Exceedance of air quality standards in urban areas

Assessment versions

Published (reviewed and quality assured)

• No published assessments

Rationale

Justification for indicator selection

High population densities in urban areas and related economic activities result in increased emissions of air pollutants, which in turn lead to higher ambient concentrations of these pollutants and greater exposure to them. Towns and cities (urban areas) across the European Union (EU) provide a home to more than 70 % of the population of the EU-28 (Eurostat, 2014b).

The latest World Health Organization (WHO) review of the health effects of air pollution (WHO, 2013) concluded that there was new scientific information on the adverse health effects of particulate matter (PM), ozone (O₃) and nitrogen dioxide (NO₂) observed at levels commonly present in Europe.

A 2013 assessment by the WHO’s International Agency for Research on Cancer (IARC) (D. Loomiset al., 2013) concluded that outdoor air pollution is carcinogenic to humans, with the particulate matter component of air pollution most closely associated with an increased incidence of cancer, especially cancer of the lung. This is in addition to the role air pollution plays in the development of heart and respiratory diseases, including acute respiratory infections and chronic obstructive pulmonary diseases.

This indicator focuses on those pollutants that are more relevant in terms of their health effects and urban concentrations: PM — both PM₁₀ (particles with a diameter of 10 micrometres or less) and fine PM or PM_2.5 (particles with a diameter of 2.5 micrometres or less); O₃; NO₂; sulfur dioxide (SO₂); and benzo[a]pyrene (BaP).

According to different WHO studies (WHO, 2000, 2006, 2013, 2014), exposure to PM can cause or aggravate cardiovascular and lung diseases, heart attacks and arrhythmias. It can also affect the central nervous system, the reproductive system and cause cancer. Exposure to high O₃ concentrations can cause breathing problems, trigger asthma, reduce lung function and cause lung diseases. Exposure to NO₂ increases symptoms of bronchitis in asthmatic children and reduces lung function growth. SO₂ can affect the respiratory system and the functioning of the lungs, and causes irritation of the eyes. Finally, BaP is carcinogenic and is used as an indicator of the carcinogenic effect of the total polycyclic aromatic hydrocarbons (PAHs).

Scientific references

• Dana Loomis, Yann Grosse, Béatrice Lauby-Secretan, Fatiha El Ghissassi, Véronique Bouvard, Lamia Benbrahim-Tallaa, Neela Guha, Robert Baan, Heidi Mattock, Kurt Straif, on behalf of the International Agency for Research on Cancer Monograph Working Group IARC, Lyon, France, The carcinogenicity of outdoor air pollution. The Lancet Oncology, Volume 14, Issue 13, Pages 1262 - 1263, December 2013.
• EC, 2006 - Development of a methodology to assess the population exposed to high levels of noise and air pollution close to major transport infrastructure, prepared by Entec UK Limited (Appendix D).
• EC, 2007 - EUROPE IN FIGURES — Eurostat yearbook 2006-07.
• EU, 1997 - Council Decision 97/101/EC on the exchange of information and data on ambient air quality, replaced by (EU, 2011).
• EU, 2004 - Directive 2004/107/EC of the European Parliament and of the Council of 15 December 2004 relating to arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbons in ambient air.
• EU, 2008 - Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe.
• EU, 2011 -  Commission Implementing Decision 2011/850/EU laying down rules for Directive 2004/107/EC and 2008/50/EC of the European Parliament and of the Council as regards the reciprocal exchange of information and reporting on ambient air.
• EU, 2013a - Environment Action Programme to 2020 ‘Living well, within the limits of our planet’.
• EU, 2013b - Communication from the Commission to the Council, the European Parliament, the European Economic and Social Committee and the Committee of the Regions - "A Clean Air Programme for Europe", COM(2013) 918 final.
• Eurostat, 2014a - Population by sex, age group and country of citizenship.
• Eurostat, 2014b - Statistics on European cities. 
• Eurostat, 2014c - Urban Audit.
• WHO, 2000 - Air Quality Guidelines for Europe. Second edition. WHO Regional Publications, European Series, No. 91. World Health Organization, Regional Office for Europe, Copenhagen, Denmark.
• WHO, 2006 -  WHO air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulphur dioxide. Global update 2005. World Health Organization, Regional Office for Europe, Copenhagen, Denmark.
• WHO, 2013 - Review of evidence on health aspects of air pollution - REVIHAAP Project, Technical Report. World Health Organization, Regional Office for Europe, Copenhagen, Denmark
• WHO, 2014 - A mbient (outdoor) air quality and health, Fact sheet N°313, Updated March 2014. World Health Organization.
• ETC/ACM, 2011 - Evaluation of current limit and target values as set in the EU Air Quality Directive, ETC/ACM Technical Paper 2011/3  

