The study objective was to examine how the changes in outdoor concentration of traffic-related aerosol at schools occurring due to an anti-idling campaign affect the indoor aerosol concentration. Four urban public schools featuring different average numbers of school buses and cars and located at different distances from major urban traffic arteries were selected in Cincinnati (Ohio, USA). For each school, the indoor and outdoor air monitoring was conducted before and after the implementation of anti-idling campaign. Additionally, the ambient air was monitored at four “community” sites representing the background aerosol. The PM2.5 samples were collected and subjected to gravimetric, carbon and elemental analysis. The outdoor concentrations of PM2.5 and its relevant elemental constituents were consistent with the data reported in previous studies. Following the anti-idling campaign, the outdoor concentration of most elements decreased at two schools although only a fraction showed statistically significant changes. After accounting for the background aerosol changes, the positive impact of the anti-idling campaign was demonstrated only for one school – with the most intense local traffic. For this school, reduction of the outdoor elemental concentrations was followed by a decrease in the corresponding indoor concentrations (R2 = 0.47, p < 0.05). The data suggests that in cases when traffic emission is the dominant pollution source in the school vicinity, the changes in outdoor air quality associated with the anti-idling campaign are capable of reducing the children exposure to traffic aerosols inside the schools.