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The actual outdoor exposure of the population to ozone is difficult to estimate. In addition to estimating the spatial distribution and time variation of the ozone concentration, the location and physical activity level of the population should be known. Since detailed data about these variables is not available, the description of exposure must be limited to estimating the number of people which possibly experienced ('were potentially exposed to') at least one exceedance of ozone concentrations above the information threshold value during summer 1996.

For rural areas, the number of people exposed to at least one exceedance could not be calculated. The interpolated exceedance 'field' (Map 3) is too fragmented. Moreover, no rural population database is available at the moment.

For urban areas, using a Geographical Information System, the location of urban stations was combined with an urban population database provided by Eurostat [6]. This database lists ca. 2100 cities in the EU with more than 25 000 inhabitants. It is estimated that from a total EU population of ca. 362 million, ca. 195 million people live in these cities. Approximately 66 million people live in cities in which at least one ozone station was operational during the 1996 summer season (34% of the total EU urban population or 18% of the total EU population). 142 cities reported at least one exceedance of the population information threshold value. Approximately 31 million people in these cities (46% of the urban population living in cities with operational monitors) were potentially exposed to at least one exceedance. The urban exposure estimates are presented in Figure 8.

The results should be interpreted with great care. In many cities, only one station is operational and exceedances at such a station were attributed to the whole urban population. The current description of station types is unprecise and for a substential number completely missing. Nevertheless, it is obvious that urban population exposure estimates cannot be based on monitoring results alone, since monitoring stations cover only 34% of the total urban population.

Figure 8: Number of exceedances (in days) and frequency distribution of urban population potentially exposed to hourly ozone concentrations exceeding 180 µg/m3, April-July 1996. (Representative for a urban population of 66 million (34% of the EU urban population) living in cities with more than 25 000 inhabitants; 18% of the total EU population).

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