Legionella bacteria are naturally occurring microorganisms found in freshwater environments. These bacteria can pose a risk to human health when they grow and spread in human-made water systems, potentially leading to Legionnaires’ disease, a severe form of pneumonia. Understanding the temperature conditions that favor or inhibit Legionella growth is important for preventing outbreaks. Controlling water temperature is a primary method for managing the presence of these bacteria in various settings.
Ideal Conditions for Growth
Legionella bacteria multiply most rapidly within a specific temperature range. The optimal temperature for their growth is between 20°C (68°F) and 45°C (113°F). Within this range, Legionella can thrive, particularly between 32°C (89.6°F) and 42°C (107.6°F), with maximum growth often observed around 37°C (98.6°F). This temperature window aligns with temperatures commonly found in various human-made water systems.
The presence of nutrients, such as rust, scale, and organic matter, further supports bacterial growth. Stagnant water also contributes to an environment where Legionella can multiply to high levels. Systems with water held within this warm range, especially with limited flow and available nutrients, create an ideal breeding ground for these bacteria.
Temperatures That Prevent Growth
Temperatures outside the optimal growth range can either inhibit Legionella multiplication or kill the bacteria. Below 20°C (68°F), Legionella becomes dormant, meaning it survives but does not multiply. While dormancy reduces the immediate risk, the bacteria are not killed and can reactivate if the water temperature rises again.
Conversely, higher temperatures are effective at killing Legionella. At 50°C (122°F) and above, Legionella begins to die. At 50°C, a significant reduction in bacteria can occur within a few hours. Raising the temperature to 55°C (131°F) can achieve a 90% kill rate in approximately 20 minutes, with complete elimination over several hours. At 60°C (140°F), most Legionella bacteria die within minutes, and at 70°C (158°F) or higher, they are killed almost instantly.
Where Legionella Thrives
Legionella can flourish in various human-made water systems where temperatures fall within their ideal growth range. Common sources include cooling towers, hot and cold water systems, including showers, sink faucets, and hot water tanks.
Other environments where Legionella can thrive due to favorable temperatures include hot tubs and whirlpools, which often maintain warm water. Decorative fountains, water features, humidifiers, and certain medical devices can also harbor the bacteria if water temperatures are suitable and conditions allow for aerosol generation.
Temperature-Based Prevention Strategies
One strategy involves maintaining hot water at temperatures that kill the bacteria. Hot water storage cylinders should keep water at 60°C (140°F) or higher. Hot water should be distributed so that it reaches outlets at 50°C (122°F) or higher within one minute.
Equally important is keeping cold water cold, ideally below 20°C (68°F). This ensures that Legionella remains in a dormant state. Regular flushing of infrequently used outlets, such as showers and taps, helps prevent water stagnation and maintain appropriate temperatures. Proper maintenance, including cleaning and descaling of system components like showerheads, also helps prevent Legionella growth.