On June 30, 2026, air quality in Basel was characterized by elevated ozone levels. The maximum ozone concentration of 142.5 µg/m³ ranked 15th highest among the last 365 days. This value exceeded the regulatory threshold of 120 µg/m³ for the maximum hourly average, which is permitted only once per year under Swiss air quality regulations. The average ozone concentration for the day was 118.7 µg/m³, which is notably higher than the seasonal average of 77.6 µg/m³ and the 30-day average of 76.2 µg/m³.

Nitrogen dioxide and particulate matter (PM10) concentrations remained low. The maximum NO₂ concentration was 10.4 µg/m³, with an average of 6.2 µg/m³, both well below the daily average limit of 80 µg/m³. The maximum PM10 concentration was 18.3 µg/m³, with an average of 12.2 µg/m³, also far below the daily average limit of 50 µg/m³. These values are consistent with typical levels for the season.

Compared to historical data since 2018, the maximum ozone concentration of 142.5 µg/m³ is well below the all-time maximum of 206.3 µg/m³ recorded on June 27, 2019. The current maximum ranks 136th out of 2948 days. The maximum NO₂ concentration of 10.4 µg/m³ is far below the all-time maximum of 83.2 µg/m³ recorded on February 15, 2019. The maximum PM10 concentration of 18.3 µg/m³ is also far below the all-time maximum of 165.1 µg/m³ recorded on August 24, 2023.

Overall, air quality on June 30, 2026, was not consistent with historical norms due to the elevated ozone levels. The high ozone concentration is likely the result of strong solar radiation and high temperatures typical of summer, which promote photochemical ozone formation. While NO₂ and PM10 levels were low and unremarkable, the ozone exceedance indicates a notable deviation from typical conditions and warrants attention, particularly for sensitive individuals.