5.3 Exceedance of EU threshold values

5.3.1 Protection of human health

In this section we concentrate on the exceedances observed during the 12:00 - 20:00 hours period. From the four 8-h periods defined by the Directive, the highest number of exceedances may be expected during this interval. From data collected in 1994 and 1995 (Figure 11) it is concluded that the EU15 threshold set for the protection of human health (110 μg.m-3, 8-h mean) is exceeded substantially and in all countries (de Leeuw et al., 1995; de Leeuw and van Zantvoort, 1996). The 1995 subset of urban/street stations is estimated to provide representative values of the exposure of an urban population of about 41 million EU15 inhabitants. Per individual country, the coverage ranges from 28 million people in Germany to none in Sweden. More than 90% of these 41 million citizens were exposed to an exceedance of the threshold at least once in 1995. More than 80% of these people experienced exceedances during more than 25 days. The maximum 8-h average recorded in 1995, was 240 μg.m-3 and occurred in both Italy and Greece. If it is assumed that the ozone climatology observed by the current network can be extrapolated to the full EU15 population, then approximately 330 million people may be exposed to a minimum of one exceedance per year. A further analysis of the health impact of ozone exposure, in terms of hospital admissions, is presented in section 6.

In its first considerations the current Directive states that "the numerical values of [thresholds] should be based on the findings of work carried out in the framework of WHO". The initial WHO guideline for the protection of human health was defined at 100-120 μg.m-3 during an (any) 8-h period (WHO, 1987). The data reported in the framework of the Directive don't allow to investigate whether the choice of 4 fixed 8-h periods has diverted results much from the original WHO definition. Table 11 and 12 of Appendix 2 indicate that average over all Member States the number of exceedances reported over the 12:00-20:00 period is 11 to 24% higher than the number reported over the other three 8-h periods. The relative composition of various networks from street, urban and background sites is expected to play a role here. Stations showing a strong diurnal cycle will show a much larger number of exceedances in the 12:00-20:00 period than during the other 3 intervals. A preliminary analysis of the 1994 EMEP database (100 sites) shows that the number of exceedances over 110 μg.m-3 as a floating 8-h average is about 18% higher than the number of exceedances observed during the fixed 12:00-20:00 h period, over the same threshold.

5.3.2 Protection of vegetation

In 1994 and 1995 the threshold value set for the protection of vegetation (65 μg.m-3 24-h mean) was exceeded substantially (Figure 12), by up to a factor of 3, widely and frequently (de Leeuw et al., 1995; de Leeuw and van Zantvoort, 1996). In 1995 several countries reported exceedances during more than 150 days at some sites. In the same year, the full EU15 area of coniferous forest and arable land experienced exceedances of this threshold. In less than 1% of the area of broad-leaved forest exceedances were not observed. Note however that recent work (WHO, 1996b) indicates that this particular threshold may be of little relevance in assessing the potential for ozone effects on vegetation.

There is one more EU15 vegetation protection threshold (200 μg.m-3 1-h mean) from which measured data only were available. In 1995 this threshold was exceeded largely and widely, in the 14 countries reporting on this parameter. Approximately 40, 75, and 70% of the area of coniferous forest, broad-leaved forest and arable land respectively were not protected against exceedances of this threshold in 1995.

5.3.3 Information and warning thresholds

The threshold value for providing information to the public (180 μg.m-3 1-h mean) has been exceeded in almost all Member States during a limited number of days in 1994, 1995 (Figure 13) and 1996 (de Leeuw and van Zantvoort, 1996; Sluyter, 1996). An estimate from the 1996 data revealed that this concerned about 31 million Europeans, which approximates to 46% of the urban population living in cities with operational monitoring in that year.

The warning threshold (360 μg.m-3 1-h mean) is reached occasionally, in particular in the southern Member States. In 1994 a maximum hourly value of 490 μg.m-3 was observed in Italy.

5.3.4 Experiences with public information

The Air Quality Directives adopted earlier than Directive 92/72/EEC on air pollution by ozone laid down air quality limit values, which obliged Member States to take measures to improve the air quality in case of exceedances. In contrast, Directive 92/72/EEC did not set limit values, but defined air quality information and warning thresholds, obliging Member States to disseminate information to the public via the media as soon as any exceedance was observed. This also stimulated the national authorities to give background information on ozone to the population via brochures, teletext and other media, explaining the causes and the effects, and advising what to do during episodes of high ozone. In some countries more detailed brochures were provided to physicians. Further, the Directive proved to be a stimulus to ongoing work to develop ozone forecasting systems. In many Member States the Directive has brought about a change in the public perception of the ozone problem. The population information threshold (180 μg.m-3, 1-h mean) was exceeded several times per year in most Member States, while the population warning threshold (360 μg.m-3, 1-h mean) was attained in a few cases. The information that was issued when a threshold was exceeded received considerable attention from the media, and the increased public awareness has brought the ozone problem higher on the list of political priorities.

