FOI request detail

Air refresh

Request ID: FOI-0219-2122
Date published: 01 June 2021

You asked

I have seen a poster on the TfL network saying that “air is refreshed on our tubes and trains is refreshed every three minutes” and “thanks to ventilation systems and the frequent opening of doors at stations, air on our tubes and trains is refreshed every three minutes so that you can travel safely” Please provide any evidence that this helps to travel safely. Please provide correspondence relating to the origin of this advert. Is any of this air clean? is it just the same air moved around from one place to the other? What filtration devices are in use and the evidence about them.

We answered

Our ref: FOI-0219-2122/GH

Thank you for your request received by Transport for London (TfL) on 1 May 2021 asking for information about air refresh on the Tube.

Your request has been considered under the requirements of the Environmental Information Regulations and our information access policy. I can confirm that we do hold the information you require.

Please provide any evidence that this helps to travel safely.
Advice from the scientific advisory group for emergencies (SAGE) environmental and modelling group is that COVID 19 spreads through respiratory droplets, smaller aerosols and surface contact routes with aerosol spread able to be mitigated by adequate ventilation which helps to dilute potential covid-19 aerosols. However, ventilation needs to be applied as part of a hierarchy of controls (Face, hands, space, ventilation), alongside the use of face coverings, social distancing, sanitisation of common contact surfaces and hand hygiene to interrupt the known transmission mechanisms. We have carried out an engineering assessment on the ventilation provision on our Tube trains to ensure we achieve the minimum ventilation rates recommended by SAGE/rail industry bodies (RSSB (Rail Safety and Standards Board) recommends 6-12 air changes per hour [ACH]). The air inside a Tube carriage changes more than 20 times every hour, more than the 6-12 times per hour recommended. A copy of the assessment is attached. This is further supplemented by our regular cleaning of common contact surfaces using chemical anti-viral coating cleaners and the requirement for customers to wear face coverings when on the TfL network (with the exception of those with medical exemptions). 

Please provide correspondence relating to the origin of this advert.
Please see the attached emails. The advert originated when Rail Delivery Group colleagues shared with us their advertising featuring ventilation which aimed to reassure customers of the ventilation systems in place across rail services. It was agreed within TfL that this message would also be useful for our customers. The detail of the message was then shaped with our Safety, Health and Environment team based on the ventilation systems in place across TfL services. Finally, please find a link here to Government guidance stressing the importance of good ventilation:

Please note that in accordance with TfL’s obligations under Data Protection legislation some personal data has been removed, as required by section 40(2) of the FOI Act. This is because disclosure of this personal data would be a breach of the legislation, specifically the first principle which requires all processing of personal data to be fair and lawful. It would not be fair to disclose this personal information when the individuals have no expectation it would be disclosed and TfL has not satisfied one of the conditions which would make the processing ‘fair’.

Is any of this air clean?
What filtration devices are in use and the evidence about them.
is it just the same air moved around from one place to the other?
The ventilation of the London Underground public area and tunnel network is driven primarily by train induced airflow, generated by trains entering and leaving tunnels pushing and pulling air in and out of the tunnels similar to the way a piston works in a cylinder. This is supplemented in the deep tube tunnels by mid tunnel ventilation fans (mechanical ventilation) and natural ventilation shafts which typically extract air. In both cases, this air movement draws ‘fresh air’ from the outside, in through the station and tunnel entrances which then circulates around the tunnel network.

We have coarse grade G3 filters fitted on our LU trains, although these are used to reduce the level of tunnel dust in the carriage and do not provide mitigation against Covid spread.  Based on the SAGE guidance, the principle to note is that provision of adequate ventilation helps by diluting potential Covid-19 aerosols to such a level that transmission is a low risk. 

If this is not the information you are looking for, or if you are unable to access it for some reason, please do not hesitate to contact me.

If you are considering submitting a further FOI request please think carefully about whether the request is essential at this current time, as answering FOI requests will require the use of limited resources and the attention of staff who could be supporting other essential activity. Where requests are made, please note that our response time may be impacted by the current situation.

If you are not satisfied with this response please see the attached information sheet for details of your right to appeal.

Yours sincerely

Graham Hurt
FOI Case Officer
FOI Case Management Team
General Counsel
Transport for London


Back to top

Want to make a request?

We'll email you the response within 20 working days.

We'll publish the response online without disclosing any personal information.