4. Exposure to exceedances

4. EXPOSURE TO EXCEEDANCES

4.1 Population exposure

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 who possibly experienced ('were potentially exposed to') at least one exceedance of ozone concentrations above the information threshold value during summer 1997.

For urban areas, using a Geographical Information System, the location of urban stations was combined with information on the European population provided by Eurostat (EUROSTAT, 1996). This database lists ca. 2100 cities in the EU with more than 25 000 inhabitants. It is calculated that from a total EU population of ca. 362 million, ca. 195 million people live in these cities. Approximately 75 million people live in cities in which at least one ozone station was operational during the 1997 summer season (38% of the total EU urban population or 21% of the total EU population).

157 cities reported at least one exceedance of the population information threshold value. Approximately 25 million people in these cities¹ (34% 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 9.

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 imprecise and for a substantial number completely missing. Nevertheless, it is obvious that urban population exposure estimates cannot be based on monitoring results alone, since monitoring stations cover only 38% of the total urban population.

The number of people "potentially exposed" as calculated for this report cannot be compared with the calculated number published in last year's report (Sluyter and van Zantvoort, 1996). This year calculations were made for the period April-August (1996: April-July). Moreover, the population database used was updated and the number of stations (coverage) has increased.

Figure 9: Number of exceedances (in days) and frequency distribution of urban population potentially exposed to hourly ozone concentrations exceeding 180 µg/m³, April-August 1997. (Representative for an urban population of 75 million (38% of the EU urban population) living in cities with more than 25 000 inhabitants; 21% of the total EU population).

For rural areas, the number of people potentially exposed to at least one exceedance could not be calculated. The interpolated exceedance 'field' (Map 3) is rather fragmented. Moreover, no rural population database is available at the moment. However, as ozone levels are in general higher in rural areas than in cities, it is anticipated that a larger fraction of the rural population will be potentially exposed to exceedances than in cities.

4.2 Territorial exposure

The interpolated exceedance map for rural locations with an assumed radius of representativeness is too fragmented to draw firm conclusions on spatial patterns. The rural stations cover approximately 48% of the EU territory. The area where exceedances were observed, calculated on the basis of the interpolated map, is approximately 49% of the total area covered by background stations which is estimated at 23% of the total EU territory. These figures are comparable to those observed during summer 1996.

¹ This figure cannot be compared directly with figures provided in earlier reports covering summer 1994 and summer 1995 as the basis for calculation was not the same.