycling/walking on the same routes. Within-vehicle air pollution exposure displays significant dependence upon both the ventilation setting and route selected. Vehicle occupants can, therefore, modify their personal exposure through these choices. Finally, vehicle occupants inhale more mass of NO₂ than PM_2.5 with a trip-average inhalation dose ratio of 6.4 (NO₂ dose/PM_2.5 dose). These results may have significant health impacts upon highly exposed groups such as professional drivers.