1: Methodology box: how to measure air pollution?

Air pollution is difficult to measure and there is not one single way to present pollution or to assess the quality of air. While some work focuses on the pollution stemming from specific pollutants such as NO2 or particulate matter, other methods look at several different pollutants within one index.

It is important to differentiate between emissions and concentration data. Emissions data is mostly used for the identification of the source and its origin (such as transport or domestic combustion). But, as local emissions are only part of the story, it is the concentrations of pollution that give an indication of how polluted a place is. Concentration levels are measured at monitoring sites, located either nearby (roadside) or further away (background) from roads.32

To give a comprehensive, comparative picture of air quality across the UK, this chapter uses different approaches to measure air pollution. It uses data from the Met Office’s Daily Air Quality Index, which uses five different pollutants33 to give an overview of air pollution in the UK, and complements this by looking in more detail at nitrogen dioxide and particulate matter 2.5.

For nitrogen dioxide concentration, the UK is not currently meeting the legally-binding target of an annual mean of 40 micrograms per cubic metre (µg/m3). And, while legal limits for PM2.5 (set at 25 µg/m3) are not being breached, they do exceed World Health Organisation guidelines of 10 µg/m3.


  • 32 While roadside concentration helps measure traffic-related concentration levels, it is important to consider components coming from outside the local source, in order to obtain a broader picture of air pollution in an area, hence the use of background concentration. It can be defined as the remaining pollution concentration if all the local sources were removed. These two elements provide an accurate estimate of air pollution concentration in an area.
  • 33 The index is based on five pollutants: nitrogen dioxide (NO2), sulphur dioxide, ozone, and two types of particulate matters, PM10 and PM2.5.