Indicator Specification

Exceedance of air quality limit values in urban areas

Indicator Specification

Indicator codes: CSI 004

Published 01 Dec 2008 Last modified 06 Oct 2020

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The indicator shows the fraction of the urban population that is potentially exposed to ambient air [1] concentrations of pollutants [2] in excess of the EU limit value set for the protection of human health. The urban population considered is the total number of people living in cities with at least one monitoring station at a background location. The population data applied for the indicator derives from the Urban Audit , which is conducted at the initiative of the Directorate-General for Regional Policy at the European Commission, in cooperation with Eurostat and the national statistical offices of the 27 current Member States. Currently, the Urban Audit involve more than 620 European cities in 30 EEA member countries. The Urban Audit contains data for over 250 indicators across nine domains (e.g. demography, social aspects, environment, travel and transport). The Urban Audit aims at a balanced and representative sample of cities in Europe. To obtain such a selection, a few simple rules are applied: 1. Approximately 20% of the national population should be covered by the Urban Audit. 2. All capital cities were included. 3. Where possible, regional capitals were included. 4. Both large (more than 250 000 inhabitants) and medium-sized cities (minimum 50 000 and maximum 250 000 inhabitants) were included. 5. The selected cities should be geographically dispersed within each Member State. The selection of cities was prepared in close collaboration between the Directorate-General for Regional Policy, Eurostat and the national statistical institutes. To ensure that large and medium-sized cities are equally represented in the Urban Audit, in some of the larger Member States not all large cities could be included. The Urban Audit works with three different spatial levels: the core city, the larger urban zone (LUZ) and the sub-city district (SCD). For CSI 004 only the the core city level is considered, which is the most important level. To ensure that this level is directly relevant to policy makers and politicians, political boundaries were used to define the city level. In many countries these boundaries are clearly established and well-known. As a result, for most cities the boundary used in the Urban Audit corresponds to the general perception of that city. Due to the highly diverse nature of political boundaries in the European Union, for some cities the political boundary does not correspond to the general perception of that city. In a few cities, Dublin for example, the political boundary of the city is narrower than the general perception of that city. Exceedance of air quality limit values occurs when the concentration of air pollutants exceeds the limit values specified in the first Daughter Directive of the Air Quality Framework Directive for SO 2 , PM 10 [3], NO 2 and the target values for O 3 as specified in the third Daughter Directive. Where there are multiple limit values (see section on Policy Targets), the indicator uses the most stringent case: Sulphur dioxide (SO 2 ): the daily limit value; Nitrogen dioxide (NO 2 ): the annual limit value; Particulate matter (PM 10 ): the daily limit value; Ozone (O 3 ): the target value. [1] 'Ambient air' shall mean outdoor air in the troposphere, excluding work places. [2] 'pollutant' shall mean any substance introduced directly or indirectly by man into the ambient air and likely to have harmful effects on human health and/or the environment as a whole. [3] 'PM 10 ' shall mean particulate matter which passes through a size-selective inlet with a 50 % efficiency cut-off at 10 microgram aerodynamic diameter.

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Assessment versions

Published (reviewed and quality assured)

No published assessments

Rationale

Justification for indicator selection

This indicator of the exposure of urban populations to air pollution focuses on sulphur dioxide, particulate matter (PM), nitrogen oxides and ground-level ozone. Sulphur dioxide (SO₂) is directly toxic to humans, its main action being on the respiratory functions. Indirectly, it can affect human health as it is converted to sulphuric acid and sulphate in the form of fine particulate matter.

Epidemiological studies have reported statistical significant associations between short-term, and especially long-term exposure to increased ambient PM concentrations and increased morbidity and (premature) mortality. PM levels that may be relevant to human health are commonly expressed in terms of PM₁₀ meaning particulate matter which passes through a size-selective inlet with a 50 % efficiency cut-off at 10 microgram aerodynamic diameter. Health effect associations for the PM_2.5 fraction are even more clearly evident. Although the body of evidence concerning the health effects of PM is increasing rapidly, it is not yet possible to identify a concentration threshold below which health effects are not detectable.

PM₁₀ in the atmosphere results from direct emissions (primary PM₁₀) and from emissions of particulate precursors (nitrogen oxides, sulphur dioxide, ammonia and organic compounds) which are partly transformed into particles by chemical reactions in the atmosphere (secondary PM₁₀).

