USC researchers find car ventilation setting critical to in-cabin exposure to particulate pollutants; new model for aiding exposure assessments
|In-vehicle-to-outside (I/O) ratios for four pollutants under different ventilation settings. Credit: ACS, Hudda and Fruin (2013). Click to enlarge.|
Researchers at USC have found that using recirculation rather than outside air ventilation in a car can effectively reduce in-cabin exposure to on-road particle pollution. In a study published in the ACS journal Environmental Science & Technology, they also report that in addition to the benefits of recirculation settings, exposures are lower in newer cars; at slower speeds; and on arterial roads, where pollutant concentrations are lower than on freeways.
Scott Fruin, assistant professor of preventive medicine, and Neelakshi Hudda, research associate in environmental health, both of the Keck School of Medicine of USC, performed a systematic study of in-vehicle exposure to include a full range of car types and operating conditions, as well as all types of particulate pollution. According to the researchers, concentrations of particle pollutants on freeways are often five to 10 times higher than elsewhere.
Traffic-related particulate pollutants studied included ultrafine particles to the larger particulate matter (PM) sizes such as PM2.5 that are linked to heart disease and premature mortality. Pollutants also included black carbon and particle-bound polycyclic aromatic hydrocarbons, both high in diesel emissions, considered by the state of California to produce cancer.
Under closed-window driving conditions, the in-vehicle-to-outside (I/O) concentration ratio for traffic-related particulate pollutants ranges from nearly 0 to 1 and varies up to 5-fold across a fleet of vehicles, thus strongly affecting occupant exposures.
Concentrations of five particulate pollutants (particle-bound polycyclic aromatic hydrocarbons, black carbon, ultrafine particle number, and fine and coarse particulate masses) were measured simultaneously while systematically varying key influential parameters (i.e., vehicle type, ventilation, and speed). The I/O ratios for these pollutants were primarily determined by vehicle air exchange rate (AER), with AER being mostly a function of ventilation setting (recirculation or outside air), vehicle characteristics (e.g., age and interior volume), and driving speed. Small (±0.15) but measurable differences in I/O ratios between pollutants were observed, although ratios were highly correlated.—Hudda and Fruin (2013)
By random sampling, Fruin and Hudda found that for a typical car (the national average is 7 years old), recirculation settings reduce in-vehicle particle pollution for very small particles from 80% (of on-road levels) to 20%, and from 70% to 30% for larger particles, compared to air ventilation settings, which bring in outside air.
(Windows were always assumed to be closed for this test. Keeping windows open while driving quickly raises inside pollutant concentrations to the same levels as on-road levels.)
However, the researchers found that leaving the windows closed over 30-minute or longer drives with several passengers also raised carbon dioxide (CO2) levels in compact, new cars, matching the levels found in stuffy meeting rooms.
Fruin and Hudda used this study to build on prior studies of ultrafine particle number I/O ratios to develop predictive models for other particulate pollutants. They found that the models explained more than 60% of measured variation, using ventilation setting, driving speed, and easily obtained vehicle characteristics as predictors.
The models developed in this study are useful for allowing estimation of I/O ratios for PB-PAH, PM2.5, and PM10 (BC I/ O ratios appeared to be similar to those for PB-PAH).
...It is not feasible in large cohort studies to conduct in-vehicle measurements for all subjects, and in smaller panel studies, it is difficult to measure in-vehicle exposures for multiple pollutants. Therefore, our models that can predict I/O ratios for multiple pollutants based on information that can be gathered through questionnaires can aid exposure assessment.—Hudda and Fruin (2013)
N. Hudda and S. A. Fruin (2013) Models for Predicting the Ratio of Particulate Pollutant Concentrations Inside Vehicles to Roadways. Environmental Science & Technology doi: 10.1021/es401500c
Neelakshi Hudda, Evangelia Kostenidou, Constantinos Sioutas, Ralph J. Delfino, and Scott A. Fruin (2011) Vehicle and Driving Characteristics That Influence In-Cabin Particle Number Concentrations. Environ. Sci. Technol. 45 (20), pp 8691–8697 doi: 10.1021/es202025m