Legionella in Water Systems: Why This Waterborne Bacteria Deserves Your Attention

Legionella is a genus of bacteria that occurs naturally in freshwater environments — rivers, lakes, ponds — at low levels that don't typically cause disease. The problem arises when Legionella enters engineered water systems and finds conditions that allow it to multiply to concentrations high enough to cause infection.

The disease caused by Legionella is called Legionnaires' disease (a severe form of pneumonia) or Pontiac fever (a milder, flu-like illness that resolves without treatment). Legionnaires' disease was first identified in 1976 following an outbreak among American Legion convention attendees at a Philadelphia hotel — 221 people were infected and 34 died, prompting the original investigation that identified the bacteria.

Legionella is not spread from person to person. You can't catch it by touching someone who is infected or by drinking water that contains Legionella. The infection route is inhalation of water droplets or aerosols containing the bacteria — fine mist from a shower, spray from a cooling tower, water from a humidifier or decorative fountain, or aerosols from a hot tub. The bacteria reach the lungs and, in susceptible individuals, cause pneumonia.

Why doesn't everyone who inhales Legionella get sick? Healthy adults have immune defenses that typically handle low-level Legionella exposure. Illness is more common when the exposure concentration is high (the water system has allowed significant bacterial amplification) and when the individual's immune defenses are reduced. People at elevated risk include those over 50 years old, current or former smokers, people with chronic lung disease, people with diabetes, and those who are immunocompromised due to disease or medication.

Legionnaires' disease symptoms typically appear 2 to 10 days after exposure and include high fever, chills, cough (which may produce mucus or blood), shortness of breath, muscle aches, headache, and sometimes diarrhea or confusion. It requires antibiotic treatment — it does not resolve on its own the way some mild pneumonias do — and hospitalization is common.

Understanding the conditions that favor Legionella growth helps explain both why it's a concern in certain settings and what prevents it.

Legionella thrives in warm water. Its optimal growth temperature range is roughly 77°F to 113°F (25°C to 45°C). At temperatures above 140°F (60°C), the bacteria are killed within a few minutes. At temperatures below 68°F (20°C), growth is inhibited. This temperature sensitivity is the basis for most prevention strategies — keeping water either too cold or too hot for Legionella to multiply.

Legionella also benefits from biofilm — the thin layer of microorganisms, organic material, and minerals that forms on pipe surfaces in water systems. Biofilm provides nutrients for Legionella and can protect it from disinfectants. Stagnation — water sitting in pipes without flowing — encourages biofilm development and allows Legionella to multiply without the dilution that would occur with active flow.

The high-risk settings in buildings:

Hot water systems are the most common source. Hot water tanks that don't maintain adequately high temperatures throughout, long runs of hot water pipe where the water cools before reaching the tap, recirculation systems with dead-end branches where water stagnates, and corroded or scaled hot water heaters all create conditions for Legionella growth.

Cooling towers — the large evaporative cooling systems on the rooftops of commercial buildings — have historically been the source of many large Legionella outbreaks because warm, aerosolized water is their operating principle. Well-maintained cooling towers with proper biocide treatment have much lower Legionella risk.

Hot tubs and whirlpool spas that aren't properly maintained are a known Legionella risk, both in commercial settings and residential use. The combination of warm water, aerosolization from jets, and reduced chlorine effectiveness at higher temperatures creates favorable growth conditions.

Decorative water features — fountains, misters, and water walls in shopping centers, hotels, and office buildings — have been linked to several outbreaks. The aesthetic design of these features (irregular flow, surfaces that promote biofilm) often isn't optimized for Legionella prevention.

Residential settings: while most attention focuses on commercial buildings, homes can also harbor Legionella. Hot water heaters set too low, long stagnation periods during vacations, old water heaters with significant sediment accumulation, and shower heads with accumulated biofilm are all potential growth points.

Home Legionella risk is generally lower than in large commercial buildings, for reasons related to scale — a home's water system is simpler, the volume of water is smaller, and stagnation periods are typically shorter. But residential Legionella exposure does occur, and there are practical steps that reduce the risk.

Water heater temperature. The most important single factor in residential Legionella prevention is water heater temperature. The Occupational Safety and Health Administration (OSHA) and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommend that water heaters be set to at least 140°F (60°C) to prevent Legionella growth throughout the hot water system.

There's a real tension here with scalding safety — water at 140°F can cause a serious burn in 5 seconds, and the risk of scalding — particularly for young children and elderly adults — is a legitimate concern. The standard resolution is to set the water heater to 140°F and install a thermostatic mixing valve (TMV) at point-of-use fixtures that blends in cold water to deliver water to taps at a safer 120°F. This keeps the tank and pipes at Legionella-inhibiting temperatures while protecting against scalding at the tap.

If you're uncomfortable with the complexity of installing a thermostatic mixing valve, keeping the water heater at 120°F is a reasonable compromise that significantly reduces (though doesn't eliminate) scalding risk while reducing (though not eliminating) Legionella risk compared to lower temperatures.

Flush stagnant water. If your home has been vacant for a week or more — vacation, extended travel — flush all hot and cold water fixtures before using them for drinking, cooking, or showering. Run each tap, shower, and bathtub for 2–3 minutes. This replaces stagnated water in pipes with fresh water from the main supply and flushes any Legionella that may have amplified during the stagnation period.

Showerhead maintenance. Showerheads can accumulate biofilm that supports Legionella growth. Periodic cleaning — disassembly and soaking in a disinfectant solution, or replacement with a new showerhead — is worthwhile, particularly for people with compromised immune systems. Annual cleaning is a reasonable maintenance frequency.

