Forty percent of the world’s population was exposed to extremely high ozone levels in 2026, the European Union’s atmosphere monitoring service reported on Wednesday, in a study that traces how pollution rides the wind across borders and oceans.

The analysis, called Atmosphere Watch, comes from the Copernicus Atmosphere Monitoring Service, known as CAMS, and covers the first eight months of the year. It draws on millions of data points from satellites, ground networks and aircraft that feed the agency’s global forecasting system. It is the first time Copernicus has mapped transboundary pollution in this way.

The pattern has a clear geography. Asia fared worst: 60 percent of its population faced extremely high ozone over the eight months, alongside what the report called significant pollution from particles, nitrogen dioxide and sulfur dioxide in industrialized, densely populated regions. In Europe, successive heat waves from June to August left half the population breathing air rated poor, very poor or extremely poor for ozone. Oceania was the least affected continent.

Within Europe the damage tracked the sun. Northern Italy, the Mediterranean coast and Guadalquivir valley of Spain, and Greece stood out as the most affected. Laurence Rouil, the director of CAMS, called the north-south split unsurprising: “Southern, Mediterranean countries are more exposed to high ozone concentrations because of sunlight and high temperatures.”

The year itself offered few breaks. Heat ran above normal because of climate change, and since June an El Niño event registering record levels has amplified it, driving both ozone and fire-related pollution.

Tropospheric ozone — the kind people breathe — forms when heat and sunlight meet chemical precursors, chiefly nitrogen dioxide from traffic and fossil fuels and volatile organic compounds “emitted by industry, solvents, some household products and vegetation,” Ms. Rouil said. That makes it stubborn. Since 2005, Europe has cut emissions of those precursors, yet ozone pollution has not fallen. Ms. Rouil calls it “the ozone paradox”: the precursors can travel thousands of miles on the wind, and ozone itself stays active for days. Cutting precursor emissions is still essential, she said, but “response in concentration terms is not always proportional.”

The report found that only 5 percent of the world’s population enjoyed good long-term ozone levels this year, and 15 percent reasonable ones.

Wildfires supplied the year’s sharpest episodes. The study notes that while global biomass-burning emissions have declined over recent decades, exposure to smoke is rising because more extreme fires are burning near populated areas, “turning a local emergency into a regional or continental air-quality episode.” On July 22, large fires broke out in the Gironde and Landes regions of France, burning nearly 50,000 hectares and forcing 40,000 people to evacuate — one of the largest evacuations in France since World War II. Almost simultaneously, a fire started in Spain’s Ávila province; together with another in Madrid it burned more than 50,000 hectares, led to 56,000 evacuations and kept nearly 30,000 people confined indoors. Both Madrid and Ávila recorded their highest wildfire carbon emissions since at least 2003.

The smoke traveled. In August, Madrid’s regional government told residents to stay indoors as smoke arrived from the Sierra Oeste and Ávila fires, and days later the vast Niebla fire in Huelva province collapsed air quality in Seville. “Air quality in our cities, towns or countries is not only influenced by local activities but can be affected by pollution from neighboring or quite distant nations,” Ian Marnane, head of the Environment and Health Unit at the European Environment Agency, said in an interview.

Desert dust rounds out the picture. Around 2 billion tons of sand and dust enter the atmosphere each year, much of it from a belt stretching from the Gobi Desert across the Arabian Peninsula to the Sahara, whose dust regularly crosses the Mediterranean to Europe and the Atlantic to the Caribbean. Several major transport events in February hit Spain particularly hard.

Of the 20 European cities of more than 250,000 people with the most dust-alert days in 2026, the report found, 12 are Spanish. Las Palmas tops the list with 20 days and Santa Cruz de Tenerife follows with 18, ahead of Córdoba with 17 and Palma de Mallorca and Seville with 15 each.