Legionnaires’ disease (LD) is a serious form of pneumonia caused by exposure to the Legionella bacteria. This naturally occurring bacterium is found in freshwater environments but becomes a health concern when it grows and spreads in human-made water systems. People contract LD not by drinking contaminated water, but by inhaling microscopic water droplets, known as aerosols, that contain the bacteria. Since domestic showers efficiently create these inhalable aerosols, understanding prevention is paramount for home health and safety.
How Legionella Thrives in Water Systems
Legionella bacteria multiply rapidly within a specific temperature range, creating a danger zone in plumbing systems. The optimal growth window is between 77°F and 113°F (25°C to 45°C). Water temperatures in this tepid range, often found in water heaters or pipes leading to a shower, allow the bacteria to proliferate quickly.
A second factor enabling bacterial growth is biofilm, a slimy layer of microorganisms, scale, and sediment that adheres to the inside of pipes and fixtures. Biofilm provides a protective barrier and a food source for Legionella, shielding the bacteria from disinfectants like chlorine. Water stagnation, especially in infrequently used showers or long pipe sections, accelerates biofilm formation and bacterial growth. When the shower is turned on, water pressure aerosolizes the contaminated water, allowing the bacteria to be inhaled.
Controlling Water Temperature Settings
Controlling the temperature of the water system is the most effective method for managing Legionella risk. The goal is to store hot water above the maximum growth temperature and deliver it to fixtures at a temperature that minimizes bacterial survival. Health organizations recommend setting the water heater thermostat to a minimum of 140°F (60°C). At this temperature, 90% of Legionella bacteria are killed within two minutes, disinfecting the water stored in the tank.
However, this high temperature introduces a significant scalding risk, especially for young children, the elderly, or those with mobility issues. Water at 140°F can cause severe burns in seconds. To mitigate this dual hazard, homeowners should not lower the water heater setting but instead install thermostatic mixing valves (TMVs).
These valves are installed close to the point of use, such as the shower, and blend the superheated water from the tank with cold water before it reaches the fixture. The TMV allows hot water to be stored safely at 140°F while delivering water to the tap at a safer temperature, typically set to a maximum of 120°F (49°C). Maintaining hot water recirculation above 120°F throughout the distribution system prevents bacterial regrowth in the pipes leading to the shower. This strategy balances bacterial control with preventing accidental burns.
Preventing Water Stagnation and Biofilm Buildup
Preventing water from sitting unused in the plumbing system disrupts the Legionella life cycle. Stagnant water quickly loses residual disinfectant and warms into the bacteria’s optimal growth range. If a shower or faucet has gone unused for a week or more, such as after a vacation, it should be thoroughly flushed.
To flush a seldom-used shower, run the hot water at its maximum setting for several minutes, ideally for at least three minutes. This introduces fresh, disinfected water into the line, and the high temperature assists in killing any bacteria that may have grown in the pipe. When performing this flushing, minimize water spray by directing the showerhead toward the drain and leaving the bathroom to avoid inhaling potential aerosols.
Physical maintenance of the shower fixture directly addresses the biofilm challenge. Showerheads and hoses accumulate limescale and mineral deposits that act as a protected habitat for Legionella. Regularly descaling the showerhead removes this biofilm and the nutrients it contains, reducing the risk.
To remove mineral buildup, soak the showerhead in white vinegar, which dissolves limescale. For a detachable head, soak it overnight in a bucket of white vinegar. For fixed showerheads, secure a plastic bag filled with vinegar around the fixture to submerge the faceplate. After soaking, use a small brush to scrub away residue, then flush the head thoroughly with water.
Indicators That Professional Intervention Is Needed
While routine temperature control and cleaning suffice for many simple domestic systems, certain conditions warrant professional assessment. Homeowners in multi-unit buildings or those with complex plumbing networks should consider a specialized risk assessment. These systems often involve long pipe runs, multiple water heaters, and more opportunities for stagnation and temperature fluctuations difficult to manage with simple household maintenance.
If a home has an occupant who is immunocompromised, elderly, or has a chronic respiratory condition, the risk of severe illness from Legionella is higher. Consulting a water management professional for risk evaluation is prudent, even with a standard plumbing setup. This is true if there is a known local case of Legionnaires’ disease or if the home’s water quality is compromised.
Professionals can perform specialized water testing to confirm the presence of Legionella and recommend advanced remediation. Disinfection methods beyond temperature control include hyperchlorination (temporarily increasing chlorine concentration) or the installation of copper-silver ionization units. These interventions are complex and must be managed by certified specialists to ensure safety and effectiveness.