Ozone irritates the mucous membranes and attacks the respiratory tract, eyes and lung tissue in particular; when inhaled, ozone penetrates deep into the lungs. In addition to changes in lung function parameters, higher ozone concentrations cause subjective symptoms such as watery eyes, headaches, poor concentration and irritation of the respiratory tract, which are accompanied by a decrease in physical performance as concentrations continue to rise. It is estimated that around ten percent of the population are particularly sensitive to increased ozone concentrations.

The EC threshold values for ozone are defined in EC Directive 92/72/EEC as follows:

• Average value during 8 hours (health protection): 110 µg/m3
• Mean value during 1 hour (informing the public): 180 µg/m3
• Mean value during 1 hour (triggering the warning system): 360 µg/m3

At ozone levels of 180 µg/m3 and above, people with sensitive health are advised to avoid strenuous outdoor activities. Sporting endurance activities should also be avoided.