On 2026-06-25, the most notable air quality issue in Basel was elevated ozone (O3) concentrations. The maximum ozone level of 168.7 µg/m³ was the fourth highest recorded in the past 365 days, placing it in the top 1% of daily maximums for that period. This value also exceeded the regulatory threshold of 120 µg/m³ for the maximum hourly average, which is only permitted to be exceeded once per year according to the Luftreinhalte-Verordnung. The average ozone concentration for the day was 105.6 µg/m³, which is significantly higher than the 30-day average of 78.8 µg/m³ and the seasonal average of 75.9 µg/m³, indicating a clear spike in ozone levels.

Nitrogen dioxide (NO2) and fine particulate matter (PM10) levels were not elevated. The maximum NO2 concentration of 28.0 µg/m³ was well below the daily average threshold of 80 µg/m³, and its rank of 136th in the past year indicates it was a typical, low reading. The average NO2 of 10.8 µg/m³ was slightly above the 30-day average of 6.3 µg/m³ but remains very low. For PM10, the maximum concentration of 41.4 µg/m³ ranked 18th in the past year, and the daily average of 26.9 µg/m³ was below the regulatory limit of 50 µg/m³. However, the PM10 average was notably higher than the 30-day average of 15.8 µg/m³ and the seasonal average of 13.7 µg/m³, suggesting a moderate increase in particulate matter alongside the ozone event.

Compared to historical data since 2018, the maximum ozone level of 168.7 µg/m³ is well below the all-time maximum of 206.3 µg/m³ recorded on June 27, 2019. Its rank of 22nd all-time places it within the top 1% of all recorded daily maximums, confirming it as an unusually high value for the location. The maximum PM10 concentration of 41.4 µg/m³ is far below the all-time maximum of 165.1 µg/m³ from August 24, 2023, and its rank of 138th all-time indicates it was not an extreme event in a historical context.

Overall, the air quality on June 25, 2026, was characterized by a significant ozone episode. The day was not consistent with historical norms for ozone, as the maximum concentration was unusually high and exceeded regulatory standards. The elevated ozone levels are typical for a hot, sunny summer day with strong photochemical activity, which promotes the formation of ground-level ozone from precursor pollutants. While PM10 levels were also above recent averages, they did not breach regulatory limits. The overall assessment is that the air quality was unusual due to the pronounced ozone exceedance, which is a common but notable summer air quality concern.