Indicator definition

This indicator shows the fraction of the EU-28 urban population that is potentially exposed to ambient air concentrations of certain pollutants (PM_2.5, PM₁₀, O₃, NO₂, SO₂ and BaP) that are in excess of the EU limit or target values (EU, 2004, 2008) set for the protection of human health, and to concentrations of these pollutants in excess of the WHO Guidelines (WHO, 2000, 2006).

It also shows the evolution of urban and suburban background levels of PM_2.5, PM₁₀, O₃ and NO₂ at EU-28 level.

The indicator is based on measurements of air pollutants as reported under the Air Quality Directives (EU, 2004, 2008) and the Decisions on the exchange of information (EU, 1997, 2011)

Units

The following units are used in this indicator:

Concentration:

• micrograms (mg) of pollutant per cubic metre for PM_2.5, PM₁₀, O₃, NO₂ and SO₂.

• Nanograms (ng) of pollutant per cubic metre for BaP.

Urban population (POP): number of inhabitants in the 'core city' and, from 2016 on, 'greater city' of the Urban Audit cities represented by the urban stations taken into account in the calculations.

Percentage of the urban population.

Policy context and targets

Context description

This indicator is relevant for current European air quality legislation related to the protection of human health in the Air Quality Directives 2004/107/EC and 2008/50/EC (EU, 2004, 2008).

It is related to the WHO Air Quality Guidelines (WHO, 2000, 2006) for protecting public health.

The Seventh EU Environment Action Programme (EU, 2013a) includes priority objectives that aim, among others, to protect, conserve and enhance the EU's natural capital, safeguard EU citizens from environment-related pressures and risks to health and well-being, and enhance the sustainability of EU cities.

The EU Clean Air Policy Package, adopted by the European Commission on 18 December 2013, proposes in the Communication 'A Clean Air Programme for Europe' (EU, 2013b) the short-term objective of achieving full compliance with existing legislation by 2020 at the latest; and the long-term objective of no exceedances of the WHO guideline levels for human health.

Targets

• A target value for PM_2.5 of 25 µg/m³ as an annual average. In force since 1 January 2010.

• A limit value for PM_2.5 of 25 µg/m³ as an annual average, with a margin of tolerance of 20 % on 11 June 2008, decreasing on the following 1 January and every 12 months thereafter by an equal annual percentage to reach 0 % by 1 January 2015.
• A limit value for PM₁₀ of 50 µg/m³ as a daily average, not to be exceeded more than 35 times in a calendar year. In force since 1 January 2005.
• An additional limit value for PM₁₀ of 40 µg/m³ as an annual average. In force since 1 January 2005.
• A long-term objective for ozone of 120 µg/m³ as a maximum daily 8-hour mean within a calendar year (not to be exceeded any day). No attainment date specified.
• A target value for ozone, equal to the long term objective, not to be exceeded more than 25 days in a calendar year, averaged over 3 years. It should have been met as of 2010 (average 2010-2012).
• A limit value of NO₂ of 200 µg/m³ as an hourly average, not to be exceeded more than 18 times in a calendar year. In force since 1 January 2010.
• An additional limit value for NO₂ of 40 µg/m³ as an annual average. In force since 1 January 2010.
• A limit value of SO₂ of 350 µg/m³ as an hourly average, not to be exceeded more than 24 times in a calendar year. In force since 1 January 2005.
• An additional limit value for SO₂ of 125 µg/m³ as a daily average, not to be exceeded more than three times in a calendar year. In force since 1 January 2005.

• A target value for benzo[a]pyrene of 1 ng/m³ as an annual average. It had to be met by 1 January 2013.

