On June 20, 2026, the air quality in Basel was notable due to elevated ozone levels. The maximum ozone concentration of 136.8 µg/m³ ranked 19th highest among the last 365 days. This value exceeded the regulatory threshold of 120 µg/m³, which is the maximum hourly average allowed under the Swiss Clean Air Ordinance. The average ozone concentration for the day was 94.1 µg/m³.

Nitrogen dioxide and PM10 levels remained low. The maximum NO2 concentration was 12.7 µg/m³, and the average was 6.4 µg/m³, both well below the daily average limit of 80 µg/m³. The maximum PM10 concentration was 28.6 µg/m³, and the average was 14.5 µg/m³, also below the daily average limit of 50 µg/m³.

Compared to historical data since 2018, the maximum ozone concentration of 136.8 µg/m³ is well below the all-time maximum of 206.3 µg/m³ recorded on June 27, 2019. The average ozone concentration of 94.1 µg/m³ is above the seasonal average of 68.6 µg/m³ and the 30-day average of 76.3 µg/m³, indicating that ozone levels were higher than typical for this time of year. The maximum NO2 concentration of 12.7 µg/m³ is far below the all-time maximum of 83.2 µg/m³ recorded on February 15, 2019, and is consistent with the low levels seen recently. The maximum PM10 concentration of 28.6 µg/m³ is below the all-time maximum of 165.1 µg/m³ recorded on August 24, 2023, and is within the typical range for the season.

Overall, the air quality on June 20, 2026, was characterized by an ozone exceedance, while NO2 and PM10 levels were low. The elevated ozone concentration was higher than the seasonal average and ranked relatively high within the past year, but it was not extreme compared to the long-term historical maximum. This pattern is typical for a warm summer day with strong sunlight, which promotes ozone formation. The low NO2 and PM10 levels suggest limited local traffic or industrial emissions.