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Legionella risk is reduced by controlling the water systems where the bacteria can multiply and by preventing people from inhaling contaminated mist. The most useful actions depend on who controls the water: residents can flush little-used fixtures and manage a home heater safely; hot-tub users can check disinfectant and pH and question the operator; building owners and contractors need a documented water-management program.
Legionella occurs naturally in freshwater, but it becomes a health concern when it grows in human-made systems and devices. Showers, faucets, hot tubs, decorative fountains, cooling towers and similar equipment can generate mist. Aspiration of contaminated water is possible but less common. No consumer test or routine precaution proves that a water source is free of Legionella.
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The nine prevention measures at a glance
| Way | Main setting | Person responsible | Core control |
|---|---|---|---|
| 1. Identify how exposure occurs | Any setting | Everyone | Avoid breathing contaminated mist and recognize higher-risk situations |
| 2. Check hot-tub chemistry | Public or private hot tubs | User and operator | Verify disinfectant and pH with test strips |
| 3. Question the hot-tub operator | Hotels, gyms and spas | Guest and operator | Check inspection, testing, cleaning and filter practices |
| 4. Flush stagnant home fixtures | Houses and apartments | Resident or facilities staff | Flush cold and hot lines after extended nonuse |
| 5. Set water-heater temperature with scald protection | Homes and buildings | Owner and qualified professional | Balance Legionella control against burn risk |
| 6. Maintain water heaters and plumbing | Homes and small facilities | Owner or plumber | Follow heater-flushing and maintenance instructions |
| 7. Establish a building water-management program | Multi-unit and commercial buildings | Owner, manager and water team | Control growth and spread through a written, monitored plan |
| 8. Control temperature, stagnation and disinfectant | Building water systems | Design and maintenance teams | Keep water moving, temperatures appropriate and equipment effective |
| 9. Apply extra checks while traveling or operating rentals | Hotels, vacation rentals and event tubs | Guest, host and vendor | Ask about practices and follow the correct operating range |
1. Understand where Legionella exposure happens
Legionella is usually a systems-management problem rather than a problem with a single glass of water. The bacteria can multiply when water sits in a favorable temperature range, nutrients and biofilm accumulate, disinfectant is inadequate, or equipment is poorly maintained. A person is generally exposed by breathing a fine spray or mist that contains the bacteria.
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- Hot tubs and whirlpool spas
- Showers and sink faucets, especially where water has stagnated
- Decorative fountains and other aerosol-producing water features
- Cooling towers and evaporative equipment
- Building plumbing systems with dead legs, low flow or ineffective temperature control
People aged 50 and older, current or former smokers, people with chronic lung disease and people with weakened immune systems—including those taking medicines that reduce the ability to fight infections—are more likely to develop Legionnaires’ disease. These groups should be especially cautious around poorly maintained aerosol-producing devices.
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Prevention lowers risk; it does not create a guarantee. A normal-looking, clear or odorless water source can still have a water-management failure.
2. Check disinfectant and pH before using a hot tub
CDC consumer guidance recommends carrying water test strips and checking the hot tub before entering. For that consumer check, the water should have free chlorine of at least 3 parts per million (ppm), bromine of at least 4 ppm and a pH of 7.0–7.8. These are CDC guidance thresholds, not a substitute for the health rules that apply in your jurisdiction.
How to perform the check
- Use a strip designed to measure the sanitizer used by that tub—chlorine or bromine—and pH.
- Collect the sample and read it exactly within the time stated on the strip instructions.
- Compare the result with the CDC consumer thresholds and any posted local requirements.
- If the sanitizer or pH is outside the required range, do not enter. Tell the owner or operator.
Test strips measure water chemistry. They do not detect Legionella and cannot certify that a tub is safe. Do not treat a passing strip result as proof that the filters, circulation system or cleaning program is working.
