On July 31, 2026, ozone was the primary pollutant of concern in Basel. The maximum ozone concentration reached 141.8 µg/m³, which exceeded the regulatory threshold of 120 µg/m³. This value ranked as the 23rd highest daily maximum in the past 365 days. The daily average ozone concentration was 109.0 µg/m³. Nitrogen dioxide levels remained low, with a maximum of 19.5 µg/m³ and an average of 8.3 µg/m³, both well below the 80 µg/m³ daily average limit. Particulate matter (PM10) showed a maximum of 48.8 µg/m³ and an average of 31.0 µg/m³, staying just under the 50 µg/m³ daily average threshold.

Compared to the last 30 days, the maximum ozone reading was lower than the period’s peak of 175.3 µg/m³, but the daily average of 109.0 µg/m³ was above the 30-day average of 95.8 µg/m³. The PM10 average of 31.0 µg/m³ was more than double the 30-day average of 13.6 µg/m³, and its maximum of 48.8 µg/m³ ranked 14th in the last year, indicating a notably elevated day for particulate matter. Nitrogen dioxide values were consistent with recent low levels.

Looking at the historical record since 2018, the ozone maximum of 141.8 µg/m³ was unremarkable, ranking 151st out of 2979 days. The all-time ozone maximum remains 206.3 µg/m³ on June 27, 2019. The PM10 maximum of 48.8 µg/m³ ranked 75th historically, far below the record of 165.1 µg/m³ set on August 24, 2023. Nitrogen dioxide continued a long-term trend of low concentrations, with its maximum ranking 2015th overall.

Overall, air quality on this day was characterized by a considerable pollution rating, driven by ozone exceeding its hourly threshold and PM10 levels that were unusually high compared to recent weeks. The elevated ozone is typical for summer conditions with strong sunlight and stagnant air, while the increased PM10 may reflect local sources or regional transport. The combination of these two pollutants made the day stand out from typical seasonal patterns.