The Copernicus Atmosphere Monitoring Service (CAMS) has reported a significant increase in ground-level ozone (or tropospheric ozone) in Europe in June 2025, with these episodes happening earlier than normal.
CAMS forecasts for Europe indicated rising ground-level ozone concentrations, which are harmful to human health and ecosystems, across the Mediterranean Basin since June 9, with levels increasing significantly in nearly all European countries – excluding those on the Scandinavian Peninsula.
While higher ozone concentrations during the European summer are not uncommon, episodes of this intensity so early in the season are unusual, according to CAMS. This trend is likely linked to the relatively high temperatures experienced across the continent in recent weeks.
During this episode, ozone levels in several areas exceeded the thresholds established by the European Council to protect human health against harmful effects of this pollutant. The revised Ambient Air Quality Directive (EU/2024/2881), adopted in November 2024, sets a target value to protect human health: the maximum daily eight-hour mean should not exceed 120µg/m³ more than 18 times per calendar year. The Directive also defines an information threshold of 180µg/m³ for the hourly mean.
Laurence Rouil, director of CAMS, said, “Increases in temperatures are one of the main factors explaining ground level ozone concentrations increase. Ozone results from photochemical reactions that transform pollutants emitted by human activities and natural sources. Long range transport of this pollutant plays an important role as well.
“CAMS provides Europe with the best available tools to monitor and forecast the evolution of these episodes and provide timely alerts, which are essential for implementing agile and appropriate measures to mitigate health risks and environmental impacts.”
Ground level ozone (or tropospheric ozone) results from complex photochemical reactions, stimulated by sunlight, involving nitrogen oxides (NOx) and volatile organic compounds (VOCs). Both precursors are emitted by human activities (transportation, industry, residential), but some VOCs are also emitted by vegetation.
Learn more about this episode of ground level ozone pollution here