The bacteria Legionella becomes a health concern when it enters and multiplies within man-made water systems. In these systems, the bacteria can thrive under specific conditions. The resulting illness, known as Legionnaires’ disease, is a serious form of pneumonia contracted by inhaling small water droplets, or aerosols, that contain the bacteria. Preventing this infection relies entirely on controlling the conditions—primarily temperature and water flow—that allow the bacteria to multiply.
Preventing Legionella in Domestic Water Systems
Controlling water temperature is the primary method for preventing bacterial growth within domestic plumbing. The bacteria multiply most efficiently between 20°C and 45°C (68°F and 113°F), with growth slowing below 20°C and the bacteria dying above 60°C (140°F). Therefore, hot water storage cylinders should be set to maintain water at a minimum of 60°C to ensure the bacteria cannot survive.
The heated water should then be distributed throughout the system at temperatures of 50°C (122°F) or higher to inhibit growth in the pipes. Because water at 50°C and above poses a scalding risk, thermostatic mixing valves (TMVs) should be installed as close as possible to the point of use, like taps and showers, to deliver safer water at around 45°C to 50°C. Conversely, cold water must be stored and distributed consistently below 20°C to keep the bacteria dormant.
Water stagnation encourages bacterial growth by allowing sediment and biofilm to accumulate, which provide nutrients and protection for the microbes. To combat this, infrequently used outlets, such as guest bathroom taps or showers, should be flushed with hot water for at least 15 seconds every week. This replaces the stagnant water in the pipes with freshly treated water, minimizing colonization risk.
The physical components of the plumbing system also require attention, particularly where water is aerosolized. Showerheads and faucet aerators should be disassembled, cleaned, and descaled quarterly to remove mineral deposits and biofilm. Additionally, any unused sections of pipework, known as “dead legs,” should be removed or shortened to prevent water stagnation, which contributes significantly to bacterial growth.
Safe Operation and Cleaning of Water-Holding Devices
Water-holding devices that intentionally create a fine mist or aerosol, like hot tubs and humidifiers, require management protocols to prevent bacterial dispersal. Hot tubs and spas operate within the bacteria’s optimal growth temperature range, making them a significant risk if not properly maintained. Proper maintenance requires consistently monitoring and maintaining disinfectant residuals, such as free chlorine at a minimum of 3 parts per million (ppm) or bromine at 4 ppm, with the water pH maintained between 7.0 and 7.8.
The water quality should be tested multiple times per day, especially during heavy use, to ensure sanitizer levels remain effective. Hot tub filters must be cleaned weekly and replaced according to manufacturer’s recommendations, as dirty filters can harbor biofilm and reduce circulation. Additionally, the entire system should be periodically drained, scrubbed, and refilled; this cleaning may involve hyperchlorination to kill deeply embedded bacteria.
For cool-mist or ultrasonic humidifiers, which generate aerosols from standing water, the risk is mitigated by using only distilled or demineralized water instead of tap water. The device’s reservoir should be emptied, rinsed, and allowed to dry completely every day to prevent the formation of biofilm. Regular disinfection, often with a diluted vinegar or bleach solution, followed by a thorough rinse, is necessary to maintain a clean internal environment.
Decorative water features and fountains, both indoors and outdoors, also create aerosols and require a maintenance schedule. These systems should be operated continuously, ideally for at least six hours daily, to prevent water stagnation and reduce biofilm formation. The water must be treated with an appropriate biocide, such as chlorine, and the feature should be drained and scrubbed weekly to remove any accumulated sediment or organic material.
Mitigation Strategies for Large and Complex Buildings
In large or complex buildings, such as hospitals, hotels, and industrial facilities, the risk of widespread contamination is significantly higher, often stemming from large water systems like cooling towers. Cooling towers are particularly prone to colonization because they create the ideal mix of warm water, organic material, and aerosols. Control starts with a comprehensive Water Management Plan (WMP) that identifies all potential sources of bacterial growth and defines control measures.
The WMP mandates routine cleaning and disinfection, typically involving the scheduled application of chemical biocides to kill bacteria and disrupt biofilm. Facility managers must use a combination of oxidizing biocides, like chlorine, and non-oxidizing biocides to prevent bacteria from developing resistance. Physical cleaning, including scrubbing basins and heat exchange surfaces to remove organic buildup, should be performed at least twice a year.
System design is also an integral part of risk reduction. This requires the use of high-efficiency drift eliminators on cooling towers to minimize the dispersal of contaminated water droplets into the air. Furthermore, cooling towers should be located at least 25 feet away from building air intakes to prevent contaminated mist from being drawn into the ventilation system.
During construction or renovation projects, existing plumbing can be disrupted, stirring up sediment and creating temporary areas of stagnation. This requires preemptive disinfection of the affected water lines.
If contamination is suspected, individuals should report concerns to building management or public health authorities and avoid misty areas near ventilation exhausts. The effectiveness of all prevention measures must be verified through routine monitoring and testing of the water for bacterial presence. This professional oversight ensures that control limits for disinfectants and temperature are consistently met, providing a multi-barrier approach to public safety.