Ambient pollutant trends

The Welsh Government and the Welsh Air Quality Forum (WAQF) work closely with air quality experts and the Department for Environment, Food and Rural Affairs (Defra) to monitor and reduce air pollution in Wales. The figure below illustrates the long-term trends for nitrogen dioxide, particulate matter up to 10 µm in size (PM10) and ozone concentrations in Wales. Apart from ozone this shows a steady improvement in pollutant concentrations since the 1990s. As ozone is a regional pollutant that is transboundary in nature, it is outside the direct control of the Welsh Government and Local Authorities.

Ambient pollutant trends in Wales 1990-2022
Ambient pollutant trends in Wales 1990-2022

Local Authority Monitoring

Air quality monitoring in Wales is undertaken by Local Authorities and through national networks managed by the Welsh Government. There are two main types of air pollution monitoring –  automatic monitoring and passive sampling. Automatic monitoring uses continuous analysis techniques to measure and record ambient concentrations of a range of air pollutants. Passive samplers (such as diffusion tubes) contain a chemical reagent that adsorbs the pollutant from the air. Samplers are exposed for a period of time and analysed in a laboratory. At the start of 2022, there were a total of 42 automatic monitoring sites distributed across the country that were operated by Local Authorities, by the end of 2022 this fell to 39 sites.

These sites contain equipment that automatically measures carbon monoxide, nitrogen oxides, sulphur dioxide, ozone, PM10 and particulate matter up to 2.5 µm in size (PM2.5). In addition to these, there were several hundred diffusion tubes measuring monthly mean nitrogen dioxide levels. Overall, data capture for the automatic instruments operated by Local Authorities during the year was 88%.

In 2022, Ambient concentrations of PM10 were “Moderate” on 40 days, “High” on 8 days and “Very High” on 1 day (as defined by the Daily Air Quality Index bandings). For nitrogen dioxide, there was 1 day with “Moderate” concentrations; there was 3 days with “High” levels recorded and no days recording “very high”. For SO2, there were no “Moderate”, “High” or “Very High” levels recorded. There were 37 days with "Moderate" ozone, 3 days with “High” and no days recorded as “Very High”, as measured by the monitoring sites operated by Local Authorities. Overall, pollution levels in Wales were low for 284 days, moderate for 66 days, high for 14 days and there were no very high days. So, for 78% of the time, pollution levels were low across the whole of the Wales. Details of the Daily Air Quality Index banding system used to describe pollution levels for the public during 2022 can be found at

Summary of Exceedances

Exceedance statistics generated from the 'Air Quality in Wales' website show that no monitoring sites in Wales exceeded any Air Quality Strategy (AQS) Objectives (or corresponding EU limit values) for PM10, PM2.5, carbon monoxide, sulphur dioxide, benzene or lead during 2022. No Welsh monitoring sites exceeded the annual mean objective of 40 µg m-3 for nitrogen dioxide.

However, one site (Marchlyn Mawr) exceeded the AQS Objective for hourly mean NO2 concentration (200 µg m-3 for more than 18 hours) 40 times. These exceedances were caused by a diesel generator being in service close to the monitoring site for 4 days in December 2022. Eight sites in Wales exceeded the AQS Objective for O3 (100 µg m-3 as a maximum daily 8-hour mean) on more than the permitted 10 occasions. These were Aston Hill, Cardiff Centre, Narberth, Swansea Morriston Roadside, Marchlyn Mawr, Port Talbot Margam, Swansea Cwm Level Park and Cwmbran Crownbridge. These exceedances mainly occurred occurred across two prominent high-ozone periods during heatwave conditions in July and August 2022.