On 2026-06-23, the most notable air quality issue in Basel was elevated ozone (O3) concentrations. The maximum ozone level reached 175.8 µg/m³, which is the highest value recorded in the past 365 days, ranking first. This maximum also exceeded the regulatory threshold of 120 µg/m³, as indicated by the ozone exceedance flag. The average ozone concentration for the day was 113.0 µg/m³, which is well above the 30-day average of 77.7 µg/m³ and the seasonal average of 73.2 µg/m³.

Nitrogen dioxide (NO2) and fine particulate matter (PM10) levels were not elevated. The maximum NO2 concentration was 31.4 µg/m³, and the average was 10.5 µg/m³, both well below the regulatory daily average limit of 80 µg/m³. The maximum PM10 concentration was 36.1 µg/m³, and the average was 26.7 µg/m³, also below the daily average limit of 50 µg/m³.

Comparing the current measurements with historical data since 2018, the maximum ozone level of 175.8 µg/m³ is unusually high. It ranks 11th all-time out of 2,945 days, placing it in the top percentile of recorded values. The all-time maximum ozone concentration was 206.3 µg/m³, measured on June 27, 2019. The average ozone concentration of 113.0 µg/m³ is also significantly above the historical seasonal average of 73.2 µg/m³. For NO2 and PM10, the current values are within typical historical ranges and are not considered anomalous.

Overall, the air quality on 2026-06-23 was not consistent with historical norms due to the notably elevated ozone levels. The high ozone concentrations are likely the result of strong photochemical activity, which typically occurs during periods of high temperature and intense sunlight. This condition is a common summer phenomenon in urban areas and can lead to respiratory irritation for sensitive individuals.