Ozone in the ambient air may induce adverse health effects. It is a potent oxidant which can damage all parts of the respiratory tract. Ozone exposure leads to health effects as diverse as reduced pulmonary function, increased airway reactivity, lung inflammation, increased respiratory symptoms/illness and higher mortality. Several of these acute effects have been observed in experiments involving controlled exposure at concentrations as low as 160 micrograms/m3, and in epidemiologic studies with ozone concentrations of 100 micrograms/m3 or higher. As yet, it is unclear whether repeated exposure to ozone causes irreversible pulmonary changes in humans, but such effects have been indicated in studies in animals. Marked interindividual variation in sensitivity and responsiveness to ozone has been demonstrated. To protect the population the Norwegian Pollution Control Authority recommends 100 micrograms/m3 and 80 micrograms/m3 as maximum ozone air quality guidelines for 1-hr and 8-hr averages, respectively. As a consequence of the EEA agreement, Norway is required to announce to the public when 1-hr average ozone concentrations exceed 180 micrograms/m3.