Legionella in Home Plumbing: Water Heater Settings, Showerheads, and Who's at Risk

Legionella is a bacterium that lives naturally in fresh water and can grow inside building plumbing when water is warm, stagnant and low in disinfectant. People get sick mainly by breathing in tiny droplets of contaminated water, such as shower mist, not by drinking it. The CDC says most healthy people who are exposed don’t get sick, but Legionnaires’ disease, the pneumonia it causes, can be serious, especially for people 50 and older, smokers and people with weakened immune systems. At home, the main defenses are sensible water heater settings, flushing pipes after time away, and keeping showerheads, filters and humidifiers clean.

How people get sick

The CDC describes two ways Legionella causes illness:

Breathing in mist (most common). Water containing the bacteria can form droplets small enough to inhale, from sources like showerheads, sink faucets, hot tubs and decorative fountains.
Aspiration (less common). Drinking water can accidentally go down the wrong way into the lungs, mainly in people who have trouble swallowing.

In general, the CDC says, people don’t spread Legionnaires’ disease to each other.

Legionella causes two illnesses:

Legionnaires’ disease is a serious pneumonia. The CDC says symptoms usually begin 2 to 14 days after exposure and can include cough, fever, headache, muscle aches and shortness of breath. It requires treatment with specific antibiotics, and most cases are treated successfully. Still, the CDC reports that about 1 in 10 people who get Legionnaires’ disease die from complications; for people who get it during a stay in a healthcare facility, it is about 1 in 4.
Pontiac fever is a milder illness with fever, muscle aches and headache.

If you develop pneumonia symptoms and you’re in a higher-risk group, tell your doctor about any recent exposures, such as a hot tub, a hotel stay or a home that sat empty.

Who’s at higher risk

According to the CDC, the risk of getting sick after exposure is higher for:

People 50 or older
Current or former smokers
People with chronic lung disease, cancer, diabetes, kidney failure or liver failure
People with a weakened immune system, including from medicines such as chemotherapy or post-transplant drugs

The CDC’s home water guidance also lists infants 6 months or younger among people more likely to get sick from waterborne germs in general.

Why home plumbing can grow Legionella

Your utility treats water to kill germs and keeps a small amount of disinfectant in it. The EPA regulates Legionella at the utility level through treatment requirements, with a health goal of zero. But conditions inside a building can undo some of that protection.

The CDC lists the conditions that help Legionella grow:

Lukewarm temperatures, roughly 77°F to 113°F
Low disinfectant, which happens as water sits
Slow or no water movement
Biofilm, the slimy layer that forms inside pipes and fixtures

The EPA gives a similar range, saying Legionella grows best between 80°F and 120°F, and notes that stagnant water can change temperature and lose disinfectant.

Large buildings with complex plumbing are the classic setting for outbreaks, but home systems aren’t immune. Water heaters set low, rarely used bathrooms, vacation homes and devices that hold warm water all create the conditions above.

The water heater trade-off: scalds vs. bacteria

This is the hardest part of Legionella prevention at home, because the temperature that best controls bacteria is hot enough to burn.

The case for hotter:

The CDC says keeping a home water heater between 130°F and 140°F can kill germs like Legionella.
The EPA says setting the thermostat at or above 140°F can often keep temperatures high enough to limit growth throughout the plumbing.

The case for cooler:

The U.S. Consumer Product Safety Commission’s childproofing guidance says that to prevent burns, a water heater can be set to 120°F or lower, and that anti-scald devices on faucets and showerheads also help.
The CDC itself warns that 130°F to 140°F increases the risk of burns, and recommends talking with your healthcare provider before setting a heater that high.

How to get both: The CDC and EPA both point to thermostatic mixing valves. These let the tank stay hot while blending hot and cold water at the faucet or shower so it comes out at a safe temperature. The CDC recommends them if your heater is set above 120°F, especially in homes with young children, older adults or anyone at higher risk of burns. A licensed plumber can install them, and should check your local code.

What this means in practice:

Most households: A setting around 120°F follows CPSC scald guidance. Keep water moving (see below) and maintain the heater.
Households with someone at high risk for Legionnaires’ disease: Talk with that person’s doctor about whether a higher tank temperature with mixing valves makes sense.
Households with young children or older adults: If you raise the temperature, mixing valves are not optional.

Flushing after travel or time away

Stagnant water is one of the biggest risk factors, which makes coming home after a trip a good time to act. The CDC recommends flushing any faucet or showerhead that hasn’t been used for a week or more before using it again:

Run the cold water fully for 2 minutes, then turn it off.
Run the hot water fully until it starts to feel hot, then turn it off.

Do this at every faucet and shower that sat unused. The EPA also suggests flushing seldom-used taps, such as in a guest bathroom, regularly.

After a longer absence, check whether your water heater’s manufacturer recommends draining or flushing the tank. The CDC notes that most manufacturers recommend regular flushing, commonly yearly and after plumbing work.

Showerheads, faucets and filters

Clean showerheads and faucet aerators whenever you can see buildup, following the manufacturer’s instructions, the CDC advises. Mineral scale and biofilm give bacteria a place to live.
Maintain and replace water filters on schedule. This includes pitcher, refrigerator, under-sink and shower filters. Many filters remove chlorine, which improves taste but also removes the disinfectant that protects water sitting downstream.

That last point showed up in a real case. In a June 2026 report, CDC investigators described a 10-month-old with a weakened immune system who developed Legionnaires’ disease. The likely source was a home appliance that stores water and dispenses mixed infant formula. Water in the appliance’s reservoir sat at about 106°F and had not been fully drained in more than 30 days, and Legionella was found in the reservoir and, at lower levels, at the kitchen faucet that fed it, where an under-sink filter had removed chlorine. The infant recovered. The CDC report noted that boiling water kills Legionella and that, for infants with weakened immune systems, ready-to-feed formula or formula made with water heated to at least 158°F is the safer approach. If your baby has a weakened immune system, ask your pediatrician how to prepare formula. See our guide on tap water for baby formula.

Other household devices that hold warm water

Humidifiers. The CDC says to empty all water from humidifiers every day, clean them regularly, let them air dry, and consider distilled or boiled-then-cooled water.
CPAP machines. Clean the mask, humidifier and tubing regularly, and use the water type the manufacturer recommends (most recommend distilled).
Hot tubs. The CDC lists hot tubs among common sources of Legionella exposure. Keep disinfectant levels and maintenance up to date.

Is my tap water the problem?

Usually not at the utility level. Legionella risk at home is mostly about what happens to water after it enters your house: temperature, stagnation and loss of disinfectant. That said, low disinfectant levels in the distribution system can contribute, so it’s worth knowing how your utility is doing. You can look up your water system to see its EPA compliance record, and our posts on chlorine and chloramine and is tap water safe to drink explain how disinfection works.

Quick checklist

Water heater set to a temperature you’ve chosen deliberately, with mixing valves if above 120°F
Flush all taps and showers after a week or more of no use: cold 2 minutes, then hot until hot
Run water in seldom-used bathrooms regularly
Clean showerheads and aerators when you see buildup
Replace filters on the manufacturer’s schedule
Empty humidifiers daily
If you or someone at home is at higher risk, talk with a doctor about extra steps