Tropospheric ozone (O3) is a secondary pollutant gas that causes a significant adverse effect on forest tree species. Damage to vegetation occurs when O3 enters the stomata following gas exchanges during photosynthesis. Ozone-induced stomatal sluggishness, i.e., a delay in stomatal responses to fluctuating stimuli, can potentially change forests' carbon and water balance. In particular, the O3-induced stomatal sluggishness becomes more critical in water-limited environments such as Mediterranean Europe, where stomatal regulation of water losses under low soil water availability is critical. Free-air controlled exposure (FACE) facilities are considered an ideal tool that can provide realistic estimates of tree response to O3 under real-world conditions. Since 2015, the last generation of the O3 FACE facility has been available at the CNR experimental garden in central Italy (FO3X - Free air O3 eXposure), a unique facility in Europe within an AnaEE (Analysis and Experimentation on Ecosystems) European research platform. This facility permits the exposure of plants to three levels of O3 concentrations (ambient, 1.5- and 2.0-times ambient concentration, denoted as AA, 1.5×AA and 2.0×AA, respectively), with main environmental variables continuously monitored. Twentynine plant species were fumigated at FO3X to assess the effects of only O3 or combined with other stressors on plant attributes (drought, nitrogen) evaluated O3 damages. In this presentation, we will introduce our FACE facility from a future perspective, and recent experimental results about O3 critical levels (CLs) to estimate the cumulative O3 stomatal flux (Fst) at a level that causes direct injury to vegetation (i.e. a 4% decrease in biomass) will be summarized.