The ambient air includes plenty of ozone in the springtime and summer. Concentrations in the Helsinki Metropolitan Area are at their highest outside the suburbs and far from emission sources. The abbreviation O3 refers to ozone.

The concentrations are high if the hourly value exceeds 140 µg/m3. There are information and warning thresholds for such ozone concentrations when the public must be informed about health hazards. The thresholds are 180 and 240 µg/m3 per hour. From a health perspective, the objective is that the 8-hour average remains below 120 µg/m3. This level is exceeded every year.