Ozone is a colourless and toxic gas, and one of the most important trace gases in the atmosphere. The natural ozone layer which occurs in the atmosphere at an altitude of between 20 and 30 km protects life on earth against harmful ultraviolet radiation.

Ground level ozone, by contrast, is harmful to human health and the environment. It is produced in the presence of intensive solar radiation via photochemical processes from the precursor substances nitrogen oxides and volatile organic compounds. High ozone concentrations can reduce pulmonary function and result in lung diseases. Plants suffer leaf damage, and long term exposure affects growth and crop yield.

Thresholds, target values and long-term objectives

An information threshold of 180 µg/m3 and an alert threshold of 240 µg/m3 applies for ozone concentration in ambient air. As target value for the protection of human health, a maximum daily 8-hour mean of 120 µg/m3 is established. This value is not to be exceeded on more than 25 days per calendar year. The long-term objective is a maximum daily 8-hour mean of 120 µg/m3 without permitted exceedances.