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Ozone (O 3 ) is a secondary pollutant mainly produced in the troposphere by photochemical reactions with high impact on human health. In this work hourly average O 3 concentrations from the Portuguese air quality network collected during 24 years were analysed. The background time series were assessed by environment type (urban, suburban and rural) and considering several criteria: the annual mean O 3 concentration, the Directive 2008/50/EC O 3 long-term objective and the 2005 WHO Guideline for protection of human health and, finally, the hourly information threshold exceedances. The trend of annual mean O 3 concentration data and the maximum daily eight-hour mean ozone concentration data were both annually compared using an one-way ANOVA followed by a Tamhane post-hoc comparison test. In both series, although of an instable trend, a tendency for an increase of the mean O 3 concentrations was found. On the other hand, after 2006, for all stations types, the normalized number of exceedances for public O 3 information are significantly decreasing (p<0.05). In this paper, the O 3 trend will be analyzed and discussed, contributing to improve knowledge of long time series of O 3 concentrations in Portugal.