The bacteria Legionella naturally exists in freshwater sources, but it becomes a health concern when it multiplies within human-made water systems. This organism is the cause of Legionnaires’ disease, which is a severe form of pneumonia that can be fatal, as well as a milder, flu-like illness called Pontiac fever. The disease is contracted by inhaling aerosolized water droplets containing the bacteria, such as mist from showers or fountains. Preventing the growth and spread of Legionella in water systems is the primary defense against this illness.
Identifying Water Systems Prone to Growth
Legionella bacteria thrive under a specific set of environmental conditions. The most favorable temperature range for their rapid multiplication is between 68°F and 131°F (20°C and 55°C), which is often referred to as the “danger zone.” Within this range, warm water provides an ideal habitat, particularly when other factors are present.
Water stagnation, where water moves slowly or not at all, allows for the accumulation of nutrients and sediment. Stagnant areas, like unused piping or “dead legs,” increase the water’s age and lead to a loss of disinfectant residual, creating a perfect breeding ground. Biofilm, a protective layer of slime that forms on surfaces, also shields Legionella from disinfectants and provides a steady food source.
These conditions are frequently found in large, complex plumbing systems, centralized hot water storage tanks, and various aerosol-generating devices. The risk is high in any system that creates a fine mist or spray, such as cooling towers, decorative fountains, and certain spa equipment. The Centers for Disease Control and Prevention (CDC) recommends a water management program to monitor these risk factors, including temperature, water age, and disinfectant levels.
Utilizing Temperature for Water Safety
Temperature control inhibits Legionella growth by maintaining water outside its preferred range. To ensure safety in hot water systems, water should be stored in the heater at a minimum of 140°F (60°C). This high temperature is sufficient to rapidly kill the bacteria and prevent colonization within the storage tank.
The distribution system must be designed to deliver hot water to all outlets at or above 131°F (55°C) to maintain the bactericidal effect throughout the plumbing. For cold water, the goal is to keep the temperature consistently below 68°F (20°C) throughout the system, as temperatures below this point keep the bacteria dormant.
The risk of scalding is a consideration when using high temperatures, especially for vulnerable populations. Thermostatic mixing valves (TMVs) should be installed as close as possible to the point of use, such as a shower or sink. These valves blend the superheated water with cold water, providing a safe temperature at the tap while allowing the system’s main pipes to remain hot enough to prevent Legionella growth.
Chemical Disinfection and Routine Treatment
Beyond temperature, chemical disinfection serves as a control measure, particularly in large or intricate water systems. Chlorine is often used for shock treatment. Shock treatment, or hyperchlorination, involves introducing a very high concentration of disinfectant to rapidly kill the bacteria, often during remediation or after system repairs.
For routine residual maintenance, alternative disinfectants are used. Monochloramine, formed by combining chlorine and ammonia, is one persistent oxidant that can reach the furthest points of a distribution system. Chlorine dioxide is another effective oxidizer, frequently generated on-site, that is particularly valued for its ability to break down and remove biofilm.
Copper-silver ionization, where low levels of metal ions are introduced into the water, and stabilized silver hydrogen peroxide are also used. These advanced methods are applied in complex institutional settings, such as hospitals, where maintaining a continuous disinfectant residual is paramount. Chemical treatment programs require professional management to ensure proper concentration levels and consistent monitoring.
Home System Maintenance for Prevention
Preventing water stagnation is accomplished by regularly flushing all hot and cold water taps, especially those that are infrequently used, such as guest showers or basement sinks. After an extended period of absence, such as a vacation, run all faucets and showers for several minutes to flush out any stagnant water.
Removing scale and sediment is important, as this material can harbor bacteria and protect it from heat. Showerheads and faucet aerators should be routinely disassembled and cleaned to remove any mineral deposits or biofilm accumulation. Home hot water tanks should be periodically drained to flush out accumulated sediment from the bottom, which can compromise the effectiveness of the water temperature control.
Appliances that create mist require specific attention. Home humidifiers and CPAP machines that use water should be cleaned and disinfected according to the manufacturer’s instructions, using only distilled or sterile water when required. Hot tubs, which maintain water within the Legionella growth range, must be cleaned, drained, and treated regularly with an approved disinfectant to keep bacterial levels controlled.