Reducing the risk of Legionnaires Disease after lockdown

As building occupancy levels decreased in response to lockdown measures, the risk of legionella bacteria proliferation within water systems is likely to have increased. Whilst these should have been maintained and normal usage simulated as much as reasonably practicable throughout the lockdown, the risk of Legionnaires Disease should be considered prior to re-occupation of the buildings. 


Next steps

Before occupants start to return, occupiers must review their current legionella risk assessment. If this has passed its scheduled review date or has not been completed previously, then one should be completed as soon as possible.

Cooling Towers

  • Cooling towers should already have strict shut-down and start-up procedures listed within the building’s water logbook. These should be followed if cooling towers have been shut down.
  • If towers have remained operational and a good monitoring regime has been maintained (e.g. biocide levels in range, no elevated dip slides, TVC or legionella results) then it is likely no further action is required.
  • In the unlikely case that cooling towers have not been maintained during this time and have remained on, then immediately shut them down and contact a water treatment specialist.

Domestic Water Services

A building is likely to be safe if, during lockdown, the following maintenance occurred:

  • Weekly or more frequent flushing of all outlets
  • Good water storage tank conditions and turnover levels maintained e.g. daily turnover
  • If temperatures have remained compliant with HSG274 Part 2 with hot water being supplied above 50°C to outlets and cold water supplied below 20°C or within 2-3°C of incoming mains.
  • If calorifier temperatures have been maintained at or above 60°C and the return maintained above 50°C.
  • If water treatment systems (where applicable) have been operating correctly and chemical residuals have been recorded within parameters.

If a building’s water services have not been maintained during lockdown, or the building has legionella positives either pre or during lockdown, then extra precautions should be considered:

  • A clean and chlorination of the domestic water services including all cold water storage tanks
  • Pasteurising hot water systems to 70°C and drawing water through to all outlets (refer to HSG274 Part 2 for Thermal Disinfection procedures)
  • Any shower and spray outlets should be cleaned and disinfected
  • Consider point of use micron filters on showers.

Additional steps

Repeat chemical disinfections may be required in buildings with water services that have not been maintained during lockdown as biofilms within the water services may have developed which can shield bacteria from the disinfection chemicals.

Microbiological sampling should be undertaken to identify if there is still a risk associated with the building’s water systems and to prove the effectiveness of actions taken during lockdown. Samples should be taken at least 2-7days after any chemical or thermal disinfection. Reassurance samples should also be taken 2-4 weeks later to ensure that the controls put in place to reopen have been effective. Occupancy levels are likely to fluctuate in offices for several months so ensuring a thorough legionella prevention regime is in place is a priority.

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Get in touch


David Coulston

Technical Director - Legionella T: +44 (0) 20 3691 0500 Email
London - Farringdon Street | UK

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