Exceedance of air quality limit/target values occurs when the concentration of air pollutants exceeds these limit/target values. When there are multiple limit values, the indicator uses the most stringent case:

• PM₁₀: the daily limit value. In this case, instead of the number of exceedances, the 90.4 percentile is used. This percentile represents, in a complete series of 365 elements, the 36th highest value. If it is below or equal to 50 µg/m³ it indicates that the PM₁₀ daily limit value has not been exceeded;
• O₃: the target value. In this case, instead of the 3-year average, only 1-year values are considered; and instead of the number of exceedances, the 93.2 percentile is used. This percentile represents, in a complete series of 365 elements, the 26th highest value. If it is above or equal to 120 µg/m³ it indicates that the long term objective has not been exceeded on more than 25 days;
• NO₂: the annual limit value;
• SO₂: the daily limit value. In this case, instead of the number of exceedances, the 99.2 percentile is used. This percentile represents, in a complete series of 365 elements, the 4th highest value. If it is below or equal to 125 µg/m³ it indicates that the SO₂ daily limit value has not been exceeded.

WHO Air Quality Guidelines

• Annual mean of PM_2.5: 10 µg/m³
• 24-hour mean of PM_2.5 (99th percentile of the annual daily series (3 days per year)): 25 µg/m³
• Annual mean of PM₁₀: 20 µg/m³
• 24-hour mean of PM₁₀ (99th percentile of the annual daily series (3 days per year)): 50 µg/m³
• Daily maximum 8-hour mean of ozone: 100 µg/m³
• Annual mean of NO₂: 40 µg/m³
• 1-hour mean of NO₂: 200 µg/m³
• 24-hour mean of SO₂: 20 µg/m³
• 10-minute mean of SO₂: 500 µg/m³
• BaP is used as an indicator of the carcinogenic risk of PAHs in ambient air. Since the WHO has not defined an air quality guideline for BaP, a reference level was estimated assuming the WHO unit risk for lung cancer for PAH mixtures and an acceptable risk of additional lifetime cancer risk of 1/100 000. This estimated reference level (also called 'indicative concentration' for the purposes of this indicator) is fixed at 0.12 ng/m³ (ETC/ACM, 2011).

Exceedance of the WHO guidelines or the reference level occurs when the concentration of air pollutants exceeds the corresponding values. For PM, the annual average is suggested to take precedence over the 24-hour average since, at low levels, there is less concern about remaining episodic excursions. For NO₂ the annual mean is taken; and for SO₂ the 24-hour mean, so these values can be compared with the corresponding limit value used in EU legislation. 

Related policy documents

• Directive 2004/107/EC
  Directive 2004/107/EC of the European Parliament and of the Council of 15 December 2004 relating to arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbons in ambient air.
• Directive 2008/50/EC, air quality
  Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe.

Methodology

Methodology for indicator calculation

Urban population exposure

Information on cities is obtained from the Urban Audit (UA) data (Eurostat, 2014c). UA data collection, maintained by Eurostat, provides information and comparable measurements on the different aspects of the quality of urban life in selected European cities. The urban population considered is the total number of people represented by any of the urban monitoring stations in the 'core city' and, from 2016, the 'greater city' of the UA cities taking part in the calculations.

Initially, stations in the EEA air-quality database are spatially joined with UA core and, from 2016, greater cities in a geographical information system in order to select those stations that fall within the boundaries of the cities included in the UA collection. The selected stations include station types classified as 'urban traffic', 'suburban traffic', 'urban background' and 'suburban background'. Stations classified as 'industrial' are influenced by other local emissions and such environments are generally not representative for residential areas. The industrial stations are therefore not selected for the indicator calculations.

According to a study for the European Commission by Entec UK Limited (EC, 2006), in Europe, on average, 5 % of the city population lives closer than 100 metres from major roads and is therefore potentially exposed to concentrations measured at traffic stations. The remaining 95 % of the city population is assumed to be exposed to urban and suburban background concentrations.

These percentages vary from country to country. To calculate them, national data on the population living closer than 100 metres from major roads have been taken from Appendix D (EC, 2006). These data have been divided by the total population figures for 2001 according to the Eurostat census (Eurostat, 2014a). For Croatia, Malta and the United Kingdom there are no data on the 2001 population in that census, so the totals in (EC, 2007) have been used. Furthermore, for Cyprus and Malta there are no data for people living close to roads in (EC, 2006), so for them, and also for Turkey, the average value of 5 % was used.

