This study applies a traffic exhaust air dispersion model (the ExTra index) to 403 children enrolled in a French multicentric case-control study, the VESTA study (Five [V] Epidemiological Studies on Transport and Asthma). The ExTra index (previously validated by our team) was used to assess lifelong average traffic-related air pollutant (TAP) concentrations (nitrogen oxides) children in the study were exposed to in front of their living places. ExTra index took into account traffic density, topographical parameters (building height, road and pavement width), weather conditions (wind direction and strength) and background pollution levels. Topographical and traffic data were collected, using a specific questionnaire for each home, school or nursery address, attended by children. The assessment of time-weighted NOx levels in front of the children's living places highlighted significant disparities: mean ExTra index values and share attributable to proximity traffic were, respectively, 70+/-42 and 14+/-22 microg/m3 NOx equivalent NO2 for the 403 children in our study. Not only would this diversity not have been revealed using urban background pollution data provided by air quality networks, it would have resulted in 40% of the children being misclassified with regard to their TAP exposure by underestimating it in half of the cases and overestimating it in the other half. Such errors of classification, which are highly prejudicial in epidemiology, argue strongly for the use of an index such as the ExTra, which enables TAP exposure to be reconstructed within the framework of retrospective or prospective epidemiological studies.