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A guest cannot inspect a hotel or spa’s entire plumbing system, but direct questions can reveal whether routine controls are in place. CDC consumer guidance suggests asking:
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- “What was the most recent health inspection score for the hot tub?”
- “Are the disinfectant levels and pH checked at least twice per day?”
- Are checks performed more often during heavy use?
- How often are slime and biofilm removed?
- When were the filters last cleaned or replaced?
- When was the water last replaced, and is that schedule based on the manufacturer, health department or another applicable requirement?
An operator should be able to explain who records readings, what happens when a result is out of range and how the tub is taken out of service for cleaning. If answers are vague, the inspection information is unavailable or the chemistry is visibly poor, choose another activity and report the concern to the facility.
4. Flush faucets and showerheads that have been unused
Stagnant water gives biofilm and bacteria more opportunity to develop. CDC home guidance recommends flushing a faucet or showerhead that has not been used for a while, such as a week or more. Flush after noticing a drop in water pressure as well, because a pressure change can signal a plumbing or flow problem.
Two-minute cold-then-hot procedure
- Remove items from the sink or shower area that could be splashed.
- Open the cold water fully and let it run for two minutes.
- Then run the hot water until it starts to feel hot.
- For a single-handle fixture, keep the handle at full cold for the two-minute flush, then move it to full hot until the water starts to feel hot.
- Clean and disinfect the fixture if the manufacturer or building procedure requires it, and avoid breathing the spray.
Open windows or use ventilation where practical, and keep your face away from the stream. In a multi-unit property, include vacant apartments, guest rooms, rarely used janitor sinks and seasonal spaces in the return-to-service checklist.
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CDC says maintaining a home water heater between 130°F and 140°F can kill germs such as Legionella in the water, but the higher setting increases the chance of serious burns. This is not blanket advice to raise every thermostat.
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Make the decision household by household
- Ask a healthcare provider whether a setting of 130°F or above is appropriate for the household.
- If the heater is set above 120°F, use thermostatic mixing valves that blend cold water with hot water at the tap to reduce delivery temperature and burn risk.
- Give extra weight to scald prevention where young children, older adults or anyone with reduced mobility or sensation lives.
- Have a qualified professional verify the heater, mixing valve and fixtures after any change.
A mixing valve reduces the temperature at the outlet; it does not remove the need for heater maintenance or water-system management. Follow local plumbing codes and the equipment manufacturer’s instructions.
6. Flush and maintain the water heater
Mineral deposits, sediment and biofilm can interfere with a heater’s performance and create places where germs grow. CDC recommends regular water-heater flushing and says to follow the manufacturer’s instructions or have a professional perform the work.
When manufacturers commonly call for flushing
- About annually, if that is the schedule in the heater’s instructions
- Before moving into a home
- After plumbing work
- When tap water is discolored
Those situations are examples reported by CDC, not a universal schedule. Tankless and storage heaters have different procedures, and some systems require isolation, pressure relief and drain-hose controls. Never open a drain or relief valve without understanding the manufacturer’s procedure; hot water can scald and a poorly controlled discharge can damage property. Record the date, work performed and any abnormal finding so the next maintenance visit has a usable history.
7. Create a building water-management program
For buildings at increased risk, CDC describes a water-management program as the primary strategy for controlling Legionella growth and spread. This is a coordinated process, not a one-time disinfection or a thermostat setting.
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What the program should assign
- Responsible people: Name an owner, facilities lead, maintenance staff, water-treatment personnel and, when needed, an outside specialist.
- System map: Document incoming water, heaters, storage tanks, recirculation loops, showers, cooling equipment, fountains, spas, dead legs and rarely used outlets.
- Hazard controls: Identify where water can stagnate, where temperatures enter the growth range and where aerosols are produced.
- Monitoring: Set inspection and measurement frequencies, define acceptable results and keep records.
- Corrective actions: Specify who isolates a fixture, cleans equipment, restores flow, adjusts treatment or contacts public health officials.