The question whether the population information threshold is of any real benefit to the public is difficult to answer. We address this question by comparing how often the health protection threshold (12:00-20:00 h) is exceeded without the population information threshold being reached. Table 9 and 12 in Appendix 2 indicate that the ratio of the number of exceedances of the health protection threshold and the population information threshold ranges between 3 and 87 in individual national networks during the three years covered. The reader should note that exceedances of these thresholds are not necessarily observed at the same stations. The data show that this ratio has been higher than 3 in all EU15 countries during all years and higher than 10 in Sweden, Denmark and Finland. High hourly peaks are seldom observed in the latter three countries and consequently their population is rarely warned. The data also indicate that the Spanish and Austrian population experiences a relatively low number of warnings; though we expect that this has other explanations. The Spanish network is composed of a relatively large number of street sites where hourly peaks are suppressed by local emissions of NO. The Austrian network includes a number of high altitude sites where a bigger influence of air from the free troposphere, relatively rich in ozone, is seen but where high hourly concentrations are observed only occasionally.

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Figure 11: Number of exceedances of the threshold value for protection of human health (110 μg.m-3 8-h mean) observed at urban/street or other/unspecified stations in January to December, 1995

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Figure 12: Number of exceedances of the threshold value for vegetation (65 μg.m-3 24-h average) observed at background stations in January - December, 1995

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Figure 13: Number of exceedances of the threshold value for information of the population (180 μg.m-3 1-h mean) observed at urban/street and other stations in January to December 1995

5.4 Exceedance of WHO guidelines

5.4.1 WHO guidelines

The 5EAP (EC, 1992) states that air quality values from WHO should become mandatory at EC level not later than 1998. Since WHO has recently revised its air quality guidelines (WHO, 1996a, 1996b) it is reasonable to evaluate the status of photochemical pollution in Europe in the light of these new guidelines. The establishment of the WHO guidelines for vegetation have taken place in close collaboration with UN-ECE (WHO, 1996b; Kärenlampi and Skärby, 1996). Table 8 lists the current WHO air quality guidelines.
Table 1: Air quality guidelines set by WHO

Threshold value set by Description Criteria based on Value
WHO Protection of public health maximum of floating 8-h average concentration 120 μg.m-3
Protection of agricultural crops of yield loss of 5% accumulated ozone dose above a cut-off of 40 ppb (AOT40) for daylight hours (i.e. >50 W.m-2 potential global radiation) over 3 months (May-July) 3 ppm.h
Protection of natural and semi-natural vegetation accumulated ozone dose above a cut-off of 40 ppb (AOT40) for daylight hours (i.e. >50 W.m-2 potential global radiation) over 3 months (May-July) 3 ppm.h
Protection of forest trees accumulated ozone dose above a cut-off of 40 ppb (AOT40) for daylight hours (i.e. >50 W.m-2 potential global radiation) over 6 months (Apr-Sept) 10 ppm.h
Evaluation of ecological risk AOT40 values for receptor specific annual time periods should be average over 5 years receptor specific values
Protection of sensitive species from short-term acute effects accumulated ozone dose above a cut-off of 40 ppb (AOT40) for daylight hours (i.e. >50 W.m-2 potential global radiation) over 5 days when vapour pressure deficit > 1.5 kPa 0.5 ppm.h
Protection of sensitive species from short-term acute effects accumulated ozone dose above a cut-off of 40 ppb (AOT40) for daylight hours (i.e. >50 W.m-2 potential global radiation) over 5 days when vapour pressure deficit < 1.5 kPa 0.2 ppm.h

The concept of AOT40 may need some clarification. This effect parameter is calculated as the accumulated ozone exposure during daylight hours above a threshold value of 40 ppb (» 80 μg/m3, 1-h values). The excess is expressed as the number of ppm.h above the threshold. An AOT40 of 3 ppm.h, calculated over daylight hours during May-July is the current guideline for crops, while an AOT40 of 10 ppm.h, calculated for the period April-September, is the guideline for forests (WHO, 1996b).

The debate on whether or not an AOT type statistic is suitable to assess exposure of the population to exceedances of the WHO guideline for protection of public health of 120 μg.m-3 (60 ppb) is not yet completed. AOT60=0 ppb.h is mathematically equivalent to zero exceedances of the WHO guideline. The WHO/UN-ECE workshop on 'health effects of ozone and nitrogen oxides in an integrated assessment of air pollution' agreed that AOT60 could be used as an exploratory indicator for exceedances of the WHO threshold.

The data reported under the current Ozone Directive are not expressed in terms of the new WHO guidelines. Consequently guidelines exceedances cannot be derived from those data. In the following, measurement results from the EMEP network (Hjellbrekke, 1997) and concentrations expressed in terms of AOT calculated by the EMEP model (Simpson et al., 1997) are used to provide a preliminary assessment. It is noted that results from the EMEP model are also used in the work of Amann et al., (1997) to support the development of the EU Ozone Reduction Strategy Development and the UN-ECE emission reduction protocols.

