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.

The index bands

The index bands table shows the system of coloured bands and ranges that was developed to demonstrate how diffusion tube measurements (latest and historic) are interpreted. 

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

 

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

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

Considerations

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

  • the latest World Health Organisation Recommended Guidelines, (published in November 2021) which establish a much more stringent set of concentrations and interim targets for NO2
  • the most recent Local Air Quality Management (LAQM) Technical Guidance, that Local Authorities are required to follow (LAQM TG 22)
  • the UK’s current air quality limits and objectives for NO2
  • the World Health Organisation (WHO) recommends that annual mean concentrations of NO2 should not exceed the value of 10 ug/m3