Inadequate Temperature & Throughput = Ideal Legionella Conditions

There is much talk about reducing in-premise hot water temperatures in order to save energy and move towards net zero goals.  However, there is little evidence to support adequate control of waterborne pathogens or suggested compensatory control measures would have a lower energy or carbon requirement.  These parameters remain critical regarding large buildings with complex water systems, and even more so if it is within healthcare buildings housing vulnerable users.

A Spanish team from Barcelona, who have previously published on the importance of consistent temperature, hydraulic balance and daily outlet use (Gavaldà et al., 2019), have recently reported an update of their Legionella risk management plans based on temperature and improved microbiological surveillance.  Their hospital building, with approximately 800 beds, has 3 towers ranging in height from 9 – 19 floors which is run by two independent hot water systems:  One system covers lower floors (“low Zone”) which are traditionally designed with typical number of tap and shower outlets, and a completely separate other hot water system covers the upper floors (“high zone”) where the number of taps and showers supplied are considerably lower. 

Their study described the persistence and variability of Legionella pneumophila colonisation in their systems over the 2012 – 2017 period.  Eighty different Legionella pneumophila isolates were assessed from this 6 year period, all were Legionella pneumophila serogroup 6 (known to be pathogenic) and comprised of 4 genotypes. The two independent systems, fed by the same influent mains water, had significantly (p < 0.001) different proportions of these genotypes, with one system predominantly type A (75%) and the other system predominantly type  B (81%).  The reasons why a species or genotype with a particular molecular pattern dominates within a water system is not well known, however it is likely that it is positively selected for by its environmental pressures and stress – such as system design, materials in contact with the water pathway, temperature,  any secondary disinfectant present and throughput/stagnancy.

Most hospitals undertake some level of regular environmental surveillance of their water systems for Legionella spp., and the plate culture method remains the most common despite availability of the more accurate and faster reporting molecular methods.  One problem with plate culture testing is that it cannot report the Viable But Not Culturable subpopulation.  Legionella,  indeed most waterborne pathogens, changes their metabolic state under environmental stress and will not grow on the culture plate. The bacteria do however retain its cell membrane and its virulence. 

The authors report that temperature achieved in the “High Zone” hot water system were 56.8 oC (range 50.0 – 62 oC) whereas in the “Low Zone” hot water system were 54.5 oC with a range of 42 – 61.5 oC.  Legionella strains found within water samples were exposed to thermic treatments at 50 oC, 55 oC and 60 oC for a 24 hour period under constant agitation, followed by both plate culture method and cytometry analysis to detect live cells.  No colonies were detected on the culture plates for any of the temperatures tested, although live VBNC bacteria were detected in all cases by cytometry as follows:

Thermic Treatment (24 hour exposure)% Legionella pneumophila serogroup 6 types found “alive”,  with intact cell membranes
Following 50 oC exposure62% in A, 57% in B and 29% in C  
Following 55 oC exposure21% in A, 26% in B and 19% in C
Following 60 oC exposure<5% of cells were found with intact membranes

When analysing thermotolerance of Legionella pneumophila serogroup 6 types found within the two independent water systems, the percentage of cells exhibiting membrane integrity were:

  • High Zone:  59% @ 50 oC; 20% @ 55 oC; 3% @ 60 oC
  • Low Zone:   52% @ 50 oC; 25% @ 55 oC; 5% @ 60 oC

The authors were particularly concerned regarding the growth of Legionella in water at temperatures of up to 58 oC in the “high zone” installation, which leads to suspicion that thermotolerant Legionella subtypes are present and persisting in the areas where higher temperatures have remained constant over the 6 year investigative time period.  Results show that live cells with intact membranes were present and could revert to an active metabolic state. With zero colonies detected on the culture plate, microbiological surveillance of hot water through conventional culturing underestimates the presence and risk from Legionella by failing to detect the VBNC forms.  Nevertheless these organisms pose a risk to patients due to their ability to revert to an infective status.

Healthcare facility water systems have significant lengths of hot water pipework at the periphery which regularly fall well below 55 oC due to the intermittent nature of outlet usage and the positioning of thermostatic mixing valves.  Often the water within outlets and their immediate peripheral supply pipework spends more than 95% of the time at ambient temperature.  So what is the solution?  Should we consistently maintain the hot water above 60 oC throughout the circulatory system and supplied to each outlet to prevent systemic amplification of Legionella?  Would this drive thermotolerance in Legionella further and how could we best monitor that?  What is the answer for the periphery and fittings which trend to ambient temperature as soon as the outlet is turned off?  Removal of outlets ensures higher usage in those that remain, but what is optimal to achieve higher temperatures at the periphery and to reduce Legionella amplification? How is it best to understand hydraulic balance and sufficient frequency of outlet use? 

