The string of intense and prolonged heat waves has pushed tropospheric ozone to unusually high levels in Spain. This summer, described by analyses as the hottest on record, has acted as a chemical accelerator that, in the presence of solar radiation , multiplies the formation of so-called "bad" ozone.
Data collected by organizations and public bodies indicate that concentrations have reached ten-year highs in numerous areas. With current standards already exceeded at multiple monitoring stations, the implementation of the new European Union limits for 2030 would further expand the map of non-compliance, while under WHO guidelines, virtually the entire territory has fallen above the recommended threshold.
What is happening and why is ozone rising?
Tropospheric ozone does not originate from a specific chimney or car exhaust pipe; it forms in the lower atmosphere when sunlight strikes precursor pollutants. These include nitrogen dioxide (NO2)—primarily linked to road traffic—and volatile organic compounds from various activities.
This mixture is compounded by the increasing presence of methane (CH4) , associated in Spain primarily with intensive livestock farming, and episodes of increased carbon monoxide levels due to large forest fires. It is a typically seasonal phenomenon: where there is more radiation (spring and summer), there is more reactivity and, therefore, more ozone peaks , especially in suburban and rural areas affected by urban and industrial pollution.
Where the largest increases have been detected
Records show that the warmest summer on record has pushed the seasonal average above the current legal limit across the Community of Madrid , much of Extremadura, southwestern Castilla-La Mancha, inland areas of Catalonia, the Region of Murcia, and various parts of Andalusia. With the new EU maximum legal value for 2030, virtually all of Andalusia and a large swath of Castilla-La Mancha would fall into the category of areas exceeding the limit.
If the analysis is done using the WHO's health criteria, the picture is even more severe: almost the entire population would have been exposed to levels above the recommended limits during the summer. Regarding peak hourly levels, legal limits were exceeded at stations in Madrid, the western half of Castilla-La Mancha, Extremadura, Andalusia, parts of Catalonia, and Mallorca.
In the three-year period from 2023 to 2025, more than twelve million people breathed air with ozone levels exceeding the new European limits for 2030, according to Ecologists in Action. With the current legal target still in effect, the figure would be around eight million . Meanwhile, as of September 30, 2025 had already seen 320 exceedances of the public information threshold , concentrated mainly in Madrid and Catalonia.
The Community of Madrid stands out for its unprecedented deterioration: the benchmark report describes the highest levels since the 1990s , with a notable increase compared to the 2012-2019 period, and a large number of exceedances in June, July, and August . Along the Mediterranean coast, episodes and warnings were reported in Castellón and inland Valencia—with several activations of the information threshold— ; in 2025, the consulted source did not identify any population exposed above the annual legal target in the Valencian Community, although isolated episodes with very high concentrations below the WHO guidelines were recorded.
Health risks and health response
Ozone exposure is associated with inflammation and oxidative stress in the respiratory system, exacerbation of cardiac and respiratory conditions, neurological damage, and, according to various studies, effects on mental health. The Carlos III Health Institute estimates around 23.000 emergency room admissions annually related to ozone in Spain; NO2 alone causes more, with approximately 28.000 per year.
In 2025, the same organization attributed 3.840 deaths to excessive temperatures and increased exposure to pollutants such as ozone. In the Community of Madrid, hundreds of deaths were attributed to heat and ozone during the summer, confirming that this is a major public health problem . In people over 65, ozone accounts for more than half of hospital admissions for upper respiratory tract infections.
Furthermore, health services warn that these episodes do not affect everyone equally: children, the elderly , pregnant women, and those with cardiorespiratory diseases are especially vulnerable. Therefore, the Carlos III Health Institute recommends addressing the root cause: reducing urban traffic , which is key due to its link with NO2 and other precursors.
Impact on the field and ecosystems
Ozone not only harms people: when absorbed by leaves, it damages plant tissues , slows growth and reproduction, and reduces agricultural and forestry yields, impacting biodiversity , according to the European Environment Agency.
A study by the European Topic Centre confirmed significant losses associated with ozone peaks in 2022: more than three million tons of wheat , worth around €1.300 billion , in addition to approximately €680 million in potato losses. Southern European countries—with longer and more intense radiation seasons—such as Spain and Italy are particularly vulnerable. In Spain, environmentalists are denouncing the lack of a sufficient official assessment of the impact on vegetation and ecosystems.
Public information and plans in progress
The network of alerts and warnings to the public showed mixed results during the 2025 episodes. There were hundreds of exceedances of the information threshold , but shortcomings were detected: in Aragon, Asturias, Extremadura and the Basque Country, ozone levels were not warned as required, and in other communities the warnings were routine and ineffective in reaching the entire population.
Ecologists in Action denounces the fact that seven autonomous communities still lack specific plans to combat ozone pollution and demands the urgent approval of a National Ozone Plan . In Madrid, the group is pursuing legal action for administrative inaction and is calling for the activation of protocols during periods of high pollution, in line with other European capitals.
What measures are proposed
The priority, according to experts and organizations, is to cut precursor emissions. Among the most frequently cited measures are reducing urban motorized traffic (including the implementation of effective Low Emission Zones), promoting public transportation, and technologically and fiscally discouraging diesel vehicles and aviation.
It is also proposed to replace organic solvents with water in industrial processes, improve efficiency and energy savings , plan the deployment of renewable energy in an orderly manner, and make progress in the Mediterranean Sea and Northeast Atlantic Emission Control Areas . Regarding background ozone, the need to address methane emissions is emphasized : composting bio-waste and establishing a moratorium on new large-scale farms that increase these emissions.
However, health authorities insist on everyday prevention: checking levels before doing outdoor sports on days of intense radiation, avoiding midday hours, and protecting vulnerable groups during episodes.
With an exceptionally warm summer as a catalyst, the surge in tropospheric ozone has left more people exposed, more frequent episodes, and greater pressure on health and agriculture. Curbing its precursors—especially NO2 from traffic and methane—improving public information, and implementing effective plans at the national and regional levels are crucial to preventing another spike in this pollutant next summer.
