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 1990-2018
Ambient pollutant trends in Wales 1990-2018

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 2018, there were a total of 41 automatic monitoring sites distributed across the country that were operated by Local Authorities, by the end of 2018 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 87%.

In 2018, Ambient concentrations of PM10 were “Moderate” on 35 days, “High” on 5 days and there were no “Very High” days (as defined by the Daily Air Quality Index bandings). For nitrogen dioxide, there were 23 days with “Moderate” concentrations; there was 1 day with “High” levels recorded in 2018 and no days recording “very high”. For SO2, there were no “Moderate”, “High” or “Very High” levels recorded. There were 55 days with "Moderate" ozone and no days with “High” or “Very High” ozone, as measured by the monitoring sites operated by Local Authorities. Overall, pollution levels in Wales were low for 263 days, moderate for 96 days, high for 6 days and there were no very high days. So, for 72% 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 2018 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 carbon monoxide, sulphur dioxide, benzene or lead during 2018. Four Welsh monitoring sites (Rhondda Mountain Ash, Caerphilly Hafodyrynys, Newport M4 Junction 25 and Swansea Station Court High Street) exceeded the annual mean objective of 40 µg m-3 for nitrogen dioxide. Caerphilly Hafodyrynys also exceeded the AQS Objective for hourly mean nitrogen dioxide concentration on more than the permitted 18 occasions in 2018. Eight sites in Wales exceeded the AQS Objective for ozone (100 µg m-3 as a maximum daily 8-hour mean) on more than the permitted 10 occasions. These were Aston Hill, Cardiff Centre, Cwmbran, Narbeth, Port Talbot Margam, Swansea Cwm Level Park, Swansea Morriston Roadside, Swansea St. Thomas DOAS.  These exceedances are most likely due to the prolonged hot weather in the summer of 2018.