The final percentages for each country (also for the five non EU-28 EEA member countries not considered in this indicator but in the air pollution country fact-sheets) are shown in the following table  

For PM₁₀, PM_2.5, O₃, NO₂ and SO₂, only stations with at least 75 % of valid data per calendar year are used. That is, in the case of daily values, those having more than 274 valid daily values per calendar year (or 275 days in a leap year). And in the case of hourly values, having more than 6 570 valid hourly values per calendar year (or 6 588 hours in a leap year). For BaP, the minimum data time coverage accepted is 14 % (51 days), according to the data quality objectives related to indicative measurements in the Directive 2004/107/EU (EU, 2004).

For every year, each city (i) in country (j), and every pollutant, the total number of urban or suburban traffic stations (n_it) and the total number of urban or suburban background stations (n_ib) are obtained. P_tj % of the total population of the city (Pop_i) is proportionally assigned to each of the traffic stations and P_bj % of Pop_i is proportionally assigned to each of the background stations. So, every traffic station has an allocated population equal to ((P_tj / 100) * Pop_i / n_it) and every background station has an allocated population equal to ((P_bj /100) *Pop_i / n_ib).

EU LIMIT AND TARGET VALUES

Particulate matter (PM_2.5)

The annual mean concentration is calculated for each of the selected stations fulfilling the valid data criteria.

Depending on the mean concentration, each station (and its allocated population) is then classified uniquely in one of the two concentration classes (below or equal to the target value (25 µg/m³), or above it).

The percentage of the urban population allocated to these two concentration classes is calculated by dividing the population represented by the stations assigned to each concentration class by the sum of the population assigned to each station.

Particulate matter (PM₁₀)

For each selected station that fulfils the valid data criteria, the 90.4 percentile (P90.4) of the daily mean concentration series is calculated. P90.4 represents, in a complete series of 365 elements, the 36^th highest value. When P90.4 is below or equal to 50 µg/m³, it indicates that the daily limit value (DLV) would not have been exceeded on more than 35 days.

Depending on the value of P90.4, each station (and its allocated population) is then classified uniquely in one of the two concentration classes (P90.4 > 50 µg/m³, i.e. above the DLV and P90.4 ≤ 50 µg/m³, i.e. below the DLV).

The percentage of the urban population allocated to these two concentration classes is calculated by dividing the population represented by the stations assigned to each individual concentration class by the sum of the population assigned to each station.

Ozone (O₃)

For each selected station fulfilling the valid data criteria, the 93.2 percentile (P93.2) of the daily maximum 8-hourly mean concentration series is calculated. P93.2 represents, in a complete series of 365 elements, the 26^th highest value. When P93.2 is below or equal to 120 µg/m³, it indicates that the long term objective would have not been exceeded on more than 25 days.

Depending on the value of P93.2, each station (and its allocated population) is then classified uniquely in one of the two concentration classes (P93.2 > 120 µg/m³, i.e. exceedance of the long term objective on more than 25 days, and P93.2 ≤ 120 µg/m³, i.e. exceedance of the long term objective is less than or equal to 25 days).

The percentage of the urban population allocated to these two concentration classes is calculated by dividing the population represented by the stations assigned to each individual concentration class by the sum of the population assigned to each station.

Nitrogen dioxide (NO₂)

The annual mean concentration is calculated for each of the selected stations that fulfills the valid data criteria.

Depending on the annual mean concentration, each station (and its allocated population) is then classified uniquely in one of the two concentration classes (below or equal to the limit value (40 µg/m³), or above the limit value).

The percentage of the urban population allocated to these two concentration classes is calculated by dividing the population represented by the stations assigned to each concentration class by the sum of the population assigned to each station.

Benzo[a]pyrene (BaP)

The annual mean concentration is calculated for each of the selected stations fulfilling the valid data criteria.

Depending on the mean concentration, each station (and its allocated population) is then classified uniquely in one of the two concentration classes (below or equal to the target value (1.0 ng/m³), or above the target value).

The percentage of the urban population allocated to these two concentration classes is calculated by dividing the population represented by the stations assigned to each concentration class by the sum of the population assigned to each station.

