Air pollution is monitored by the London boroughs within their own area. There are a number of different monitoring methods available, including passive diffusion tubes. These diffusion tubes are small, passive samplers which measure the concentration of nitrogen dioxide (NO2) in the air at a certain location, providing a monthly average measurement. The diffusion tubes supplement the existing network of more accurate reference analysers across London.
London boroughs collate and report their diffusion tube monitoring data to the GLA and individually publish the results on their own websites. The concentrations measured are reflective of the sites selected by the boroughs and therefore comparing levels between boroughs is not an equal or fair comparison. This page is updated annually.
Note that the 2022 monitoring data covers a period prior to the London-wide Ultra Low Emission Zone (ULEZ) expansion in August 2023. Reductions in NO2 concentrations have been seen as a result of the Inner London ULEZ expansion; for more information, see the One Year Report.
For technical guidance on the use of diffusion tubes, refer to the London Local Air Quality Management (LLAQM) Technical Guidance document.
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