Guidance · Reviewed September 2026
What Legionella is, how people catch Legionnaires’ disease, who is most at risk, the conditions that let the bacteria multiply in a water system, and the four controls that stop it.
The short answer
Legionella bacteria are common in natural water and usually harmless there. They become dangerous in man-made water systems where water sits between 20°C and 45°C, stagnates, and is then turned into breathable droplets by a shower, tap, spa pool or cooling tower. Control comes down to keeping hot water hot, cold water cold, water moving and the system clean — and the law expects whoever controls the premises to make sure that happens.
What Is Legionnaires’ Disease?
Legionellosis is the collective name for the illnesses caused by Legionella bacteria. The most serious is Legionnaires’ disease, a potentially fatal form of pneumonia. Pontiac fever and Lochgoilhead fever are similar but milder conditions caused by the same group of bacteria.
People catch it by breathing in small droplets of water (aerosols) that contain the bacteria. It is not known to spread from person to person. The disease takes its name from an outbreak among people attending an American Legion convention in Philadelphia in 1976, which led to the bacterium being identified.
Who Is Most at Risk?
Everyone is susceptible to infection, and the risk increases with age. Some groups are at higher risk:
- people over 45 years of age
- smokers and heavy drinkers
- people with chronic respiratory or kidney disease
- people with diabetes, lung or heart disease
- anyone with an impaired immune system
That is why care homes, hospitals and other healthcare premises are treated as higher-risk settings: the people using the water are the people least able to fight off the infection.
What Are the Symptoms?
Symptoms usually appear between two and ten days after exposure. The early symptoms are similar to flu:
- high temperature, feverishness and chills
- cough
- muscle pains
- headache
It can go on to cause pneumonia, diarrhoea and signs of mental confusion. The illness is treated with antibiotics, and early treatment matters.
If you think someone has been exposed
Anyone with these symptoms who may have been exposed should get medical advice promptly and mention the possible source. Legionnaires’ disease is a notifiable disease, so the diagnosing doctor reports the case to public health, and a linked workplace case in an employee who works on water systems is also reportable under RIDDOR.
Where Does Legionella Grow?
Legionella bacteria are widespread in rivers, ponds and other natural water, but conditions there are rarely right for people to catch the disease. Outbreaks come from purpose-built systems where water is held at a temperature that encourages growth. The risk increases where:
- the water temperature in all or part of the system is between 20°C and 45°C
- breathable water droplets can be created and dispersed, for example by a cooling tower, shower or tap
- water is stored or recirculated
- there are deposits that feed the bacteria, such as rust, sludge, scale, organic matter and biofilm
| System | Why it matters |
|---|---|
| Hot and cold water systems | Found in almost every building. Tanks, calorifiers, long pipe runs, dead legs and little-used outlets are the usual weak points. |
| Cooling towers and evaporative condensers | Warm, recirculated water and a fan that throws droplets into the open air. Behind most large outbreaks, and subject to the strictest controls. |
| Spa pools and hot tubs | Warm, agitated water producing aerosol at face height, including display models that nobody bathes in. |
| Other risk systems | Humidifiers, misting systems, vehicle washes, fountains and water features, and sprinkler or fire-fighting systems. |
How to Prevent Legionella Growth
Control the temperature
Store hot water at 60°C or above and deliver it at 50°C or above (55°C in healthcare premises). Keep cold water below 20°C. Fit thermostatic mixing valves where scalding is a risk.
Keep water moving
Remove dead legs and redundant pipework, and flush infrequently used taps and showers at least weekly.
Keep the system clean
Inspect and clean tanks and calorifiers, descale showerheads and hoses quarterly, and avoid materials that encourage growth.
Limit the spray
Reduce the release of aerosol where you can, and treat the water where temperature alone cannot control the risk, as in cooling towers and spa pools.
Water sampling has a place, but it is not the control. For most hot and cold water systems the risk assessment decides whether and how often to sample; cooling towers are sampled at least quarterly.
What the Law Requires
Employers, landlords and anyone in control of premises have duties under the Health and Safety at Work etc. Act 1974, the Management of Health and Safety at Work Regulations 1999 and COSHH 2002. HSE’s Approved Code of Practice L8 sets out what that means in practice:
- identify and assess the sources of risk
- appoint a competent responsible person to manage it
- prepare and implement a written scheme to prevent or control the risk
- monitor the controls and keep records
- notify the local authority of any cooling tower or evaporative condenser
Next step: the risk assessment
Everything else follows from the assessment. Our separate guide covers who has to carry one out, what a good one contains, who is competent to do it and how often to review it. What comes afterwards — the written scheme, the monitoring regime and the responsible person — is in the legionella control guide.
Health and Social Care Settings
Care homes, hospitals and other healthcare premises combine susceptible people with complex water systems, so the expectations are higher. Hot water should reach 55°C at outlets rather than 50°C, thermostatic mixing valves are needed wherever vulnerable people have access to baths and showers, flushing of unused rooms and outlets needs to be more frequent, and any detection of Legionella in a sample should be investigated. In England, healthcare premises also work to Health Technical Memorandum 04-01.
Training
Anyone who manages, monitors or works on a water system needs training proportionate to their role, and a record of it.
| If you need to… | Usual route |
|---|---|
| Understand the basics | Legionella and Legionnaires’ Disease Awareness. |
| Act as the responsible person | Legionella Risk Management for the Responsible Person. |
| Hold the dutyholder role | Legionella Management for Duty Holders. |
| Compare everything available | All legionella and water treatment courses. |
Frequently Asked Questions
Can you catch Legionnaires’ disease from another person?
It is not known to spread from person to person. People catch it by breathing in small droplets of contaminated water.
Can you catch it from drinking water?
The normal route is inhaling droplets, not drinking. The risk comes from water that is sprayed or splashed into the air, such as showers, taps, spa pools and cooling towers.
What temperature controls Legionella?
The bacteria multiply between 20°C and 45°C. They do not readily multiply below 20°C, where they can lie dormant, and they do not survive above 60°C. That is why hot water is stored at 60°C or above, delivered at 50°C or above, and cold water is kept below 20°C.
Is Legionella a risk in ordinary homes?
It can be, but the risk is usually low. A home with a combination boiler, no stored water and outlets in daily use gives the bacteria little opportunity. The risk rises with stored water, long periods of being empty and rarely used showers. Landlords still have a duty to assess it.
Do I have to test my water for Legionella?
Not necessarily. For most hot and cold water systems, routine sampling is only needed where the risk assessment calls for it, for example where temperatures cannot be reliably achieved or the occupants are especially vulnerable. Cooling towers and spa pools do have routine sampling built into their control regime.
Related Guidance
General guidance only, reviewed September 2026 against HSE’s Legionnaires’ disease guidance, the Approved Code of Practice L8 and HSG274 Parts 1 to 3. It is not medical advice and is not a substitute for a site-specific risk assessment by a competent person. Envico is an independent training intermediary, does not carry out or arrange legionella risk assessments, sampling or water treatment, and is not affiliated with the HSE.