Tropospheric Ozone Pollution Episodes

The "Tropospheric Ozone Pollution Episodes" fact sheet identifies situations in which information thresholds are exceeded for tropospheric ozone pollution, triggering a warning system for dissemination to the population and relevant organisations so that action can be taken to reduce exposure to this air pollutant.


Tropospheric ozone (O3) is a secondary pollutant and therefore not directly emitted into the air. Its formation occurs when oxygen and the pollutants that are its precursors, such as nitrogen oxides and volatile organic compounds, react under the action of sunlight. These precursors have their main origin in road transport, in electric power plants, in domestic heating, in the use of solvents and in industrial processes.

Ozone has effects on the respiratory system causing inflammation of the airways, which becomes acute for high levels of concentration, causing coughing, throat irritation and discomfort in breathing. The severity of these effects increases with the concentration of ozone in the air, the exposure time and the amount inhaled.      

The indicator relating to the number of days exceeding the public information threshold of 180 μg/m3, or the alert threshold of 240 μg/m3, assesses the short-term exposure to this pollutant
. In case of exceedance of the threshold value, a warning mechanism is triggered in order to allow the population in general and sensitive groups, in particular, to adopt a preventive attitude, through behaviours aimed at reducing the time of exposure to the pollutant.

The long-term exposure indicator of O3 is evaluated through the long-term goal of 120 μg/m3, obtained through the maximum concentrations of the daily eight-hour means. For the calculation of the weighted mean, the values obtained at all background rural and urban/suburban air quality monitoring stations, with a measuring efficiency greater than 75%, are considered. If the evaluation strategy defined for the zone is the indicative measurement, stations with a measuring efficiency of more than 14% are considered.

This fact sheet concerns mainland Portugal, the Autonomous Regions of Madeira and the Azores and will be updated annually.

Objectives and targets: 
  • Ensure compliance with European and domestic air quality objectives to avoid, prevent or limit the harmful effects of air pollutants on human health and the environment;
  • Efficiently communicate to the public the exceedance of thresholds for ozone;
  • Disseminate the system already in place to forecast ozone levels in order to contribute to preventing population exposure to this pollutant (forecast available at;
  • Ensure compliance with the legally established limits (Decree-Law no. 102/2010).
Progress analysis:

The formation of ozone occurs preferentially in the seasons of the year with greater luminosity and greater surface atmospheric stability. Favourable weather conditions result in less dispersion of pollutants, increasing their likelihood of reacting with each other, and in the presence of their precursors, may lead to higher concentrations of ozone. Such concentrations can even occur at great distances, hundreds of kilometres away from the emitting sources due to the airborne transport of the precursors.

Last update: 
Monday, 5 November, 2018