On June 21, 2026, the air quality in Basel was notable for elevated ozone levels. The maximum ozone concentration of 146.2 µg/m³ ranked 12th highest among the past 365 days. This value exceeded the regulatory threshold of 120 µg/m³ for the maximum hourly average, which is permitted only once per year under the Luftreinhalte-Verordnung. The average ozone concentration for the day was 93.7 µg/m³.

Nitrogen dioxide and fine particulate matter (PM10) levels remained well within regulatory limits. The maximum NO₂ concentration was 18.1 µg/m³, far below the daily average limit of 80 µg/m³, and its average was 8.2 µg/m³. The maximum PM10 concentration was 26.4 µg/m³, and its average was 19.3 µg/m³, both below the daily average limit of 50 µg/m³.

Compared to historical data since 2018, the maximum ozone concentration on this day was high but not unprecedented. The all-time maximum ozone value was 206.3 µg/m³, recorded on June 27, 2019. The current maximum was below that record and ranked 109th out of 2943 days in the entire dataset. The average ozone concentration was above the seasonal average of 69.8 µg/m³ and the 30-day average of 77.3 µg/m³, indicating a notably high ozone day for the season.

The maximum NO₂ concentration was low compared to historical levels, ranking 236th out of 366 days in the past year and 2128th out of 2943 days overall. The all-time maximum NO₂ value was 83.2 µg/m³, recorded on February 15, 2019. The maximum PM10 concentration was moderate, ranking 72nd in the past year and 636th overall. The all-time maximum PM10 value was 165.1 µg/m³, recorded on August 24, 2023.

Overall, the air quality on June 21, 2026, was characterized by unusually high ozone levels, which exceeded the regulatory standard. This pattern is consistent with a summer day featuring strong sunlight and high temperatures, which promote ozone formation. The elevated ozone was the primary concern, while NO₂ and PM10 levels were typical or low for the season.