On July 11, 2026, the most notable air quality issue in Basel was elevated ozone. The maximum ozone concentration of 159.8 µg/m³ ranked 7th highest among the last 365 days. This value exceeded the regulatory threshold of 120 µg/m³ for the maximum hourly average, and the report indicates an exceedance occurred.

The average ozone concentration for the day was 112.5 µg/m³, which is well above the 30-day average of 85.9 µg/m³ and the seasonal average of 80.8 µg/m³. The maximum value of 159.8 µg/m³ was close to the 30-day and seasonal maximum of 175.8 µg/m³. Compared to historical data since 2018, the maximum ozone concentration of 206.3 µg/m³ was recorded on June 27, 2019. The current maximum is below this all-time high, but the day's levels were unusually high for the period.

Nitrogen dioxide and PM10 concentrations were low and unremarkable. The maximum NO2 concentration was 16.5 µg/m³, well below the daily average threshold of 80 µg/m³, and its rank of 253rd in the last 365 days indicates a typical, low level. The average NO2 was 8.1 µg/m³, slightly above the 30-day average of 6.9 µg/m³ but still very low. The maximum PM10 concentration was 17.7 µg/m³, far below the daily average threshold of 50 µg/m³, and its rank of 185th in the last 365 days is also typical. The average PM10 was 14.9 µg/m³, consistent with the 30-day average of 15.1 µg/m³.

Overall, the air quality on July 11, 2026, was characterized by a significant ozone episode, while NO2 and PM10 remained at low levels. The elevated ozone is consistent with a summer day featuring strong sunlight and high temperatures, which promote ozone formation. The concentration was high enough to be a concern for sensitive groups, but it did not approach the all-time record for Basel.