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Legionella Risk in Potable Cold Water Systems 

Legionella, a waterborne pathogen, contaminates approximately half of all building water systems, including both potable and non-potable systems. It is typically introduced into building water systems from the municipal cold water supply. Once present, it can thrive in areas of the water system that offer ideal conditions for growth, such as warm water, sediment accumulation, stagnant water, or the presence of other microbes. Although hot water presents a greater risk, Legionella can also grow in cold water, a commonly overlooked factor. 

Legionella in hot water systems  

Potable hot water systems, particularly untreated ones, can act as major reservoirs for Legionella growth. The most suitable temperature for Legionella growth is between 90°F and 120°F, making hot water systems a high-risk environment for Legionella. 

Legionella in cold water systems   

While the conditions for growth are not as favorable in cold water systems, Legionella can survive in water temperatures below 77°F. This means potable cold water systems are less likely to harbor detectable levels of Legionella, but they are not immune to colonization. Legionella may colonize in cold water systems that provide favorable temperatures combined with low disinfection residuals or high sediment levels. Warmer regions with elevated cold water temperatures may experience cold water Legionella growth more frequently than other regions.  

Legionella species in cold water   

According to various research publications, the survival and growth of some Legionella species at low water temperatures is common, including Legionella pneumophila, Legionella anisa, Legionella longbeachae, and Legionella moriavica.    

When present in cold water, Legionella pneumophila, the species that causes more than 90% of Legionnaires’ disease cases in the United States, tends to favor higher cold water temperatures, while Legionella anisa and other non-pneumophila species are commonly found in lower cold water temperatures. Although non-pneumophila species are less commonly associated with disease, there have been documented cases of Pontiac Fever from exposure to aerosols containing Legionella anisa.   

Legionella in ice machines  

Ice machines with internal compressors can create localized environments where the internal tubing and water reservoir are warmed to Legionella growth ranges. Legionella within the ice can then be transmitted to people through aspiration as the ice is consumed. While many machines use carbon or sediment filtration and condition water to improve the taste or look of the ice, these filters do not remove Legionella or other bacteria. Supplemental disinfection of the building water system or inline microbial filters on the ice machine should be used to reduce the risk of Legionella growth in the machine.  

Controlling Legionella in cold water  

There are several options for controlling Legionella in a building’s cold water system. The optimal solution for a particular building will vary depending on several factors, such as the extent of the Legionella contamination, the vulnerability of building occupants, the location of the facility, and other factors.  

  • Copper-silver ionization: Copper-silver ionization is typically installed on the hot water system due to the higher risk of Legionella growth. It can also be installed on the cold water system and is especially beneficial in facilities with significant cold water positivity, in warmer regions with elevated municipal water supply temperatures, or in facilities with significant heat transfer between hot and cold water pipes.  
  • UV disinfection: Installing UV disinfection on the incoming cold water supply reduces Legionella levels entering the building. In most cases, UV disinfection should be coupled with a systemic treatment method, like copper-silver ionization. In these instances, UV disinfection reduces new Legionella entering and “reseeding” the water system, while the copper-silver ionization provides residual and continuous disinfection.  
  • Sediment filtrationSediment filtration installed on the incoming cold water supply can reduce the amount of sediment entering the building that can encourage bacteria growth by providing bacteria with a source of food and protection. Sediment filtration is also an effective barrier against unplanned external factors (e.g., water main breaks, construction, flooding, etc.) that impact the quality of water entering the building and reduce the effectiveness of supplemental disinfection.     

Ensuring overall water safety involves discovering and addressing the risk of Legionella in cold water systems. By taking proactive measures to manage Legionella, we can create safer and healthier environments. 


Wadowsky, R. M., R. Wolford, A. M. McNamara, and R. B. Yee. 1986. Effect of temperature, pH, and oxygen level on the multiplication of naturally occurring Legionella pneumophila in potable water. Appl. Environ. Microbiol. 49:1197-1205.   

Rogers, J., A. B. Dowsett, P. J. Dennis, J. V. Lee, and C. W. Keevil. 1994. Influence of temperature and plumbing material selection on biofilm formation and growth of Legionella pneumophila in a model potable water system containing complex microbial flora. Appl. Environ. Microbiol. 60:1585–1592.   

Mauchline WS, James BW, Fitzgeorge RB, Dennis PJ, Keevil CW. Growth temperature reversibly modulates the virulence of Legionella pneumophila. Infect Immun. 1994 Jul;62(7):2995-7.  

Soderberg, M.A., Dao, J., Starkenburg, S.R., Cianciotto, N.P. 2008. Importance of type II secretion for the survival of Legionella pneumophila in tap water and in amoebae at low temperatures. Appl. Environ. Microbiol. 74(17):5583-5588.  

De Giglio, O., D’Ambrosio, M., Spagnuolo, V., Diella, G., Fasano, F., Leone, C. M., Lopuzzo, M., Trallo, V., Calia, C., Oliva, M., Pazzani, C., Iacumin, L., Barigelli, S., Petricciuolo, M., Federici, E., Lisena, F. P., Minicucci, A. M., & Montagna, M. T. 2023. Legionella anisa or Legionella bozemanii? Traditional and molecular techniques as support in the environmental surveillance of a hospital water network. Environmental monitoring and assessment, 195(4), 496.  

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