In 2026, only one in every 20 people worldwide breathed ground-level ozone at levels considered "good" by the World Health Organization, a global analysis by the Copernicus Atmosphere Monitoring Service found. The situation pointed to a widespread and largely overlooked public health concern.
Here's what to know
The Copernicus Atmosphere Monitoring Service's Atmosphere Watch report examined ozone, wildfire smoke, and desert dust between January 1 and September 15, according to Earth.com. As for the ozone readings, 40% of people on Earth experienced ozone exposure that qualified for the worst category tracked. Asia faced the greatest exposure to the lung-irritating gas.
To estimate how many people were exposed, researchers brought population maps together with atmospheric modeling and observations collected on the ground and by satellites. CAMS measured ozone exposure using each day's highest eight-hour reading, averaged over the six worst months. While the World Health Organization's ozone guideline is 60 micrograms per cubic meter, CAMS considers exposure above 110 micrograms per cubic meter "extremely poor."
CAMS estimated that "extremely poor" ozone exposure affected 60% of people in Asia, with 90% experiencing levels classified as poor or worse. According to Earth.com, cities recording some of their continents' highest ozone exposure levels included Cairo, Delhi, Milan, Rio de Janeiro, and Mexico City.
Why ozone is bad
While ozone in Earth's atmosphere helps shelter the planet's surface from the sun's harmful ultraviolet rays, near-ground ozone can pose a serious threat to lung health. According to Earth.com, ozone forms when sunlight drives chemical reactions between nitrogen oxides, which come from burning fuel, and organic vapors released by traffic, industry, household solvents, plants, and wildfires.
Hot, still weather speeds up these reactions and lets pollution accumulate at higher concentrations. According to CAMS, repeated European heatwaves have pushed half the continent's population into poor ozone exposure or worse. In North America, more than two-thirds of people experienced poor levels or worse, with exposure concentrated along the U.S. coasts and in major cities.
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During last summer's July 28 heatwave, CAMS calculated that ozone transported to Bergamo, Italy, from other parts of the Northern Hemisphere had an impact equal to all of Europe's human-generated sources combined. According to Earth.com, ozone can remain near the ground for several days and at higher altitudes for weeks, complicating efforts to control local air quality.
What can be done?
Laurence Rouil, director of CAMS, told Earth.com that ozone forecasts have become more dependable and accessible, allowing people to adjust their activities to avoid peak pollution.
"WHO advises reducing prolonged or intense outdoor physical activity, especially in the afternoon," Rouil said, per Earth.com.
The WHO also recommends extra precautions for children, older adults, and people with respiratory or cardiovascular conditions. Beyond individual precautions, Rouil pointed to European pollution rules that have reduced sulfur dioxide, fine particles, and nitrogen dioxide.
CAMS' annual assessments showed that ozone has been the only pollutant covered by Europe's air quality law that the continent has not reduced. The service has also been researching how much of Europe's summer ozone comes from wildfires on the continent.
Where can I learn more?
Heat, wildfires, and industrial emissions make it harder to bring ozone pollution down to healthier levels. These stories examine other ozone exposure challenges from around the world.
• Heat and wildfires left a third of the world breathing extremely poor ozone levels.
• Denver recorded one of its worst ozone summers in 30 years, risking an EPA downgrade.
• Mexico City cut major pollutants, but heat may be slowing progress on ozone.
• In New Mexico, Permian oilfields have been linked to radioactive plumes and ozone.
• Ireland met EU air limits, yet pollution remained linked to 1,500 early deaths annually.
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