A weather station in Sörmland, south of Stockholm, recorded levels of ground-level ozone over the level at which Swedish authorities are obliged to warn the public. High levels were also noted in Småland and Östergötland.
Ground-level ozone can be dangerous for people with asthma or heart or lung illnesses.
“I haven’t seen such high levels over the past five years,” said Professor Yngve Brodin at the Swedish Environmental Protection Agency (Naturvårdsverket).
“What happens is that southerly winds come from central Europe with high levels of emissions, and these combine with Swedish emissions, and together with a lot of sun they create ozone,” he said.
The highest levels of ozone tend to be recorded outside urban areas, with the problem worse on hills than in low-lying areas.