This indicator shows the number of days with exceeded ozone target value and the number of hours with exceeded information value within a calendar year.

Tropospheric ozone differs from some other pollutants, as it is a product of photochemical reactions. Ozone precursors, especially nitrogen oxides and hydrocarbons, are substances from which ozone is formed in photochemical reactions. Consequently, high ozone concentrations do not occur in the immediate vicinity of large sources of precursors but in their wider surroundings. Under stable weather conditions during anticyclones, ozone clouds with high ozone concentrations can have a diameter of hundreds of kilometres and can be several kilometres high. Ozone concentrations above the information value may cause health problems (chest pain, coughing, vomiting, irritated eyes) in children and in people with chronic respiratory and cardio-vascular diseases. Repeated exposure to high ozone concentrations may cause permanent lung damage.

Ozone concentrations have a distinct annual cycle, which is associated with the way in which ozone is formed, requiring sufficient sunlight. The level of outdoor air pollution with ozone is significantly affected by transboundary transport of ozone concentrations. This is particularly characteristic of the Primorska region. Lowlands in northern Italy are among the areas in Europe where the largest amounts of ozone are formed. Thus, the highest concentrations occur during the summer along the coast and in the Primorska region, generally during periods of moderate westerly and south-westerly winds. The measurements of ground-level ozone show overages at almost all measurement sites in Slovenia except at those exposed to emissions of nitrogen oxides from traffic. At those measurement sites, ozone decomposes to O₂ oxygen molecules. The lowest ozone concentrations were measured at the Maribor centre and Zagorje measurement sites, where the volume of traffic is high. The largest number of target value overages was recorded at the rural measuring sites of Iskrba and Otlica, where emissions of ozone precursors are low, which reduces the possibilities of reactions with ozone.

A multi-year data series does not reveal a discernible trend. Ozone concentrations depend on emissions of ozone precursors and, most of all, weather conditions during the summer months. Weather dependence is also evident from the number of overages of information values by year. The Slovenian Environment Agency forecasts ozone concentrations for all of Slovenia two days in advance. When an hourly information value is exceeded, the public is informed thereof on the ARSO website and the notification is sent to various addresses (hospitals, health clinics, the media, rescue centres, municipalities, schools, kindergartens). Along with the notification, information on potential effects on health, recommendations for the general population and the predicted duration of elevated concentrations is provided.