Legionella doesn’t grow (multiply) substantially in cold water. The articles listed below were published many years ago and demonstrated that higher temperatures are required for Legionella to grow and multiply.

  1. 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. 
  2.  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. 
  3. 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.

The article by Wadowsky et al demonstrated that growth of Legionella did not occur at 77°F (25°C).  Growth was only observed at 89.6°F (32°C) and 107.6° (42°C).  Similarly, The article by Rogers et al showed that Legionella did not multiply significantly at temperatures below 77°F (25°C).  Increased growth only occurred as temperatures rose above 100° F (40° C).

Should we worry about Legionella in cold water? 
The article by Mauchline suggests that Legionella is like many other bacteria and the disease-causing ability (virulence) is regulated by temperature.  Virulence is significantly decreased at lower temperatures.It is well accepted that Legionella enters our buildings via the cold source water—usually in numbers too low to detect. We want to address significant risk where the risk has been scientifically demonstrated and corrective action is evidence-based. We do not want to set into motion expensive treatment of cold water if that treatment will not substantially mitigate the risk for Legionnaires’ disease.  Chemicals in our water bring with them exposure to disinfection by-products. This is true for chlorine as well as for other disinfectants.

The regulatory requirement in most states is to have a measurable residual oxidant level in the cold water that arrives at your building usually around 0.2 mg/L. Because Legionella are relatively resistant to chlorine and survive the water treatment process, it is widely accepted that Legionella exist in municipal water supply at levels too low to detect. Furthermore, if Legionella are in amoebae cysts they can survive exposure up to 50 ppm of chlorine. In the presence of warmer temperatures in the hot water system, chlorine essentially disappears due to the kinetics of chlorine decay.  Even if when residual chlorine is higher than 0.5 mg/L in the cold water, the residual in the hot water will approach zero.

Treating cold water to control Legionella doesn’t address the primary breeding ground for Legionella—hot water systems. Moreover, adding chemical biocides like chlorine to the cold drinking water may require monitoring for their by-products. This monitoring checks for by-products which are known carcinogens such as haloacetic acid (HAA)  and trihalomethane (THM) of chlorine.

Before you worry about Legionella in cold water here are some facts you should know that will save you time and money:  

  • Chlorine is not the “magic bullet.” Although at a high enough concentration chlorine can kill Legionella, this high level comes with a cost.  The Wadsworth VA in Los Angeles was one of the first hospitals to have an outbreak of Legionnaires’ disease and use continuous hyper chlorination for Legionella control.  A few years later the chlorination systems were removed after spending millions of dollars replacing pipes due to extensive corrosion.  
  • Avoid expensive engineering fixes. One such engineering solution that has been suggested is to keep cold water cold by installing a dedicated chilled water system. If the region of the country has higher source water temperatures, the disinfectant in the source water may be low. Address this issue during the risk assessment at each facility and determine through Legionella testing whether supplemental disinfection is needed. Don’t broadly apply such “fixes” or implement disinfection requirements that in most instances would not be needed to reduce Legionella risk but would be very expensive.