Cleaner air for Scotland: the road to a healthier future

A strategy setting out the Scottish Government's proposals for delivering further improvements to air quality.

3. Setting the Scene

What causes air pollution?

3.1 Air quality is affected by everything we do. Industry, transport, farming and power generation release pollution into the atmosphere, along with domestic activities such as heating and cooking, and natural sources such as sea salt, wildfires, volcanic activity and soil erosion (Figure 4).

Figure 4. Sources of air pollution

Figure 4. Sources of air pollution
Source – Scotland’s Environment website [17]

3.2 When talking about air pollution we need to distinguish between:

  • direct emissions of pollutants from these sources; and
  • concentrations – the levels found in ambient air once the various emissions have been mixed and transported by atmospheric processes (Figure 5).

3.3 There is also a distinction between primary pollutants – emitted directly from different sources, and secondary pollutants – formed when primary pollutants undergo changes in the atmosphere.

Figure 5. Relationships between air quality emissions and concentrations

Figure 5. Relationships between air quality emissions and concentrations
Source – ‘Air Quality and Road Transport: Impacts and Solutions’ – Ricardo Energy & Environment report for the RAC Foundation [18]

3.4 Although there has been a strong overall downward trend in emissions, the picture for concentrations is less clear. Year-to-year variations in concentrations can be strongly influenced by:

  • meteorological conditions;
  • the chemical and physical properties of the different pollutants; and
  • local factors such as building works and road closures.

3.5 Street design and the local transport mix can also affect concentrations at street level. Figure 6 shows changes in emissions of the main pollutants compared with 1990 levels. The data are normalised, which means that emissions data for each pollutant – gathered using a variety of different methods – have been converted to a common scale for ease of comparison, where 1.0 represents the situation in 1990.

3.6 Concentrations can also vary widely across relatively small areas. This is illustrated by the example of nitrogen dioxide ( NO 2) level variation at four monitoring sites in Dundee (Figure 7).
The red lines show overall trends.

Figure 6. Emissions trends normalised for key air pollutants in Scotland since 1990. NH3 – ammonia; CO – carbon monoxide; NOx – oxides of nitrogen; NMVOC – non methane volatile organic compounds; PM10 – particulate matter; SO2 – sulphur dioxide; Pb – lead

Figure 6. Emissions trends normalised for key air pollutants in Scotland since 1990. NH 3 – ammonia; CO – carbon monoxide; NO x – oxides of nitrogen; NMVOC – non methane volatile organic compounds; PM 10 – particulate matter; SO 2 – sulphur dioxide; Pb – lead
Source – Air Quality Pollutant Inventories for England, Scotland, Wales and Northern Ireland 1990-2013 [19]

Figure 7. Trends in NO2 concentration at four monitoring sites in Dundee, 2006–2014

Figure 7. Trends in NO 2 concentration at four monitoring sites in Dundee, 2006–2014
Source – Scottish Air Quality Database Annual Report 2013 [20]

Where and why does air pollution occur?

3.7 The highest levels of air pollution are found in cities and towns, but unlike the dense smogs of the past, today’s air pollution is largely invisible. The effect of air pollution can be detected throughout urban Scotland, as illustrated by the fact that there are over 30 Scottish AQMAs currently in place. The actions being undertaken by local authorities to tackle these issues are outlined in the accompanying technical document.

3.8 Significant stationary pollutant sources come from industrial and domestic energy production, along with other combustion processes and agriculture (Table 1). However, given that the majority of Scotland’s population lives and works in urban environments, transport emissions are the most important – but clearly, not the only – source of air pollution affecting human health.