5.4.2 AOT60 - human health

Figure 14 presents the number of days that the WHO guideline of 120 μg.m-3, calculated as a 8-h floating mean, was exceeded in April to September 1995. Note that the EMEP network is composed of rural and remote stations which makes it somewhat less suitable to assess human exposure. The figure shows that exceedances occur in all Member States, ranging from 1-5 exceedance days in Sweden and Finland to more than 40 in the north of Italy, Spain and Austria. A calculation of the AOT60 accumulated dose (Figure 15) from the same data reveals a different pattern. In this case there is a maximum exceeding 5 ppm.h stretching from the south of the UK, over northern France, Belgium to Germany.

AOT60 was also computed by the EMEP model (Figure 16). The modelled map (Simpson et al., 1997) was used to enlarge the spatial coverage over all EU Member States. The computed AOT60 level is presented as a mean over 5-years in order to remove most of the interannual fluctuation induced by meteorology. The calculations are performed using the latest version of the EMEP 150 km Lagrangian ozone model. Emissions were held constant at 1990 levels, but meteorological data were taken from 5 summers, i.e. April-September 1989, 1990, 1992, 1993 and 1994. This procedure prohibits a one-to-one comparison of the measured and modelled patterns, although the large scale gradient and the location of maxima and minima display reasonable agreement. The modelled results were combined with GIS maps. This calculation indicates that 99% of EU15 inhabitants may be exposed, at least once per year, to exceedances of the WHO guideline. Close to 80% of EU15 inhabitants is calculated to experience an AOT60 of 1 ppm.h, with nobody exposed in Finland and 100% exposed in Germany, France, and Benelux.

Figure 14: The number of days (April-September, 1995) that the maximum 8-h floating mean exceeded 120 μg.m-3. Courtesy: A.-G. Hjellbrekke and the EMEP programme.

Figure 15: AOT60 (April-September, 1995). Courtesy: A.-G. Hjellbrekke and the EMEP programme.

Figure 16: Modelled AOT60 as a mean over 5 summer's meteorology. 1 ppb O3 = 2 µg.m-3. Source: Simpson et al., 1997

5.4.3 AOT40 and crops

The measured and modelled exceedance of AOT40-crops are shown in Figure 17 and 18 respectively. The modelled map (Simpson et al., 1997) was used to enlarge the spatial coverage over all EU Member States. The computed AOT40 levels for crops and semi-natural vegetation is presented as a mean over 5 years in order to remove most of the interannual fluctuation induced by meteorology. Both charts show that the threshold of 3 ppm.h is exceeded in most Member States. However, parts of Sweden, Finland and the north of the UK are free from exceedances. GIS calculations based on the computed map indicate that in these countries 44%, 100% and 15%, respectively, of their arable land did not experience exceedances. The calculations also reveal that on average over all EU15 countries only 6% of arable land is not exposed to exceedances of the AOT40 guideline. In particular French, German, Belgian and Italian crops are calculated to experience exceedances of a factor of about 5 above the 3 ppm.h threshold. Similar exposure patterns for semi-natural vegetation are not available.

The EMEP model indicates that the interannual variation of single-year AOT40 calculations is quite considerable; 1985 and 1986 were 'low' ozone years, whereas during 1989 and 1990 several episodes occurred. The model computed that in each year, Sweden, Finland and Scotland had AOT40 levels below 3 ppm.h. In the other Member States, even in the 'low' ozone years, the area of exceedance covers almost their entire surface. The main difference is the magnitude of exceedance, namely a factor 2 to 3 in low ozone years and a factor 5 to 8 in episode years.

Figure 17: Measured AOT40-crops (1995) calculated from the EMEP network. The WHO guideline value is set at 3 ppm.h. 1 ppb O3 » 2 μg.m-3. Source: Hjellbrekke, 1997

Figure 18: Modelled AOT40-crops as a mean over 5-summer's of meteorology using 1990 emissions. The WHO guideline is set at 3 ppm.h. 1 ppb O3 » 2 μg.m-3. Source: Simpson et al., 1997

5.4.4 AOT40 and forests

The 5-year average modelled excess ozone over the guideline for forests (10 ppm.h) is presented in Figure 19. The map derived from the EMEP measurement network is not shown. The modelled map reveals a north-west to south-east increasing gradient. In Scandinavia, Ireland and the United Kingdom forests are almost fully free from exceedances. The north-west part of the continent experiences exceedances by a factor of 2 of the 10 ppm.h guideline, while some areas of central and south Europe are exposed to a maximum approaching 30 ppm.h. GIS computations indicate that 35% of Europe's coniferous forest experiences exceedances of the forest guideline. This is in contrast to the broad-leaved forest, of which according to a GIS calculation approximately 70% are in areas of exceedance. These different percentages reflect the large amount of coniferous forests in the Scandinavian countries and the major share of European broad-leaved trees in Germany, France and Italy.

The AOT40 guideline for forest is found to be less stringent than AOT40 for crops and semi-natural vegetation, i.e. the area of exceedance is smaller.

Figure 19: Modelled AOT40-forest as a mean over 5-summers. The WHO guideline value is set at 10 ppm.h. 1 ppb O3 » 2 μg.m-3. Source: Simpson et al., 1997

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