Published: 12:43 AM, 11 July 2026
How harmful ozone builds up near ground in heatwaves
Medics attend to an incident in Liege, Belgium, during last month's hot weather. Ozone levels were raised during the heatwave. -EPA
Widespread air pollution was a feature of June's record-breaking heat across western Europe and is likely to increase again as the UK experiences its third heatwave of the summer.
UK weather warnings for extreme heat from 22 June were followed by a rare high air pollution alert from the London mayor.
During heatwaves, ozone forms near the ground when other pollutants react together in strong sunlight. Although the ozone layer in the upper atmosphere benefits us by shielding us from harmful UV, ozone in the lower atmosphere is very harmful, the Guardian reports.
The 2003 European heatwave led to about 2,000 extra deaths in the UK, about 800 of them from ozone. The Netherlands had about 400 to 600 extra air pollution deaths.
Laurence Rouil, the director of the EU's Copernicus Atmosphere Monitoring Service, said: "Ozone remains one of the most challenging air pollution issues facing Europe and other parts of the world. Once an episode begins to develop, it becomes very difficult to halt or significantly reduce its intensity. This is why anticipation and prevention are key."
Forecast maps for last month's heatwave show how ozone built up with the heat of each day, to reach a peak in the late afternoons. The worst conditions were initially over southern England, Belgium, northern France and the Netherlands. This moved eastwards as the heat moved.
Rouil said: "Ozone concentrations were continuously raised in Europe during the June heatwave. The highest concentrations were in Germany, Benelux and Italy's Po valley, and more locally in the Paris area, Spain and Czech Republic. In those places, ozone concentrations were quite remarkable compared to the previous years."
In 1943, at the height of the second world war, residents of Los Angeles thought they were under poison gas attack in what is now recognised as one of the first ozone smogs. At that time, the ozone mainly formed from traffic exhaust and petrochemical industry pollution. Today, pesticides, coatings, printing inks, adhesives, cleaning agents and personal care products are among the main sources.
Rouil said: "Short-term exposure to ozone can cause respiratory diseases and throat irritations, worsening asthma etc. It also damages vegetation, causing browning and premature leaf drop."
In 2024, 21.5% of Europe's agricultural land - nearly half a million square kilometres - was exposed to ozone levels above EU targets.
During the June heatwave, maximum concentrations were less than half those measured during the 1976 heatwave and also less than those measured in 2003. But while peak concentrations have fallen, there is no clear direction for the average amount of ozone to which we are exposed.
Ozone in the lower atmosphere is a super pollutant. Reducing it will help with the climate crisis and our health.
Rouil said: "We have not finished with ozone. It requires coordinated mitigation strategies at international, national and in some cases even local scales. Ozone is also influenced by climate change - not only heatwaves but also because of the increasing frequency and intensity of wildfires, which release important amounts of ozone precursors. International commitments such as the global methane pledge and tighter regulations on methane can have positive impacts, but it will take time."
