Legionella is a naturally occurring microbe found in freshwater environments globally. When it colonizes human-made water systems, it poses a significant health risk, causing Legionnaires’ disease, a severe form of pneumonia. This illness is contracted when contaminated water droplets are inhaled, such as from showers or cooling towers. The primary factor determining whether this bacteria becomes a threat is the temperature of the water itself. Controlling water temperature is the most effective strategy for managing this risk.
The Critical Temperature Zone for Growth
Legionella bacteria multiply aggressively within a specific range of temperatures, often referred to as the danger zone. The minimum temperature required for the bacteria to begin proliferating is approximately 68°F (20°C). Below this threshold, active growth essentially stops, but above it, the environment becomes permissive for reproduction.
The most rapid and significant multiplication occurs within the optimal temperature range of 95°F to 115°F (35°C to 46°C). This temperature bracket is particularly concerning because it aligns perfectly with the operating temperatures of many common water features and appliances. Systems like hot tubs, decorative fountains, and certain industrial cooling towers frequently operate in this warm, nutrient-rich environment.
This temperature zone allows the bacteria to reproduce exponentially, dramatically increasing their concentration within the water system. Once the water temperature exceeds 115°F (46°C), the rate of growth begins to slow down. However, the bacteria remain highly active up to about 122°F (50°C), making any water system operating in the lukewarm range a potential source for outbreak.
Survival in Cold Water Systems
When water temperatures drop below the 68°F (20°C) minimum, Legionella enters a dormant or non-replicating state. The bacteria remain viable and capable of survival for long periods in this condition. Cold water systems, such as storage tanks and distribution lines, often operate below this threshold, suppressing bacterial growth.
Cold temperatures only inhibit multiplication; they do not kill the existing population. If the temperature of the cold water supply rises above 68°F (20°C)—perhaps due to poor insulation or proximity to hot pipes—the dormant bacteria can reactivate and multiply. Stagnant water in infrequently used pipes is susceptible to temperature fluctuations and can serve as a reservoir for contamination.
Thermal Disinfection Temperatures
To eliminate Legionella from a water system, temperatures must be raised significantly higher than the growth zone, a process known as thermal disinfection. The method’s effectiveness depends on both the water temperature and the duration of exposure. While growth stops above 122°F (50°C), a sustained temperature of at least 140°F (60°C) is required to achieve a rapid kill rate.
At 140°F (60°C), the bacteria are largely eliminated within two minutes of exposure. Raising the temperature further drastically reduces the necessary exposure time; at 158°F (70°C), the bacteria are destroyed almost instantaneously upon contact. This time-temperature relationship is a fundamental principle of effective water system remediation.
The temperatures required for quick eradication introduce a public safety concern: the risk of scalding. Water at 120°F (49°C) can cause third-degree burns in five minutes, and 140°F (60°C) can cause the same injury in less than five seconds. This conflict necessitates careful engineering, such as using mixing valves at the point of use to reduce the water temperature to a safe level before it reaches the tap.
Practical Steps for Temperature Control
Managing water temperature is the primary defense against Legionella proliferation in building systems. Hot water heaters, especially in commercial settings, should store water at 140°F (60°C) or higher. This ensures the storage tank operates outside of the critical growth zone.
The hot water should then be distributed throughout the building at temperatures no lower than 122°F (50°C) to prevent bacterial colonization within the pipes. For cold water, the goal is to maintain the temperature below the 68°F (20°C) proliferation threshold. This often involves insulating cold water lines to prevent heat transfer from nearby hot pipes or warm ambient air.
Regularly flushing unused water fixtures, such as spare showers or taps, introduces newly treated, temperature-controlled water into the system. This prevents water from becoming stagnant, which can lead to warming in cold lines or cooling in hot lines, allowing bacteria to grow. Consistent monitoring of hot and cold water temperatures at various points confirms that control measures are effective against the danger zone.