Hot tub maintenance. If you have a residential hot tub or spa, maintain proper chlorine or bromine levels and pH according to manufacturer recommendations, and drain and clean the system regularly. The combination of warm temperature and aerosolization makes hot tubs one of the higher-risk residential Legionella sources.

Point-of-use filters for immunocompromised individuals. For household members who are significantly immunocompromised — transplant recipients, people on immunosuppressive medications — point-of-use filters with absolute 0.2-micron filtration (which physically removes Legionella) can be used at specific high-use outlets like bathroom taps. These are used in hospital settings routinely for immunocompromised patients.

Renters and residents of older apartment buildings face Legionella risks that are somewhat different from single-family homeowners, and that they have less direct control over.

Large building plumbing systems have more complexity, more dead-end branches, longer runs of pipe, and more potential for temperature stratification than single-family home systems. If a building's hot water recirculation system isn't functioning properly, water in distant parts of the system may cool below Legionella-inhibiting temperatures. If a building undergoes renovation that takes certain sections of the plumbing out of service for extended periods, reintroduction of stagnated water when service is restored can deliver a high-concentration slug of Legionella into the distribution system.

The responsibility for managing Legionella risk in large buildings rests with building management. The CDC and ASHRAE have published guidance documents (ASHRAE 188 and the CDC's Developing a Water Management Program to Reduce Legionella Growth and Spread) that establish best practices for building water management programs. Many states and some cities now require written Water Management Programs for certain building types — particularly healthcare facilities, hotels, and large office buildings.

If you live in a rental building and have concerns about Legionella — particularly if you're immunocompromised, elderly, or have chronic lung disease — you can:

Ask building management about their water management program and whether they conduct periodic Legionella testing. This is a legitimate question and responsible building managers should be able to answer it.

Report any hot water temperature problems to management — if your hot water isn't reliably hot, that's worth addressing both for Legionella reasons and for general habitability.

Flush your own taps after any extended absence from the unit, and after any building-wide plumbing work. Building maintenance often involves shutting down and restoring water service, which can disturb biofilm and introduce stagnated water.

After a building has been vacant or had reduced occupancy — a pattern that became common during the COVID-19 pandemic — building management has a particular obligation to flush the system thoroughly before residents return. Several Legionella outbreaks following pandemic-related building closures were documented.

Legionella testing of water systems is more complex than standard drinking water testing because you're not just looking for presence or absence — you need to understand the concentration, the distribution within the system, and the specific strain to understand outbreak risk.

Standard water quality tests for public water systems do not routinely test for Legionella. The EPA's Safe Drinking Water Act does not have an MCL for Legionella — it's regulated as a Treatment Technique requirement rather than a specific concentration limit, reflecting the understanding that Legionella control is fundamentally about system management rather than end-point concentration limits.

When a Legionella outbreak is suspected, environmental health investigators collect water samples from multiple points in the suspect water system — different taps, the water heater, the cooling tower, showerheads — and submit them for culture and, increasingly, genetic analysis (PCR). Matching the genetic signature of bacteria found in the environment to bacteria isolated from patient samples is how investigators confirm that a particular water source caused infections.

For building managers who want to proactively assess their system, commercial Legionella culture testing of water samples costs roughly $50–$100 per sample. A comprehensive survey of a large building's water system might require 10–20 samples, making it a meaningful investment. Some jurisdictions require periodic Legionella testing for certain building types as part of their water management program requirements.

If you develop a severe pneumonia requiring hospitalization, particularly if you've recently been in a hotel, gym, or other large building with water features, mention to your doctors the possibility of Legionella infection. Legionnaires' disease can be confirmed through a urine antigen test (which detects the most common Legionella serogroup quickly) or through cultures. Early identification allows for appropriate antibiotic selection, which improves outcomes. Legionella responds to fluoroquinolones and macrolides — it does not respond to beta-lactam antibiotics like amoxicillin, which are sometimes prescribed empirically for pneumonia.

One of the most reliably actionable Legionella prevention steps for homeowners and apartment renters alike is proper flushing after any period of extended non-use. This is worth its own dedicated section because it's easy to do, costs nothing, and is frequently overlooked.

When water sits stagnant in pipes — which happens whenever a property is unoccupied — several things happen simultaneously. The disinfectant residual (chlorine or chloramine) in the water degrades, removing the protection it provides against bacterial growth. The temperature of the water gradually equilibrates to the surrounding environment — hot water lines cool, cold water lines may warm up. Biofilm organisms that are normally kept in check by disinfectant and flow begin to multiply. And Legionella, if it's present in the system at low levels, can amplify significantly during this period.

The protocol for re-occupation after a property has been vacant for a week or more:

Flush all cold water outlets first. Starting with the tap farthest from the water main, run each cold water faucet, showerhead, and bathtub faucet for 2–3 minutes each. Work your way back toward the main supply entry point. This replaces stagnated water with fresh water from the supply.

Check and restore hot water temperature. Before flushing hot water outlets, confirm your water heater is set to adequate temperature and has had time to heat fully. Verify the thermostat setting and, if possible, check the outlet temperature at the water heater using a thermometer.

Flush all hot water outlets. Run each hot water tap, shower, and bathtub for 2–3 minutes each. This replaces stagnated hot water in the pipes with freshly heated water.

Run appliances that use water. Run the dishwasher through a full cycle, flush the ice maker, and run the washing machine through a rinse cycle. These appliances have water-holding components that can harbor stagnated water.

The entire protocol takes 20–30 minutes for a typical home and should become a routine part of returning from any extended trip. For vacation properties or second homes that sit vacant for months at a time, a more thorough approach — including professional inspection of the water heater, hot water recirculation system if present, and any specialty water features — is worth considering, particularly before the property is occupied by guests.