Short-term exposure to nitrogen dioxide may result in airway and lung damage, decline in lung function, and increased responsiveness to allergens following acute exposure. Toxicology studies show that long-term exposure to nitrogen dioxide can induce irreversible changes in lung structure and function.

Exposure to high ozone concentration for periods of a few days can have adverse health effects, in particular inflammatory responses and reduction in lung function. Exposure to moderate ozone concentrations for longer periods may lead to a reduction in lung function in young children.

Scientific references

European Communities (2009) Eurostat Structural Indicators related to urban air quality: 1) Urban population exposure to air pollution by ozone, and 2) Urban population exposure to air pollution by particulate matter.
OECD (2004) OECD Key environmental indicators (KEI). Available indicator: SO x and NO x emission intensities. Medium term indicator: Population exposure to air pollution.
European Communities (2001) Towards a Local Sustainability Profile. European Common Indicators. Methodology sheet for indicator no. A.5.
WHO (2002) Air Quality Guidelines for Europe. Second edition. WHO Regional Publications, European Series, No. 91.
WHO (2005) WHO air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulphur dioxide. Global update 2005.
UNDSD (2007a) UN Division for Sustainable Development, Department of Economic and Social Affairs. Indicators of Sustainable Development: Guidelines and Methodologies - Third edition.
UNDSD (2007b) UN Division for Sustainable Development, Department of Economic and Social Affairs. Indicators of Sustainable Development: Guidelines and Methodologies - Third edition. Detailed Methodology Sheets.

Indicator definition

The urban population considered is the total number of people living in cities with at least one monitoring station at a background location. The population data applied for the indicator derives from the Urban Audit, which is conducted at the initiative of the Directorate-General for Regional Policy at the European Commission, in cooperation with Eurostat and the national statistical offices of the 27 current Member States. Currently, the Urban Audit involve more than 620 European cities in 30 EEA member countries. The Urban Audit contains data for over 250 indicators across nine domains (e.g. demography, social aspects, environment, travel and transport).

The Urban Audit aims at a balanced and representative sample of cities in Europe. To obtain such a selection, a few simple rules are applied:

1. Approximately 20% of the national population should be covered by the Urban Audit.

2. All capital cities were included.

3. Where possible, regional capitals were included.

4. Both large (more than 250 000 inhabitants) and medium-sized cities (minimum 50 000 and maximum 250 000 inhabitants) were included.

5. The selected cities should be geographically dispersed within each Member State.

The selection of cities was prepared in close collaboration between the Directorate-General for Regional Policy, Eurostat and the national statistical institutes. To ensure that large and medium-sized cities are equally represented in the Urban Audit, in some of the larger Member States not all large cities could be included.

The Urban Audit works with three different spatial levels: the core city, the larger urban zone (LUZ) and the sub-city district (SCD). For CSI 004 only the the core city level is considered, which is the most important level. To ensure that this level is directly relevant to policy makers and politicians, political boundaries were used to define the city level. In many countries these boundaries are clearly established and well-known. As a result, for most cities the boundary used in the Urban Audit corresponds to the general perception of that city. Due to the highly diverse nature of political boundaries in the European Union, for some cities the political boundary does not correspond to the general perception of that city. In a few cities, Dublin for example, the political boundary of the city is narrower than the general perception of that city.

Exceedance of air quality limit values occurs when the concentration of air pollutants exceeds the limit values specified in the first Daughter Directive of the Air Quality Framework Directive for SO₂, PM₁₀[3], NO₂ and the target values for O₃ as specified in the third Daughter Directive. Where there are multiple limit values (see section on Policy Targets), the indicator uses the most stringent case:

Sulphur dioxide (SO₂): the daily limit value;
Nitrogen dioxide (NO₂): the annual limit value;
Particulate matter (PM₁₀): the daily limit value;
Ozone (O₃): the target value.

[1] 'Ambient air' shall mean outdoor air in the troposphere, excluding work places.

[2] 'pollutant' shall mean any substance introduced directly or indirectly by man into the ambient air and likely to have harmful effects on human health and/or the environment as a whole.