What we can understand is that 60 oC currently controls the Legionella pneumophila serogroup 6 types within this Barcelona hospital water system – the  viability analysis via cytology methodology has determined that.  We can also infer that if hot water samples (> 50 oC) are returning positive plate culture results, then consistent hot water temperatures (> 50 oC ) are not being achieved.  Any level of positive result should trigger an investigation, remediation and verification cycle 

Another key Legionella paper was published last month, this one from a Las Vegas research team highlighting the critical impact of water age on Legionella pneumophila growth rates.  Water age is often used as a proxy for water quality, but less often as a direct input for assessing microbial risk within in-premise plumbing systems. Lead author, Emily Clements, highlights that reported rates of legionellosis is rising – from 1 per 100,000 US population in 2010 to 3 per 100,000 in 2018, although the true incidence has been estimated as high as 27 cases per 100,000 in 2020.  Opportunistic pathogens, such as Legionella spp, may rebound in numbers within plumbing systems due to intermittent use and long stagnation periods, temperature changes and multiple material type contact.  First flush water samples often have higher numbers of bacteria following a period of stagnation, and therefore water age is important for water quality.  Understanding how often an outlet is used and the throughput of water necessary to refresh the volume held in the periphery is difficult to calculate and assess.  Flushing is widely adopted as a strategy to reduce water age and improve water quality, however success in sufficiently reducing levels of pathogens has been variable.  In water scarce areas, and where environmental goals are set which impact volumes and frequency of flushing, it is important to understand what is effective for the individual water system.  A framework to link water age and risk with the benefit of adequate flushing would better characterise health risks and solutions

This team developed a methodology for Quantitative Microbial Risk Assessment of Legionella pneumophila within a model “home” in-premise plumbing system based around water age which was supported by “smart” purging  fixtures and randomly generated water demand scenarios.  This is a well thought out study with complexity in the scenarios covered including number of premise occupants and intermittent use impacts on sheer stress.  There are important conclusions drawn which are relevant for all types of building and user populations

  • Water-saving fixtures increase water age and therefore increases the probability of Legionella infection and illness.  This impact is worse in sinks rather than showers
  • Higher occupancy homes have a lower probability of infection with Legionella due to increased use and throughput of fixtures – however use is not “even” across all fixtures
  • Communities with immunocompromised groups should aim to reduce water age to mitigate risk
  • Smart purging devices significantly reduce the probability of infection from Legionella pneumophila
  • Water age held higher significance than aerosol generation in terms of risk of infection
  • More frequent but smaller volume purging decreased the risk more than the less frequent higher volume purging whilst also using less water overall

Water age at the periphery and at every individual outlet of the water system does not get the attention it deserves when risk assessing for waterborne pathogens. 

We hope both these papers will help Water Safety Groups think further about risks in their systems and with their user populations.  Preferentially any no and low use outlets should be removed, but where outlets are required in locations where the user population are less inclined to operate them with consistent frequency, it may be better to select an automatically “smart” purging fixture to ensure water age and temperature profiles remains within target.

Both of these papers are freely available to access, and we hope you enjoy their content and consider what improvements can be made to your water system operation, environmental surveillance plans and risk reduction in order to address hot water temperatures and throughput in your buildings.  Please contact us if you would like to discuss these papers and the risks that they highlight further.


Párraga-Niño N, Cortès-Tarragó R, Quero S, Garcia-Núñez M, Arqué E, Sabaté S, Ramirez D, Gavaldà L. Persistence of viable but nonculturable Legionella pneumophila state in hospital water systems: A hidden enemy? Sci Total Environ. 2024 Jun 1;927:172410. doi: 10.1016/j.scitotenv.2024.172410. Epub 2024 Apr 10. PMID: 38608884.

Gavaldà L, Garcia-Nuñez M, Quero S, Gutierrez-Milla C, Sabrià M. Role of hot water temperature and water system use on Legionella control in a tertiary hospital: An 8-year longitudinal study. Water Res. 2019 Feb 1;149:460-466. doi: 10.1016/j.watres.2018.11.032. Epub 2018 Nov 16. PMID: 30472548.

Clements E, Crank K, Nerenberg R, Atkinson A, Gerrity D, Hannoun D. Quantitative Microbial Risk Assessment Framework Incorporating Water Ages with Legionella pneumophila Growth Rates. Environ Sci Technol. 2024 Apr 16;58(15):6540-6551. doi: 10.1021/acs.est.4c01208. Epub 2024 Apr 4. PMID: 38574283; PMCID: PMC11025131.

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