Have the Covid-19 lock-downs inadvertently created water safety concerns that has the potential to put users of a hotels water systems at significant risk?

It is a widely accepted “fact” that water stagnation in buildings leads to Legionella growth. This has been amplified considerably following the COVID-19 building shutdowns worldwide, with calls for some intensive actions to be undertaken by building managers (flushing protocols) being translated to warnings of potentially severe health risks.

Optimizing hydraulic design and thus maintaining target controls within building networks is clearly beneficial to controlling microbial water quality.

A recent study carried out by a UKAS accredited laboratory looked at a series of hotels for whom sampling data was available before and after lockdown. Some of the hotels operated a temperature control regime and some use chlorine dioxide as an alternative to temperature.

The results confirmed a marked increase in the numbers of samples testing positive for Legionella before and after lockdown, even in systems where Legionella colonisation was not easily detectable before March 2020. For some systems two-thirds of samples taken were positive for Legionella in post-lockdown sampling.

The data shows that even in premises considered under control prior to the pandemic, Legionella become much more commonly detected bacteria in samples taken post-lockdown. Data also suggests that chlorine dioxide treated systems have much lower levels of Legionella than systems treated with temperature.

Chlorine dioxide is an oxidising biocide/disinfectant that when used correctly, has been shown to be effective at controlling both legionella and biofilm growth in hot and cold water systems. In the appropriate application, it may be used to aid legionella control where maintaining a conventional temperature regime is difficult or where the removal of all dead legs and little used outlets is impractical.

The evidence suggests that the protective effect of using an appropriate biocide, provides greater inhibition of Legionella growth in these systems, thus reducing risk in these circumstances. It is also clear that sites with historic positive results show significant increases in positive results following lockdown.

Table 1: Proportion of samples testing positive for Legionella before and after lockdown

Hotel Code ClO2 dosed? % Positive before lockdown (actual number) % Positive after lockdown (actual number)
D07401​ No​ 14.63 (41)​ 66.67 (15)​
G07401​ No​ 9.09 (22)​ 53.33 (15)​
M05610​ No​ 0 (31)​ 21.85 (119)​
M05609​ No​ 0 (39)​ 11.25 (160)​
P03201​ No​ 0 (38)​ 9.52 (42)​
S22601​ No​ 0 (24)​ 5.56 (18)​
W08801​ No​ 0 (32)​ 0 (37)​
M05608​ No​ 0 (15)​ 0 (12)​
S05302​ Yes​ 11.63 (43)​ 64.71 (51)​
S20201​ Yes​ 0 (58)​ 2.5 (80)​
A12501​ Yes​ 0 (44)​ 0 (58)​
T058​ Yes​ 0 (41)​ 0 (51)​
S00901​ Yes​ 0 (40)​ 0 (40)​
N018​ Yes​ 0 (23)​ 0 (23)​






Some people are more vulnerable than others to COVID-19. Generally, these infections can cause more severe symptoms in people with weakened immune systems, older people, and those with long-term conditions like diabetes, cancer and chronic lung disease – not unlike the susceptible groups for Legionnaires’ disease.Legionnaires’ disease is a type of pneumonia caused by the bacteria Legionella pneumophila and other Legionella species. The illness usually starts with flu-like symptoms including fever, tiredness, headache, and muscle pains. This is followed by a dry cough and breathing difficulties which may progress to a severe pneumonia. The disease is spread through the air from a water source. People become infected when they breathe in aerosols (tiny droplets of water) which have been contaminated with Legionella bacteria.

Many of the environmental factors that encourage Legionella growth also allow for growth of other are germs that grow well in drinking water distribution systems, such as Pseudomonas and nontuberculous mycobacteria.

There are no vaccines that can prevent Legionnaires’ disease. Instead, the key to preventing Legionnaires’ disease is to make sure that building owners and managers maintain building water systems in order to reduce the risk of Legionella growth and spread.

Any water system that has the right environmental conditions could potentially be a source for legionella bacteria growth.

The Health and Safety at Work Act still applies even in the current situation and as such an employer or those in control of premises must continue to manage any risk arising from their activity and this includes legionella control.


Recommissioning your water systems

It is essential that when buildings reopen following the lifting of COVID-19 restrictions, that any water system is not simply put straight back into use.   During the period of shutdown, it would be sensible to formulate a recommissioning plan for each water system to allow safe start-up and assurance to users that it is safe. 

Any plan for recommissioning buildings must consider the safety of the operatives carrying out the work.  It is foreseeable that the hazard present within water systems in this situation would be greater than normally expected.  Reasonably practicable measures such as limiting aerosol, minimising exposure and use of RPE should be considered.

Evaporative cooling systems should already have robust start-up and shut-down procedures in place and the expectation is that these will be followed.