[3] 'PM₁₀' shall mean particulate matter which passes through a size-selective inlet with a 50 % efficiency cut-off at 10 microgram aerodynamic diameter.

Units

Percentage of the urban population in Europe potentially exposed to ambient air concentrations (in microgram/m³) of sulphur dioxide (SO₂), particulate matter (PM₁₀), nitrogen dioxide (NO₂) and ozone (O₃) in excess of the EU limit value set for the protection of human health.

Policy context and targets

Context description

This indicator is relevant information for the current European air quality legislation related to the protection of human health in the adopted Daughter Directive for sulphur dioxide, oxides of nitrogen, particulate matter and lead in ambient air (Council Directive 1999/30/EC).

A combined ozone and acidification abatement strategy has been developed by the Commission, resulting in the Ozone Daughter Directive (2002/3/EC) and the National Emission Ceiling Directive (2001/81/EC). In this legislation, target values for ozone levels and for precursor emissions have been set.

Targets

EU limit values on concentrations of sulphur dioxide in ambient air

Two limit values have been set for the protection of human health. Both limit values had to be met by 1 January 2005.

a limit value of 125 microgram SO₂/m³ as an daily average, not to be exceeded more than three times a calendar year, has been set for the protection of human health in the adopted Daughter Directive for sulphur dioxide, oxides of nitrogen, particulate matter and lead in ambient air (Council Directive 1999/30/EC, Section I of Annex I).
an hourly limit value for the protection of human health has been set at 350 microgram SO₂/m³; this level may not be exceeded more than 24 times a calendar year.

EU limit values on concentrations of nitrogen dioxide in ambient air

Both limit values have to be met by 1 January 2010:

In the first Daughter Directive (Council Directive 1999/30/EC, section 1 of Annex II) an annual mean limit value for nitrogen dioxide of 40 microgram NO₂/m³ has been set for the protection of human health.
In addition, an hourly limit value of 200 microgram NO₂/m³ not to be exceeded more than 18 times a calendar year has been set.

EU limit values on concentrations of PM₁₀ in ambient air

Both limit values had to be met by 1 January 2005.

a limit value for PM₁₀ of 50 microgram/m³ (24 hour average, i.e. daily), not to be exceeded more than 35 times a calendar year is set for the protection of human health by the first Daughter Directive for sulphur dioxide, oxides of nitrogen, particulate matter and lead in ambient air (Council Directive 1999/30/EC, Annex III).
an additional limit value of 40 microgram/m³ as annual average has also been set.

EU target values on concentrations of ozone in ambient air

A combined ozone and acidification abatement strategy has been developed by the European Commission, resulting in a new Ozone Daughter Directive (2002/3/EC) and a National Emission Ceiling Directive (2001/81/EC). In this legislation, target values for ozone levels and for precursor emissions have been set.

The Ozone Daughter Directive sets a target value for the protection of human health of 120 microgram O₃/m³ as maximum daily 8 hour mean, not to be exceeded more than 25 days per calendar year, averaged over three years. This target should be met in 2010.
The Ozone Daughter Directive has also set a long-term objective of 120 microgram O₃/m³ as a maximum daily 8 hour average within a calendar year (not to be exceeded any day).

Key policy question

What progress is being made in reducing concentrations of air pollutants in urban areas to below the limit values (for sulphur dioxide, nitrogen dioxide and particulate matter) or the target values (for ozone) defined in air quality legislation?

Methodology

Methodology for indicator calculation

Initially, AirBase stations are spatially joined with Urban Audit core cities in a Geographical Information System (GIS) in order to select AirBase stations, which fall within the boundaries of the cities that take part in the Urban Audit. The selected AirBase stations include station types classified as 'urban background' and 'sub-urban background'. It is assumed that the whole city population is exposed to this type of environment. Stations classified as 'traffic' or 'industrial' are influenced either by traffic emissions or other local emissions. Such environments are generally not representative for residential areas. The traffic and industrial stations are therefore not selected for the indicator calculations.

Based on the selection of AirBase stations air quality statistics are extracted from the database via Structured Query Language (SQL) server queries in order to update the pollutant specific indicators specified below.

The geo-political domain for calculating this indicator can be that of EEA member countries, EU Member States or individual states.

