Although TfL has undertaken deep tunnel cleans, data is not yet available to assess the impact of these on the length of time the tunnels continue to remain clean. Further work needs to be carried out to establish how long a deep tunnel clean reduces dust levels. This is likely to be dependent on a number of factors, including usage, maintenance activities and connections with other Tube lines.
Transport for London (TfL) is, therefore, recommissioning two air quality monitors, which allow for long term monitoring of dust levels. TfL will use these to better evaluate how dust levels change. This could include assessing the long-term impact of a deep tunnel clean.