Diffusion Tube Index

Diffusion tubes are widely used by Local Authorities for the purpose of monitoring ambient nitrogen dioxide (NO2) levels, in the context of reviewing and assessing air quality in their local areas.


Diffusion tubes are particularly useful:

  • When simple, indicative techniques will suffice;
  • To give an indication of longer-term average NO2 concentrations;
  • For highlighting areas of high NO2 concentration
  • Where installation of an automatic analyser is not feasible
  • When dealing with sources such as traffic emissions, which do not change much from day to day


The table below shows the system of coloured bands and ranges that was developed by all the local authorities in Oxfordshire, to demonstrate how diffusion tube measurements (latest and historic) are interpreted in Oxfordshire.
Seven coloured bands have been developed to provide detail about air pollution levels (measured as NO2) in a simple way. These go from light green (representing very low concentrations) to purple (representing very high concentrations).


Colour Bands Range (µg/m3)
Very Low <10
Low 10 to 19
Moderate 20 to 29
Moderate 30 to 39
High 40 to 49
High 50 to 59
Very High >60


The current UK legal annual limit value for NO2 is 40 µg/m3.


The same colours of the Air Quality Index were used on the development of the Oxfordshire diffusion tube index bands, in order to simplify the interpretation and facilitate comparison between the two systems.


The range of intervals has been developed for each colour, taking into consideration:

The World Health Organisation (WHO) recommends that annual mean concentrations of NO2 should not exceed the value of 10 ug/m3


Click here for more information on: Air Pollutants, Diffusion Tube Monitoring and/or current air quality legislation.