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.
Executive Summary
A substantial evidence base has developed in relation to air pollutant exposure and population level health effects. This indicates that health effects occur at air pollutant concentrations which do not exceed current air quality objectives, as is the case for most of the Scottish population. While the air quality objectives are based on health evidence and are, in many ways, proxies for exposure to air pollutants, the actual relationship between measured or estimated concentrations and individuals’ exposure to those pollutants is complex.
The objectives of this study are to review existing data on pollutant concentrations and demographic features and use this 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. In particular, the outputs will focus on the potential introduction to Scottish air quality policy of an exposure reduction approach for PM2.5 and/or NO2 and potential implications of this approach for different demographic groups. A key focus of the study is to review whether the currently available information base could robustly be used to assess population exposure to PM10, PM2.5 and NO2 in Scotland now and in the future, where any gaps in evidence are and how they could potentially be filled, being structured around six research questions.
Key findings indicate that while a substantial evidence base exists, including monitoring networks, modelling, emissions inventories, and demographic data, its application for comprehensive population exposure assessment and inequality tracking has limitations. The current monitoring network, primarily designed for compliance with air quality objectives, overrepresents urban areas and, in particular, roadside locations compared to population distribution. Modelling offers comprehensive spatial coverage and predictive capabilities but requires consistent protocols for comparability. The main messages of the report are summarised below:
- For assessing compliance with a potential exposure reduction target, monitoring is the recommended approach, being 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 also 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 PM2.5’s relative spatial homogeneity and the 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.
Recommendations are provided which aim to improve the robustness of the evidence base for population exposure in Scotland, ensuring a more accurate understanding of limitations of different approaches, and ensuring that inequalities can be taken account of, which should facilitate effective interventions to protect public health.