Legionnaires’ disease is caused by Legionella bacteria that thrive in warm, stagnant water and spread when people inhale contaminated water droplets. The good news: preventing it at home comes down to a handful of practical steps, mostly involving water temperature, regular flushing, and basic maintenance of anything that creates mist or spray.
How Legionella Grows in Home Plumbing
Legionella bacteria multiply best in water between 77°F and 113°F (25°C–45°C). That range overlaps with lukewarm tap water, tepid pipes in warm climates, and poorly maintained hot tubs. Below 77°F, the bacteria survive but barely grow. Above 140°F (60°C), they die. The practical takeaway: your plumbing’s temperature settings and how often water moves through your pipes are the two biggest factors you can control.
Biofilm, the slimy buildup that forms inside pipes, showerheads, and faucet aerators, gives Legionella a protected home where it can multiply even when disinfectant is present in the water. Scale, sediment, and rust provide nutrients. Stagnant water that sits in rarely used pipes or fixtures for days or weeks creates ideal breeding conditions.
Set Your Water Heater Correctly
Most water heater manufacturers preset thermostats to 120°F, a setting chosen to reduce scalding risk. That temperature is right at the lower boundary of what controls Legionella, and in practice, water can cool as it travels through long pipe runs to distant fixtures. The CDC recommends storing hot water at 140°F (60°C) or above to prevent Legionella growth inside the tank and distribution pipes.
The tension here is real: water at 140°F can cause a serious burn in about five seconds. If you raise your water heater’s storage temperature, install thermostatic mixing valves at faucets and showers. These blend hot and cold water at the point of use, delivering a safe temperature (around 120°F) to the tap while keeping the stored water hot enough to suppress bacteria. This is especially important in homes with young children or older adults.
Flush Pipes After Any Period of Vacancy
Water sitting idle in your pipes for even a few days loses its residual disinfectant and drifts toward room temperature, both conditions that favor Legionella. After a vacation or any stretch when the house has been empty, run each faucet for at least five minutes, starting with cold water and then switching to hot. Remove faucet aerators before flushing so the water flows at full force, which does a better job scouring the inside of the pipes.
This isn’t just a post-vacation habit. Guest bathrooms, basement sinks, outdoor hose bibs, and any fixture you rarely use should be flushed regularly. Weekly is a reasonable target for most households. If someone in the home is immunocompromised, daily flushing of showerheads and taps they use is a worthwhile precaution.
Clean Showerheads, Aerators, and Faucets
Showerheads are the fixture most closely linked to Legionnaires’ disease transmission because they produce a fine mist you breathe in directly. Biofilm builds up inside the head and along the flexible hose, providing a reservoir for bacteria even when the rest of the plumbing is well maintained.
Remove and clean showerheads and faucet aerators every few months. Soak them in a diluted bleach solution (one tablespoon of household bleach per gallon of water) or white vinegar to dissolve mineral scale and disrupt biofilm. Scrub visible buildup with a brush, rinse thoroughly, and reattach. If the showerhead is old and heavily scaled, replacing it is easier and more effective than trying to clean deep deposits.
Hot Tub and Spa Maintenance
Residential hot tubs are one of the more common sources of Legionella outbreaks because they hold warm water (typically 100–104°F), generate heavy aerosols through jets and bubblers, and often sit for days between uses. The bacteria can multiply rapidly if disinfectant levels drop.
The CDC recommends maintaining free chlorine between 3 and 10 parts per million (ppm) in hot tubs, or bromine between 4 and 8 ppm, with a pH of 7.2 to 7.8. Test disinfectant and pH at least twice a day during periods of use. Standard test strips from a pool supply store work fine. Drain, clean, and refill the tub according to the manufacturer’s schedule, typically every three to four months for residential use, and scrub the shell and jets to remove biofilm each time.
Decorative Fountains and Water Features
Indoor and outdoor decorative fountains that spray or bubble water create fine droplets you can inhale. Smaller fountains (under 5 gallons) should be cleaned weekly. Medium fountains (5–25 gallons) need monthly cleaning. Larger features require routine cleaning based on water quality.
Keep the water temperature below 77°F (25°C). For fountains under 25 gallons, maintain 3 to 5 ppm free chlorine for at least one hour per day. Larger fountains need at least 0.5 ppm free chlorine for six or more hours daily. Run the fountain at least once a day to prevent stagnation. If you see algae, cloudy water, slime, or a foul odor, take the fountain out of service, drain it, scrub all surfaces, and disinfect with 10 ppm free chlorine for one hour before refilling.
Humidifiers and CPAP Machines
Any device that turns water into breathable mist is a potential source of Legionella if it’s filled with contaminated water. Use sterile or distilled water in CPAP humidifier chambers, nebulizers, and any respiratory therapy equipment. Tap water, even filtered tap water, can contain Legionella at levels too low to cause illness through drinking but high enough to cause pneumonia when aerosolized directly into your airways.
Empty and clean humidifier tanks daily. Don’t let water sit in a tank between uses. For room humidifiers, follow the same principle: use clean water, dump and dry the reservoir daily, and clean it thoroughly at least once a week. Cool-mist humidifiers are a greater concern than steam vaporizers because steam temperatures kill bacteria before the mist leaves the unit.
Home Air Conditioning Is Not a Risk
Residential air conditioners, including window units and central systems, do not use water to cool the air. They pose no Legionella risk. The large cooling towers associated with Legionnaires’ outbreaks are found in commercial and industrial buildings, not homes. You don’t need to treat or worry about your home AC system in this context.
Who Faces the Greatest Risk
Most healthy people exposed to Legionella never get sick. The bacteria cause serious illness primarily in people whose lungs or immune systems are already vulnerable. The highest-risk groups include adults over 50, people with chronic lung conditions like COPD or emphysema, those with weakened immune systems from disease or medication, and people with diabetes, kidney failure, or liver disease. Current or former smokers are also at elevated risk.
If someone in your household falls into one of these categories, the prevention steps above matter more, not less. Prioritize hot water temperature settings, consistent fixture flushing (daily for showerheads and taps the person uses), and strict sterile-water protocols for any respiratory device. These aren’t extreme measures. They’re simple habits that eliminate the conditions Legionella needs to reach dangerous levels in your home’s water.