Assessment of exposure to traffic pollution using the ExTra index: study of validation

The ExTra index, produced by the French Scientific Center for Building Physics, evaluates ambient concentrations of transport-related pollutants in front of the work and living places of urban dwellers. This study contributes to the validation of the ExTra index by carrying out measurements in four French cities. It compares nitrogen oxide concentrations (NOx) measured over 6 weeks with passive samplers, and NOx calculated concentrations using the ExTra index. The study takes into account traffic density, topographical parameters (building height, road, and pavement width), weather conditions (wind direction and strength), and background pollution levels. The model was tested at 100 street canyons sites. There were highly significant correlations (0.90 in Grenoble, 0.95 in Nice, 0.89 in Paris, and 0.89 in Toulouse) and good intraclass correlation coefficients (0.75 in Grenoble, 0.91 in Nice, 0.89 in Paris, and 0.86 in Toulouse) between the two series of values. These results suggest that if the ExTra index were to be applied to all the different residences and workplaces of a subject throughout his (her) life, it could be a useful epidemiological tool for studying the long-term health effects of transport-related emissions and for reconstructing individual exposure to such pollution in order to avoid misclassification.