Sulphur dioxide (SO₂)

For each station, the number of days with a daily averaged concentration in excess of the limit value (125 microgram/m³ as a daily mean) is calculated from the hourly values (if available) or daily values. Only time series with a data coverage of at least 75 % per calendar year are used (that is, in the case of daily values, having more than 274 valid daily values per calendar year, or 275 days if leap year). The number of exceedance days per city is obtained by averaging the results obtained for each station that falls within the city boundary.

The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.

Depending on the number of exceedances, each city (and its population) is then classified uniquely in one of the 4 classes of exceedance days (0 days, 1-3 days, 3-6 days, >6 days).
The percentage of urban population allocated to different exceedance classes is calculated by using the population in those cities assigned to each individual exceedance class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.

Particulate matter (PM₁₀)

For each station the number of days with a daily mean concentration in excess of the daily limit value of 50 microgram/m³ is calculated from the hourly values (if available) or daily values. The selected urban stations include station types 'urban background' and 'sub-urban background'. Only time series with a data capture of at least 75 % per calendar year are used (that is, in the case of daily values, having more than 274 valid daily values per calendar year, or 275 days if leap year). The number of exceedance days per city is obtained by averaging the results obtained for each station that falls within the city boundary.

The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.

Depending on the number of exceedances, each city (and its population) is then classified uniquely in one of the 4 classes of exceedance days (0 days, 0-7 days, 7-35 days, >35 days).
The percentage of urban population allocated to different exceedance classes is calculated by using the population in those cities assigned to each individual exceedance class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.

Nitrogen dioxide (NO₂)

The annual mean concentration in an Urban Audit city is calculated using the values measured at the selected stations (urban and sub-urban background). Only time series with a data capture of at least 75 % per calendar year are used (that is, in the case of daily values, having more than 274 valid daily values per calendar year, or 275 days if leap year).

The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.

Depending on the mean concentration, each city (and its population) is then classified uniquely in one of the 4 classes of concentration (0-26 microgram/m³, 26-32 microgram/m³, 32-40 microgram/m³), >40 microgram/m³).

The percentage of urban population allocated to different concentration classes is calculated by using the population in those cities assigned to each individual concentration class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.

Ozone (O₃)

For each station, the number of days with a daily maximum 8-hourly mean concentration in excess of the target value (120 microgram/m³) is calculated from the hourly values. Only time series with a data coverage of at least 75 % per calendar year are used (that is, in the case of hourly values, having more than 6576 valid hourly values per calendar year, or 6594 hours if leap year). The number of exceedance days per city is obtained by averaging the results obtained for each station that falls within the city boundary.

The above procedure is repeated for all the cities covered by the Urban Audit that have fulfilled the data coverage criteria.

Depending on the number of exceedances, each city (and its population) is then classified uniquely in one of the 4 classes of exceedance days (0 days, 0-25 days, 25-50 days, >50 days).

The percentage of urban population allocated to different exceedance classes is calculated by using the population in those cities assigned to each individual exceedance class and the total population in all Urban Audit cities that have fulfilled the data coverage criteria.

Methodology for gap filling

No gap-filling is applied for this indicator.

Methodology references

European exchange of air quality monitoring meta information in 2002 Buijsman E., van Hooydonk P.R., Mol W.J.A., Cernikovsky L. (2004). ETC/ACC Technical Paper 2004/1.

Data specifications

EEA data references

AirBase - The European air quality database provided by European Environment Agency (EEA)

External data references

Gisco - Urban Audit 2004

Data sources in latest figures

Uncertainties

Methodology uncertainty

The air quality data is officially submitted. Following the Exchange of Information Decision it is expected that data has been validated by the national data supplier (see Mol, W.J.A. (2008). Quality checks on air quality data in AirBase and the EoI data in 2008. ETC/ACC Working Paper December 2008. Available at http://air-climate.eionet.europa.eu/databases/airbase/ETCACC_WP_2008_Quality_checks_EoI2008_Airbase.pdf). Station characteristics and representativeness is in some cases insufficiently documented. The data is thought to be representative for the urban population in Europe as a whole. Locally, the indicator is subject to year-to-year variations due to meteorological variability.

