On 2026-08-13, air quality in Basel was dominated by elevated ozone levels. The maximum ozone concentration reached 166.9 µg/m³, which exceeded the regulatory threshold of 120 µg/m³ and triggered an exceedance flag. The daily average ozone was 112.5 µg/m³, also close to the threshold. Among the past 365 days, this maximum ozone value ranked 6th, indicating a notably high reading. In contrast, nitrogen dioxide and PM10 remained well within normal bounds: the maximum NO₂ was 30.9 µg/m³ (daily average 11.0 µg/m³) and the maximum PM10 was 34.4 µg/m³ (daily average 19.5 µg/m³), neither exceeding their respective limits of 80 µg/m³ and 50 µg/m³.

Compared with the last 30 days, the ozone maximum of 166.9 µg/m³ was below the 30-day maximum of 175.3 µg/m³, but the daily average of 112.5 µg/m³ was above the 30-day average of 99.4 µg/m³. Compared with the current season, the maximum was slightly below the seasonal maximum of 175.8 µg/m³, while the average was above the seasonal average of 89.9 µg/m³. For NO₂, the maximum of 30.9 µg/m³ was above both the 30-day maximum of 25.4 µg/m³ and the seasonal maximum of 31.4 µg/m³ was nearly matched; the daily average of 11.0 µg/m³ was above the 30-day average of 7.8 µg/m³ and the seasonal average of 7.1 µg/m³. PM10 values were unremarkable, with the maximum of 34.4 µg/m³ well below the 30-day and seasonal maxima of 63.2 µg/m³, and the average of 19.5 µg/m³ above the 30-day average of 16.0 µg/m³ and seasonal average of 14.4 µg/m³.

Looking at all-time data since 2018, the ozone maximum on 2026-08-13 ranked 27th out of 2992 days, placing it in the top 1% of all daily maxima. The all-time highest ozone maximum was 206.3 µg/m³ on 2019-06-27. The NO₂ maximum ranked 1202nd, a mid-range value, and the PM10 maximum ranked 295th, also unexceptional. The all-time highest NO₂ maximum was 83.2 µg/m³ on 2019-02-15, and the highest PM10 maximum was 165.1 µg/m³ on 2023-08-24.

Overall, the air quality on 2026-08-13 was characterized by a significant ozone episode, with the maximum concentration exceeding the legal threshold and ranking among the highest 2% of the past year. This is consistent with warm, sunny summer conditions that promote photochemical ozone formation. Nitrogen dioxide and particulate matter remained at typical or only slightly elevated levels, indicating that the pollution event was specific to ozone rather than a broad deterioration in air quality. The elevated ozone is notable because it can cause respiratory irritation and reduced lung function, particularly for sensitive groups, and it underscores the recurring summer ozone challenge in the Basel region.