Method for Measuring the Ratio of In-Vehicle to Near-Vehicle Exposure Concentrations of Airborne Fine Particles

Human exposure to fine particulate matter of less than 2.5 microns in aerodynamic diameter is causally linked to cardiovascular and pulmonary diseases. In-vehicle exposure may account for 10% to 20% of daily average exposure. However, exposure models are typically based on areawide air quality data that poorly predict in-vehicle concentration. A practical method is demonstrated for conducting field measurements to quantify the ratio of in-vehicle to outside vehicle concentration (I/O) for a wide range of conditions that affect intravehicle variability in exposure concentration. A field data collection study design is developed on the basis of sources of intravehicle variability in I/O that include ventilation air source, window status, fan setting, air-conditioning (AC) use, vehicle speed, road type, travel direction, and time of day. Three replicates of measurements were made for 16 combinations of these factors on 110 mi of roads comprising eight one-way routes between typical commuter origin–destination pairs. Two portable particle monitors recorded in-vehicle and near-vehicle ambient concentrations on 1-min averages for four particle size ranges. The comparability of the monitors was quantified. Near-vehicle concentrations varied with road type, time of day, and traffic conditions. The I/O ratio was approximately independent of near-vehicle concentration and varied with window status, source of ventilation air (fresh or recirculated), and for cases with recirculation and closed windows, fan setting, and AC use. The study design can be extended to additional vehicles to account for potential sources of inter-vehicle variability. Data collected here can be used to improve exposure simulation models.