Data sets uncertainty

Sulphur dioxide (SO₂)

Reliability, accuracy, robustness, uncertainty (at data level): The number of available data series varies considerably from year to year and is for the first part of the 1990s insufficient. The data is generally not representative for the total urban population in a country. Availability of data before 1990 is too low to include in the indicator; data for non EU Member States is largely missing before 1995. Locally, the indicator is subject to large year-to-year variations due to meteorological variability. When averaging over EEA member countries this meteorologically induced variation decreases in importance provided spatial data coverage is sufficient. Due to deficiencies in information on station characteristics, the selection of urban sites (i.e. of the type 'urban background' and 'sub-urban background') might not always result in a representative selection of polluted zones. As a consequence, the indicator may be biased . The representativeness of the selection is different for different cities which reduces the comparability between cities. It is not possible at this stage to select a sufficiently large set of stations covering the entire time period since the stations with available data change from year to year.

Particulate Matter (PM₁₀)

Strength and weakness (at data level): The air quality data is officially submitted to the European Commission under the Exchange of Information Decision. It is assumed that the national data supplier has validated the air quality data. Station characteristics and representativeness is often insufficiently documented. Geographical coverage and data availability still needs improvement. Data has been considered both from monitoring with the reference method (gravimetry) and with other methods. It is not documented whether countries have applied correction factors for non-reference methods, and if so, which factors have been applied. Uncertainties associated with this lack of knowledge may be several tens of percents.

Reliability, accuracy, robustness, and uncertainty (at data level): In some Member States the density of PM₁₀ monitoring stations are not sufficient to give a representative picture. The number of available data series varies considerably from year-to-year and is insufficient for the period before 1997. The data is generally not representative for the total urban population in a country. Locally, the indicator is subject to year-to-year variations due to meteorological variability. When averaging over EEA-32 this meteorologically induced variation decreases in importance provided spatial data coverage is sufficient. Due to deficiencies in information on station characteristics, the selection of urban (type 'urban background' and 'sub-urban background') sites might not always result in a representative selection of polluted areas. The indicator may be biased due to insufficient representative coverage of the pollution situation. The representativeness of the selection is likely to be different for different cities which reduces comparability.

Nitrogen dioxide (NO₂)

Reliability, accuracy, robustness, uncertainty (at data level): The number of available data series varies considerably from year-to-year and is for the first part of the 1990s insufficient. The data is generally not representative for the total urban population in a country. Availability of data before 1990 is too low to include in the indicator; data for non EU Member States is largely missing before 1995. When averaging over EEA-32 this meteorologically induced variation decreases in importance provided spatial data coverage is sufficient. Due to deficiencies in information on station characteristics, the selection of urban sites might not always result in a representative selection of polluted zones. As a consequence, the indicator may be biased. The representativeness of the selection is different for different cities which reduces the comparability between cities. It is not possible in this stage to select a sufficiently large set of stations covering the entire time period since the stations with available data change from year to year.

Ozone (O₃)

Reliability, accuracy, robustness, uncertainty (at data level): The number of available data series varies considerably from year to year and is for the first part of the 1990s insufficient. Yearly changes in indicator value may result from changes in monitoring density and/or selected cities which will influence the total monitored population. The indicator is subject to year-to-year fluctuations as it represents episodic conditions, and these depend on particular meteorological situations, the occurrence of which varies from year to year.

Data availability over the period 1980-1995 needs improvement.

Rationale uncertainty

Further work

Short term work

Work specified here requires to be completed within 1 year from now.

Long term work

Work specified here will require more than 1 year (from now) to be completed.

Work description

SO 2 Improvement of coverage in space and time; extrapolation to total (urban + rural) population. PM 10 Improve information on station type characteristics. Improve information on whether countries have applied correction factors for non-reference methods, and if so, which factors have been applied. Improvement of coverage in space and time; extrapolation to total (urban + rural) population. NO 2 Improvement of coverage in space and time; extrapolation to total (urban + rural) population. Improve consistency in station type characteristics ('urban background' and 'sub-urban background') assignments between the measurement network and national data suppliers. O 3 Improvement of coverage in space and time; extrapolation to total (urban + rural) population. consideration of spatial differences throughout Europe. Improve consistency in station type characteristics ('urban background' and 'sub-urban background') assignments between the measurement network and national data suppliers.