BACKGROUND: In the wider framework of the RESPOZE (ReSPiratory effects of OZone Exposure in Greek children) panel study, we investigated possible determinants of O exposure of school children, measured with personal passive samplers, in Athens and Thessaloniki, Greece.METHODS: Personal exposure to O was measured for five weeks spread along the academic year 2013-14, in 186 school children in Athens and Thessaloniki, Greece. At the same time, at-school outdoor measurements were performed and ambient levels of 8-h daily maximum O from fixed sites were collected. We also collected information on lifestyle and housing characteristics through an extended general questionnaire (GQ) and each participant completed daily time activity diaries (TADs) during the study period.RESULTS: Mean outdoor concentrations were higher during the warmer months, in the suburbs of the cities and in Athens. Personal exposure concentrations were significantly lower compared to outdoor. Daily levels of at-school outdoor and ambient levels of O from fixed sites were significant determinants of personal exposure to O. For a 10μg/m increase in at-school outdoor O concentrations and PM measurements a 20.9% (95% CI: 13%, 28%) increase in personal exposure to O was found. For a half an hour more spent in transportation an average increase of 7% (95% CI: 0.3%, 14.6%) in personal exposure to O was observed. Among other possible determinants, time spent in transportation (TAD variable) and duration of open windows were the ones associated with personal O exposure levels.CONCLUSIONS: Our results support the use of outdoor and ambient measurements from fixed sites in epidemiological studies as a proxy of personal exposure to O, but this has to be calibrated taking into account personal measurements and time-activity patterns.