Sulfur dioxide (SO₂)

For each selected station that fulfills the valid data criteria, the 99.2 percentile (P99.2) of the daily mean concentration series is calculated. P99.2 represents, in a complete series of 365 elements, the 4^th highest value. When P99.2 is below or equal to 125 µg/m³, it indicates that the daily limit value would have not been exceeded on more than three days.

Depending on the value of P99.2, each station (and its allocated population) is then classified uniquely in one of these two concentration classes (P99.2 > 125 µg/m³, i.e. above the daily limit value and P99.2 ≤ 125 µg/m³, i.e. below the daily limit value).

The percentage of the urban population allocated to these two concentration classes is calculated by dividing the population represented by the stations assigned to each individual concentration class by the sum of the population assigned to each station.

WHO GUIDELINES

Particulate matter (PM_2.5)

The annual mean concentration is calculated for each of the selected stations fulfilling the valid data criteria.

Depending on the mean concentration, each station (and its allocated population) is then classified uniquely in one of the two concentration classes (below or equal to the WHO air quality guideline (10 µg/m³), or above it).

The percentage of the urban population allocated to these two concentration classes is calculated by dividing the population represented by the stations assigned to each concentration class by the sum of the population assigned to each station.

Particulate matter (PM₁₀)

The annual mean concentration is calculated for each of the selected stations that fulfills the valid data criteria.

Depending on the annual mean concentration, each station (and its allocated population) is then classified uniquely in one of the two concentration classes (below or equal to the WHO air quality guideline (20 µg/m³), or above it).

The percentage of the urban population allocated to these two concentration classes is calculated by dividing the population represented by the stations assigned to each concentration class by the sum of the population assigned to each station.

Ozone (O₃)

For each selected station fulfilling the valid data criteria, the daily maximum 8-hourly mean concentration series is calculated. When there is one value above 100 µg/m³, it indicates that the WHO air quality guideline has been exceeded.

Each station (and its allocated population) is then classified uniquely in one of the two concentration classes (at least one daily maximum 8-hourly mean value > 100 µg/m³, i.e. above the air quality guideline, or all daily maximum 8-hourly mean values ≤ 100 µg/m³, i.e. below the air quality guideline).

The percentage of the urban population allocated to these two concentration classes is calculated by dividing the population represented by the stations assigned to each individual concentration class by the sum of the population assigned to each station.

Nitrogen dioxide (NO₂)

Since the WHO guideline is the same as the EU limit value, the procedure is the same as described above under 'EU limit and target values'.

Benzo[a]pyrene (BaP)

The annual mean concentration is calculated for each of the selected stations fulfilling the valid data criteria.

Depending on the mean concentration, each station (and its allocated population) is then classified uniquely in one of the two concentration classes (below or equal to the indicative concentration or reference level (0.12 ng/m³), or above it).

The percentage of the urban population allocated to these two concentration classes is calculated by dividing the population represented by the stations assigned to each concentration class by the sum of the population assigned to each station.

Sulfur dioxide (SO₂)

For each selected station that fulfills the valid data criteria, the daily concentration series is calculated. When there is one value above 20 µg/m³, it indicates that the WHO air quality guideline has been exceeded.

Each station (and its allocated population) is then classified uniquely in one of the two concentration classes (at least one daily value > 20 µg/m³, i.e. above the air quality guideline, or all daily values ≤ 20 µg/m³, i.e. below the air quality guideline).

The percentage of the urban population allocated to both concentration classes is calculated by dividing the population represented by the stations assigned to each individual concentration class by the sum of the population assigned to each station.

Urban background concentrations

Stations in the EEA air quality database are spatially joined with the Urban Audit core and, from 2016, greater cities in a geographical information system in order to select those stations that fall within the boundaries of the cities included in the Urban Audit collection. The selected stations include station types classified as 'urban background' and 'suburban background'. These types of stations, not influenced by specific local emissions such as traffic or industry, are considered representative for the assessment of change overtime of urban air concentrations. Traffic and industrial stations are not used for the calculation of this part of the indicator. Only stations with at least 75 % of valid data per calendar year are used.

Particulate matter (PM_2.5)

For all the urban and suburban background stations considered, the average of the annual mean series is obtained, together with the 10^th and 90^th percentiles.

Particulate matter (PM₁₀)

For all the urban and suburban background stations considered, the average of the series of the annual P90.4 of the daily PM₁₀ concentrations is obtained, together with the 10^th and 90^th percentiles.

