In May 2026, the Binningen monitoring station recorded no days with a KBI of 1, indicating that no days had low air pollution. There were 20 days with a KBI of 2, representing moderate pollution. More significantly, there were 5 days with a KBI of 3, classified as marked pollution, and 6 days with a KBI of 4, classified as significant pollution. No days reached the high or very high categories of KBI 5 or 6. The KBI, or Kurzzeit-Belastungs-Index, is a short-term exposure index that combines concentrations of ozone, nitrogen dioxide, and particulate matter into a single value to indicate the overall air quality load. The mean concentration for ozone was 75.5 µg/m³, with a maximum of 153.2 µg/m³. For nitrogen dioxide, the mean was 7.3 µg/m³ and the maximum was 37.8 µg/m³. For particulate matter, the mean was 12.8 µg/m³ and the maximum was 84.1 µg/m³. All values are listed in Table 1, and regulatory thresholds are listed in Table 3.

Compared to previous years for the month of May, 2026 stands out as a year with notably higher air pollution. In terms of days with KBI of 1, 2026 ranks third out of eight years, tied with several other years that also had zero such days. For days with KBI of 2, 2026 had the fewest, with only 20 days, ranking last. The maximum ozone concentration in 2026 was the highest on record, ranking first, and the mean ozone concentration also ranked first. The maximum nitrogen dioxide concentration ranked sixth, and the mean nitrogen dioxide concentration ranked seventh, indicating relatively low levels for that pollutant. The maximum particulate matter concentration was the highest recorded, ranking first, and the mean particulate matter concentration also ranked first. The monthly average and maximum values for May are reported in Table 2 and also displayed in Figures 1 to 3.