The minimum expectation for small, simple hot and cold-water systems would be flushing through with fresh mains water.  Larger buildings, those with tanks, showers, calorifiers and more complex pipework the expectation is likely to be for more extensive flushing followed by cleaning and disinfection.

During flushing all valves should be operated in the fully open position so that any particulate matter can be flushed through. Of particular importance are float-operated or other restrictive valves which need to be manually opened to ensure clearing of particulates and prevent fouling of the valves. Where a clearing velocity cannot be achieved, consideration should be given to removal of valves to enable an effective flush.

Where cleaning and disinfection is carried out, it is very important to monitor the decrease in disinfectant level over the course of the contact time.  Loss of more than 40% disinfectant concentration could indicate influence of biofilm.  See BSI PD855468 for more guidance.

Where buildings have been empty for some time and during warm weather, it is likely that some increase in bacteria levels and biofilm will occur.  These water systems may require more than a simple disinfection at 50ppm of chlorine for an hour to be successful.  Be prepared for the need to repeat some disinfections to achieve success.

In all cases where systems are being recommissioned it is sensible to have evidence to prove/reassure that the recommissioning process has been effective.  Sampling to BS7592 should be considered for recommissioning plans to validate the effectiveness of the process.  As per HSG274 part 2, samples should be taken 2-7 days following recommissioning and not on the day of disinfection.  Follow up samples may need to be considered as part of the recommissioning plan.


A typical recommissioning plan for a domestic water system

  1. Review your Legionella risk assessment and update where necessary.
  2. Carry out a system disinfection flushing through to all outlets to achieve 50 ppm free chlorine or equivalent biocide for at least an hour checking that this level is achieved at the furthest outlets, top up when required.
  3. Flush out and refill the system to achieve maximum normal operating target levels of disinfection (equivalent to at least 0.2 ppm free chlorine).
  4. Refill and reheat the calorifier to 60 °C. and when the calorifier/ storage water has been heated to 60 °C throughout, open the valves and flush through all outlets taking care to avoid any scalding risk.
  5. Monitor temperatures and biocide levels (where applicable), adjust where necessary, for at least 48 hours
  6. Collect Legionella samples. A site-specific sampling plan should be devised and should include stored (cold water storage tanks, calorifiers), sentinel and representative hot & cold-water locations to confirm the efficacy of the disinfection.  Locations should also include areas where there is doubt about the efficacy of the control regime or it is known that recommended temperatures, disinfectant concentrations, or other precautions are not being consistently achieved throughout the system.
  7. Continue to monitor systems and flush systems until laboratory results are obtained. If legionella results are unsatisfactory then you may need to repeat the process sometimes using an alternative product.
  8. Once satisfied that the hot and cold-water systems are under control then reopen the building.
  9. Ensure you keep all documentation for inspection: including the review and update of risk assessments (these can be annotated by hand) including monitoring data etc., with evidence of who carried out the monitoring, add time date and signature.
  10. Continue to maintain and manage your water system to prevent the risk of legionella and comply with the HSE’s ACoP L8 & HSG274 guidelines.

The duty holder should consider well in advance the time it will take to undertake and complete any recommissioning plan. Be mindful that legionella results will take around 14 days and that the process may need to be repeated. A building should remain closed until you are confident that your control measures have been effective.


Testing for Legionella

The HSE state within the HSG274 pt2 legionella testing “should be carried out where there is doubt about the efficacy of the control regime or it is known that recommended temperatures, disinfectant concentrations or other precautions are not being consistently achieved throughout the system.”

In both hot and cold-water systems, samples should be taken:

  • If considered necessary by the risk assessment;
  • from areas where the target control parameters are not met (i.e., where disinfectant levels are low or where temperatures are below 50°C (55°C in healthcare premises) for HWS or exceed 20°C for cold water systems);
  • from areas subject to low usage, stagnation, excess storage capacity, dead legs, excessive heat loss, cross-flow from water system or other anomalies.

In cold water systems, samples should also be taken as required:

  • from the point of entry (or nearest outlet) if the water is supplied from a private water supply or where the temperature of the incoming mains supply is above 20°C from the cold water storage tank or tanks;
  • from the furthest and nearest outlet on each branch of the system (far and near sentinel outlets).

In hot water systems, samples should be taken as required:

  • from the calorifier hot water outlet and from the base of the calorifier, if it is safe to do so, as some systems are under considerable pressure;
  • from the furthest and nearest outlet on each branch of a single pipe system (far and near sentinel outlets);
  • from the furthest and nearest outlet on each loop of a circulating system (far and near sentinel outlets).


Further information is available in the L8 Approved Code of Practice and the associated technical guidance:


Legionella Sampling Data Courtesy of Accepta Ltd

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