NO2 diffusion tube sites in Wales 2016

Analysis of nitrogen dioxide exceedances across Wales using diffusion tube (2016) data

Nitrogen dioxide (NO2) is a recognised pollutant which can cause many respiratory and cardiovascular health problems. As such, UK and EU legislation requires nitrogen dioxide (NO2) emissions to remain below an annual average of 40 micrograms per meter cubed of air (ug/m3) (the limit value), and not to exceed an hourly average of 200ug/m3 more than 18 times per year.

Local governments across the UK use continuous automatic analysers and diffusion tubes to monitor air quality in their boundaries.

Automatic analysers monitor the gases in the air and can generally provide a 15 minute average value. Automatic monitors can generally analyse a range of gases such as nitrogen dioxide and ozone, along with particulate matter (PM10 and PM 2.5). They are costly and also need to be connected to a power source. Automatic analysers are considered to be more accurate than diffusion tubes.

Diffusion tubes are small plastic tubes, around 7.5 cm in length with a diameter of 1 cm, and are designed to be physically placed on a poles using a plastic holder and ties. They are generally collected and replaced monthly, then sent to a lab for analysis, providing an average nitrogen dioxide (NO2) value for that month (and can be averaged over the year). Diffusion tubes are useful as they are low in cost and can be easily placed. However, for the results to be of value they need to be in place for many months.

Professor Paul Lewis is the director of Swansea University’s Centre for Health and Environmental Management Research and Innovation (CHEMRI). The Centre is involved in the analysis of urban air quality and the effects on human health and wellbeing.

As part of an ongoing study, Sarah Atkinson-Jones, a research officer in CHEMRI, has mapped the nitrogen dioxide annual average of over 800 diffusion tubes across Wales for 2016. Of the total 835 diffusion tubes mapped, 81 exceeded the limit value of 40ug/m3, as can be seen as red points on the map. The data suggests that NO2 exceedances are widespread across urban areas and not just limited to the M4 and major trunk roads.

The data contrasts with predictions by DEFRA modelling under the Ambient Air Quality Directive (2008/50/EC) and the Air Quality Standards (Wales) Regulations 2010 that suggested only 7 locations (motorway and trunks) in Wales are currently being exceeded for NO2.


AEA Energy and Environment. (2008). Diffusion Tubes for Ambient NO2 Monitoring: Practical Guidance for Laboratories and Users. Environment, (2), 47.

EU. (2008). Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe. Official Journal of the European Communities, 152, 1–43.

WHO. (2003). Health Aspects of Air Pollution with Particulate Matter , Ozone and Nitrogen Dioxide. Report on a WHO Working Group Bonn, Germany 13–15 January 2003, (January), 98.


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