Ozone (O₃)

For all the urban and suburban background stations considered, the average of the series of the annual P93.2 of the maximum daily 8-hour mean O₃ concentrations is obtained, together with the 10^th and 90^th percentiles .

Nitrogen dioxide (NO₂)

For all the urban and suburban background stations considered, the average of the annual mean series is obtained, together with the 10^th and 90^th percentiles. 

Methodology for gap filling

No gap filling is applied in the air quality data in the EEA air quality database.

For countries with no information on cities and/or population in the Urban Audit data collection (Lichtenstein and Turkey), population data have been retrieved from http://www.citypopulation.de/. The gap filled mapping was done using the “station_city” attribute found in the EEA air quality database (AirBase) and compared with the city names found at http://www.citypopulation.de/. The London geometry was derived from the URAU2007 data set in the first two assessment versions.

For the countries with no data on the 2001 census in (Eurostat, 2014a), data in the publication (EC, 2007) have been used.

For countries with no data on the population living close to major roads, the European average value of 5 % has been used.

Methodology references

• ETC/ACC, 2009 - Indicators on urban air quality. A review of current methodologies. ETC/ACC Technical paper 2009/8

Data specifications

EEA data references

• Air Quality e-Reporting (AQ e-Reporting) provided by European Commission
• AirBase - The European air quality database provided by European Environment Agency (EEA)

External data references

• Gisco - Urban Audit 2012
• City population

Data sources in latest figures

Uncertainties

Methodology uncertainty

This indicator is thought to be representative of the total urban population in Europe as a whole, but only the population in the Urban Audit cities collection is covered. So, populations living in smaller towns and villages are not included.

Only measurements from urban and suburban stations that are in the Urban Audit geometries are considered in the calculations. On one hand, this may lead to an incomplete assignment of stations. On the other hand, the representativeness of each station is not taken into account, instead in every city it is assumed that all stations of the same type have the same population representativeness.  

Locally, the indicator is subject to year-to-year variations because:

• only two categories are described: 'below' or 'above' the environmental objective. A slight decrease/increase in concentrations can change a station's status, leading to a change in the category of the associated population from 'above to below' (or 'below to above') the corresponding objective. 
• there is meteorological variability. When averaging over the EU-28, this meteorologically induced variation decreases in importance, provided spatial data coverage is sufficient;
• the number of available data series (monitoring stations and/or selected cities) that will influence the total number of the monitored population;
• the geographical coverage of stations, determined also by the inclusion of new countries over the years. Variations over time might be influenced by changes in coverage.

Traffic and background stations are unevenly distributed in countries and over Europe, which can introduce some bias when estimating the population exposed to traffic in the different countries.

It has been assumed that all cities in a country have the same percentage of people living in traffic and background environments, which may lead to the underestimation of the population exposed to traffic concentrations in big and compact cities with a high population density.

The percentages of people exposed to either traffic or background concentrations are assumed to remain constant, independently of possible urban developments and changes in city population density. Also, the population in cities is assumed to remain constant.

Data sets uncertainty

The air quality data are officially submitted by national authorities. It is expected that data have been validated by the national data supplier and they should be in compliance with the data quality objectives as described in the Air Quality Directives (EU, 2004, 2008). There are different methods in use for the routine monitoring of pollutants.

Station characteristics and representativeness are, in some cases, insufficiently documented. Also, the differences in the interpretation of the station characteristics and the representativeness of the selection of urban sites might introduce differences among countries and cities, reducing comparability between cities.

For BaP, data may be influenced by the fact that cities where concentrations are below the lower assessment threshold do not need to take measurements to assess the air quality in relation to BaP, and they can use models and objective estimation for this purpose. So the calculated fraction of the population exposed to levels above the regulated thresholds may be biased because most stations are placed in locations where BaP concentrations are high.

Rationale uncertainty

It is assumed that people living closer than 100 metres from major roads are potentially exposed to traffic concentrations, without taking into account the decline in concentrations as distance to the road axis increases or the fact that people may live and work in different places.

It is assumed that the percentage of the total population living close to major roads (data in EC, 2006) is the same when considering urban population alone.

Work description

Resource needs

No resource needs have been specified

Status

Deadline

Work description

Resource needs

No resource needs have been specified

Status

Deadline