Air pollutant exposure evidence review
A review of existing data on pollutant concentrations and demographic features, used to understand the strength of the evidence base on how the Scottish population is being exposed to outdoor concentrations of NO2, PM10, and PM2.5.
4 Main Messages and Recommendations
The six research questions outlined in this report aim to evaluate the existing evidence for assessing the exposure of the Scottish population to NO2, PM10, and PM2.5. The report investigates the suitability of the evidence found from an evidence review process for assessing exposure, tracking progress towards exposure reduction targets and for identifying variations in exposure among different demographic groups. The report also highlights key uncertainties and gaps in the evidence base, ultimately defining recommendations to enhance its usefulness.
Research Question 1 catalogues the existing monitoring, modelling, emissions, and population datasets in Scotland that can inform the assessment of population exposure to NO2, PM10, and PM2.5. It details the Scottish Air Quality Network's reference monitors and passive diffusion tubes, noting their distribution is not designed to map against population data. The review also covers various modelling approaches, along with emissions inventories and demographic data like the Scottish Census and Index of Multiple Deprivation. The potential use of these datasets is further evaluated in subsequent sections of the report.
Research Question 2 concludes that the current evidence base, comprising monitoring networks, modelling, and demographic data, could be used to estimate population exposure by combining pollutant concentrations with population location. However, the existing monitoring network, which largely focuses on compliance with air quality objectives, overrepresents larger urban areas and roadside locations compared to population distribution. While modelling offers comprehensive spatial coverage and predictive capabilities for future exposure scenarios, it faces challenges with regular updates and intercomparison of different models. Therefore, a combination of monitoring and modelling is recommended for assessing population exposure in Scotland, acknowledging the uncertainties inherent in each approach.
Research Question 3 examines how data sources in the evidence base could be used to track progress toward an exposure reduction target. Compliance is typically assessed through monitoring at urban background and suburban sites. While modelling can evaluate progress and assess policy success, especially regarding inequalities, it is generally not used for direct compliance due to transparency and legal robustness concerns. The recommendation is to use monitoring for compliance tracking due to its transparency and defensibility, supplemented by modelling for broader policy evaluation and impact assessment.
Research Question 4 explores the use of the evidence base to assess variations in exposure for different demographic groups, such as those in more deprived communities. The evidence base available is sufficient for analysing exposure variations by deprivation, urban-rural classification, and ethnicity. The evidence base does not currently support exposure assessment that accounts for time-activity patterns, workplace mobility or indoor environments, all of which are potentially significant in assessing how exposure varies across population groups.
Research Question 5 addresses key uncertainties and gaps in the evidence base for assessing air pollutant exposure. It highlights that measurement uncertainty increases as pollutant concentrations decrease, particularly for PM10 and PM2.5. Model uncertainty also exists due to varying emissions estimates and the complexity of secondary and transboundary PM2.5 fractions. A significant gap is the lack of systematic indoor air quality data for Scotland, making it difficult to characterise the relationship between ambient concentrations and actual personal exposure. Other gaps include limited personal exposure evidence and a lack of routine source apportionment for Scottish monitoring data.
The main messages of the report are summarised below:
- For assessing compliance with a potential exposure reduction target, monitoring is the recommended approach. This is because monitoring is a more straightforward and defensible method, easily understandable by the public, and is already established in legislation.
- To assess population exposure to air pollutants in Scotland, a combination of monitoring and modelling should be used. This approach is more robust as it covers the shortcomings of each individual method.
- There is a need for a standardised approach for assessing inequalities in pollutant exposure across different demographic groups and deprivation levels. This would potentially help local authorities integrate inequality reduction into their air quality action plans and strategies.
- There are identifiable gaps in the evidence base, such as the lack of source apportionment. These gaps should be filled, in order to help to identify the most effective measures for reducing pollutant exposure.
- While global evidence on air pollution and health is robust, specific evidence for Scotland is limited and inconclusive. Further research is needed to establish clearer links between exposure and health effects in the Scottish context.
- As efforts to represent "real" exposure become more accurate, the methods tend to become more complex and less transparent. This highlights the distinction between tracking exposure scientifically and assessing compliance with a legally defined target.
- For PM2.5, local deprivation is less closely linked to variation in ambient concentrations compared to NO2, due to the relative spatial homogeneity and dominance of secondary and transboundary contributions. Detecting and quantifying these differentials in Scotland, where concentrations are generally low, requires high-resolution data and careful treatment of uncertainty.
The recommendations are therefore:
1. The Scottish Government should use monitoring for assessing compliance with a (potential) Exposure Reduction target.
2. For assessing population exposure to air pollutants in Scotland, a combination of monitoring and modelling should be used.
3. Where modelling is used for assessment of population exposure, Scottish Government should set out a protocol to ensure consistency in approach.
4. The Scottish Government should consider outlining a standardised method for assessing inequalities in relation to different demographic groups and deprivation.
5. Scottish Government should consider sponsoring research around the relationship between ambient measurements and actual exposure.
6. Routine source apportionment should be undertaken at monitoring sites, particularly those potentially used for exposure assessment (including those assessing compliance).