- Verification: Review whether controls worked, update the plan after renovations and train staff and contractors.
CDC says a Legionella water-management program is specifically needed in multi-unit buildings with centralized hot water. Construction and renovation can change flow paths, create dead legs and disturb biofilm, so the program should be reviewed before commissioning and after plumbing changes.
CDC reported on its “About Legionella Control” page, updated March 15, 2024, that “Almost all (9 in 10) outbreaks were caused by problems preventable with more effective water management.” The statistic supports investing in a functioning program; it does not mean every case is preventable or that a program offers absolute protection.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.8. Control temperature, stagnation, disinfectant and equipment
Legionella grows most readily in water around 77°F–113°F (25°C–45°C), according to CDC building guidance. Growth may occur as low as 68°F (20°C) and above 113°F, so a single thermostat setting cannot manage risk by itself.
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System controls that work together
- Keep cold water cold and hot water hot: Insulate and balance piping, verify recirculation and prevent unintended warming of cold-water lines.
- Manage stagnation: Remove unnecessary dead legs, maintain flow through low-use branches and flush outlets after shutdowns or occupancy changes.
- Maintain disinfectant: Where treatment is used, monitor residuals and equipment performance according to the design, water authority and applicable rules.
- Operate equipment effectively: Maintain cooling towers, filters, pumps, storage tanks, spas and aerosol-producing fixtures using manufacturer and regulatory requirements.
- Control biofilm and sediment: Keep surfaces, strainers, filters and tanks on a documented cleaning schedule.
Designers should provide access for inspection and cleaning. Facilities teams should trend temperatures, flow and treatment results instead of relying on a single spot check. Local water chemistry, system size, treatment method and building use all affect the correct control limits.
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9. Use added caution in hotels, vacation rentals and event hot tubs
Questions for a hotel or vacation-rental guest
Ask the host or manager what water-management practices are used, how hot tubs are tested and cleaned, and how the property handles rooms or plumbing that have been vacant. Owners and managers should follow CDC guidance, manufacturer instructions and applicable local requirements. A guest should not assume that a rental’s recent cleaning addressed the building’s hot-water circulation or low-use fixtures.
Vacation-rental operator hot-tub range
CDC gives a separate range for vacation-rental operators: bromine 4–8 ppm, free chlorine 3–10 ppm and pH 7.2–7.8, including while the tub is not in use. These operator ranges are distinct from the consumer page’s minimum checks. Operators must also follow the tub manufacturer’s maintenance recommendations and laws that may differ from CDC guidance.
Temporary event displays
At the end of a temporary event, CDC advises vendors to clean the tub, apply a biocidal shock treatment, drain it, remove filters and let them dry, clean filters before reuse, replace filters as the manufacturer recommends, protect the tub from outside water sources and keep it as dry as possible between events. This is an operator procedure, not a household shortcut.
What to do if a source is suspected during an outbreak
Routine prevention and outbreak control are different. If public-health officials or a facility identify suspected ongoing transmission, CDC investigation guidance lists measures that may be considered: restricting showers, avoiding hot tubs, installing point-of-use microbial filters or temporarily closing an affected building, area or device.
These actions require coordinated risk assessment and should be directed by public-health authorities and the building operator. Do not install a filter, reopen a closed spa or alter a shared water system on your own. Follow official notices, preserve maintenance records and provide investigators with information about occupancy, renovations, water treatment and recent illnesses.
When to seek medical care
CDC hot-tub guidance advises anyone who used a hot tub in the previous two weeks and then develops cough, shortness of breath, fever or muscle aches to seek medical care promptly and tell the healthcare provider about the recent hot-tub use. Those symptoms do not prove Legionnaires’ disease, but the exposure history can help a clinician decide what evaluation is needed.
People in higher-risk groups should act particularly promptly. Building owners and contractors should also communicate suspected exposures and any official investigation to occupants without making a diagnosis or promising that a system is Legionella-free.
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