On July 18, 2026, air quality in Basel was characterized by elevated ozone levels. The maximum ozone concentration of 139.7 µg/m³ exceeded the regulatory threshold of 120 µg/m³, which is the maximum hourly average allowed under the Swiss Clean Air Regulations. This exceedance is noted in the data. The average ozone concentration for the day was 106.6 µg/m³. The maximum ozone concentration ranked 22nd highest out of the past 362 days, placing it within the top 10 percent of recent daily maximums.

Nitrogen dioxide and fine particulate matter (PM10) concentrations remained low. The maximum NO₂ concentration was 24.0 µg/m³, and the average was 7.7 µg/m³, both well below the regulatory daily average limit of 80 µg/m³. The maximum PM10 concentration was 12.3 µg/m³, with an average of 9.3 µg/m³, also far below the daily average limit of 50 µg/m³. Neither pollutant exceeded its respective threshold.

Comparing the current measurements to historical data since 2018, the ozone levels were notably high. The average ozone concentration of 106.6 µg/m³ was above the seasonal average of 84.9 µg/m³ and the 30-day average of 97.1 µg/m³. The maximum ozone concentration of 139.7 µg/m³ was below the all-time maximum of 206.3 µg/m³, recorded on June 27, 2019. The maximum NO₂ concentration of 24.0 µg/m³ was very low compared to the all-time maximum of 83.2 µg/m³ from February 15, 2019. Similarly, the maximum PM10 concentration of 12.3 µg/m³ was far below the all-time maximum of 165.1 µg/m³ from August 24, 2023.

Overall, the air quality on July 18, 2026, was unusual due to the elevated ozone levels, which exceeded the regulatory standard. This is consistent with a summer day with strong photochemical activity, where high temperatures and sunlight promote ozone formation. The low concentrations of NO₂ and PM10 suggest that local traffic and particulate sources were not major contributors to the day's air quality issues. The elevated ozone is the primary concern and indicates a typical summer smog event.