Diurnal patterns of ozone vary according to location, depending on the balance of the factors affecting its formation, transport and destruction. From the minimal levels of early morning, levels rise as a result of photochemical processes, peaking in the early afternoon. Peak 1-hour concentrations in excess of 500 µg/m3 are reached in parts of Southern California and in Mexico City. Mean hourly concentrations may exceed values of 240 µg/m3 for 10 hours or more in the eastern USA and Southern California (1), and in the Netherlands (7). In the Netherlands, maximal 1-hour averages range from 227 to 431 µg/m3 at different sites; maximal 8-hour averages range from 191 to 350 µg/m3 (6). During the night, ozone is scavenged by nitric oxide. Other removal processes include transport by wind and vertical mixing of the air mass. It should be noted that several of these typical days with increased photochemical activity can occur consecutively during an episode. The length of the recovery period between two successive episodes and the number of episodes in a season may also be important factors in the nature and magnitude of health and vegetation effects.