Overall Rank Sector Ammonia Carbon monoxide Nitrogen oxides Non methane volatile organic compounds PM 10 Sulphur dioxide Lead
1 Other Combustion 1.3% 38.3% 11.6% 3.8% 35.2% 7.5% 27.6%
2 Transport Sources 1.8% 30.2% 38.5% 2.4% 16.4% 2.7% 7.0%
3 Industrial Combustion 0.0% 22.6% 13.0% 1.0% 5.0% 7.5% 31.1%
4 Energy Industries 0.0% 6.5% 34.6% 0.0% 5.7% 79.4% 20.8%
5 Industrial Processes 0.2% 0.0% 0.0% 47.1% 8.7% 1.6% 10.4%
6 Agriculture 86.6% 0.0% 0.0% 9.8% 21.1% 0.0% 0.0%
7 Other 5.6% 1.7% 2.3% 1.0% 6.0% 1.4% 0.3%
8 Solvent Processes 0.0% 0.0% 0.0% 20.5% 1.8% 0.0% 0.0%
8 Fugitive 0.0% 0.6% 0.0% 14.5% 0.0% 0.0% 0.0%
10 Waste 4.5% 0.0% 0.0% 0.0% 0.0% 0.0% 2.9%
Total 100% 100% 100% 100% 100% 100% 100%

Table 1. Pollutant emissions by sector in Scotland, 2013.
Source – Air Quality Pollutant Inventories for England, Scotland, Wales and Northern Ireland 1990-2013 [21]

3.9 In relation to transport, fuel quality, emissions reduction technology and engine emission standards have all improved over the last decade or so. However, progressively tougher emissions standards are not delivering predicted real-world benefits – especially for diesel vehicles. A mismatch between regulation and real-world emissions from diesel vehicles seems to be one of the key reasons why the expected reductions in concentrations at the roadside have not materialised. This is further compounded by a growth in the proportion of diesel vehicles in the UK.

What are the health impacts of air pollution?

3.10 Fine particles of material can descend deep into the lungs where they can cause irritation and inflammation. In turn, this can worsen existing conditions, such as asthma and heart and respiratory diseases [22] . Air pollution is a significant public health issue in terms of the collective impact on society. Evidence from research is increasingly suggesting that detectable adverse health impacts occur even at, or below, the legislative limit values which were set at levels previously assumed to be protective of health. Section 5 provides more detailed information on the health impacts and benefits associated with air quality.

Legislative and policy framework: Europe and Scotland

3.11 Table 2 summarises the key legislation and policies that direct action on air quality in Scotland. Detailed information on, and further links to, policy and legislation can be found in the accompanying technical document.

Directive 2008/50/ EC on ambient air quality
and cleaner air for Europe [23]
  • Defines and fixes legal limits for air quality.
  • Sets limit values and/or alert thresholds (and/or target values for ozone) for several pollutants of concern for human and environmental health.
  • Assesses air quality according to a standardised methodology across the EU.
  • Sets out requirements for making information available to the public.
  • Compliance is legally binding on Member States.
National Emissions Ceiling Directive 2001/81/ EC [24]
  • Sets national emissions ceilings for certain pollutants.
  • Currently under review.
Industrial Emissions Directive 2010/75/ EC [25]
  • Controls emissions from industrial activities.
Clean Air Policy Package and Clean Air
Programme for Europe [26]
  • Sets out measures to ensure compliance with legislative requirements in the short term.
  • New air quality objectives to 2030.
  • Proposals to improve air quality in cities, support research and innovation, and promote international cooperation.
  • Revision of the National Emissions Ceiling Directive.
  • Proposal for a Directive on medium combustion plants.
Air Quality Strategy for England, Scotland,
Wales & Northern Ireland [27]
  • Sets out the short to medium term policy framework for air quality across the UK.
Local Air Quality Management [28]
  • Defines objectives for several pollutants of concern for human health.
  • Sets out the policy framework for improving local air quality.
  • Achievement is the responsibility of local authorities, but is not legally binding.

Table 2. Air quality legislation and policy in Europe and Scotland

Figure 8. Links between air quality and natural and physical environment

Figure 8. Links between air quality and natural and physical environment

Air quality and the environment

3.12 Besides the effects of poor air quality on human health, there can also be a significant environmental impact [29] , as shown in Figure 8.

Air quality and noise

3.13 There is also a close relationship between air pollution and environmental noise. There are several common sources (most notably road traffic in urban areas), impacts and solutions [30] . The environmental and health impacts (particularly associated with annoyance and sleep disturbance) are considered to be just behind those of air quality [31] , with external costs of noise in the EU amounting to at least 0.35% of its gross domestic product. Policies that can have a positive impact on both air quality and noise include:

  • improving local traffic flows by using intelligent transport systems (see paragraph 6.27 for further details); and
  • increased separation or buffer zones between sources and sensitive receptors.


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