What is Legionnaires’ Disease?

Legionnaires’ disease is a serious type of pneumonia caused by Legionella bacteria. It is usually caught by breathing in tiny droplets of water (aerosols) that contain the bacteria.

It is important to understand that Legionnaires’ disease is not spread from person to person in the way colds and flu are. Instead, it is linked to man made water systems, particularly in larger or more complex buildings.

For facilities managers and duty holders, Legionnaires’ disease is one of the key risks covered under UK water hygiene guidance, including HSE documents such as ACoP L8 and HSG274.

How do people catch Legionnaires’ disease?

People catch Legionnaires’ disease by inhaling aerosol droplets that contain Legionella. These droplets can be created when water is sprayed, misted, or disturbed.

Common sources of aerosol exposure

In buildings, aerosols may come from:

• showers and spray taps

• thermostatic mixing valves (TMVs)

• hot and cold outlets that are rarely used

• cooling towers and evaporative condensers

• some types of spa pools and humidifiers

The bacteria themselves thrive in water systems when conditions allow them to multiply. That usually involves a combination of:

• warm water temperatures

• stagnant water

• scale, sludge, or corrosion

• biofilm inside pipework and outlets

If you are responsible for water safety, it helps to understand how the risk is assessed in practice. You may want to explore Titan’s guidance on Legionella risk assessments here:https://www.titanwater.co.uk/water-hygiene-services/legionella-risk-assessment-services

What are the symptoms of Legionnaires’ disease?

Legionnaires’ disease often starts with flu like symptoms, but it can progress quickly.

Early symptoms

People may experience:

• high temperature

• chills

• headache

• muscle aches

• dry cough

Later symptoms

As it develops into pneumonia, symptoms may include:

• shortness of breath

• chest pain

• confusion (especially in older adults)

• diarrhoea or nausea in some cases

Legionnaires’ disease can be life threatening, particularly for people with underlying health conditions.

Who is most at risk?

Most healthy people who are exposed to Legionella do not become seriously ill. The risk increases for people who are more vulnerable.

Higher risk groups include:

• adults over 45 (especially over 50)

• smokers

• people with chronic lung conditions

• people with weakened immune systems

• people with diabetes, kidney disease, or cancer

In many commercial and public buildings, the presence of vulnerable users is one of the key reasons Legionella control is taken so seriously.

Legionnaires’ disease vs Pontiac fever

Legionella can cause more than one type of illness.

Legionnaires’ disease

This is the severe form and involves pneumonia. It can require hospital treatment.

Pontiac fever

This is a milder flu like illness caused by Legionella exposure. It does not cause pneumonia and usually resolves without treatment.

From a building safety perspective, both illnesses point to the same underlying issue: a water system where Legionella control is not effective.

Where does Legionella grow in building water systems?

Legionella bacteria can be present in low numbers in natural water sources. The risk increases when they find the right conditions in a building’s system.

Typical problem areas

In facilities, the most common risk areas include:

• cold water storage tanks, especially if poorly maintained

• pipework with low flow or redundant sections

• dead legs and capped off outlets

• calorifiers and hot water plant

• outlets that are rarely used

• cooling towers and associated pipework

If your site includes cold water storage, it is worth understanding how maintenance affects risk. You can explore Titan’s tank cleaning and disinfection service here:https://www.titanwater.co.uk/water-hygiene-services/water-tank-cleaning-services

How is Legionnaires’ disease prevented in buildings?

Legionella control is about preventing the bacteria from multiplying in the first place. This is done through a combination of design, monitoring, and maintenance.

Key control measures include:

Temperature control

Hot water should be stored and distributed at safe temperatures, while cold water should remain cold enough to prevent bacterial growth. If temperature control is inconsistent, it is often linked to plant issues, poor circulation, or faulty valves.

Facilities teams often review TMVs as part of this. If that applies to your site, you can explore TMV servicing here:https://www.titanwater.co.uk/water-hygiene-services/tmv-servicing

Reducing stagnation

Legionella risk increases where water sits still.

That includes:

• dead legs

• low use outlets

• redundant pipework after refurbishments

If you want a practical breakdown of this topic, this guide is useful: https://www.titanwater.co.uk/post/what-is-a-dead-leg

Cleaning and inspection

Tanks, strainers, and outlets can accumulate scale, sludge, and biofilm. These create the ideal conditions for Legionella to persist.

Monitoring and planned checks

This includes flushing, temperature checks, and inspections.

For a wider overview, you can explore Titan’s water hygiene monitoring page here:https://www.titanwater.co.uk/water-hygiene-services/water-hygiene-monitoring-services

Does Legionella testing prevent Legionnaires’ disease?

Testing is helpful, but it is not the starting point.

Legionella sampling (water testing) is usually used to:

• confirm whether control measures are working

• investigate suspected system issues

• support decision making after a positive result

• provide reassurance in higher risk buildings

But testing alone does not prevent Legionnaires’ disease. Prevention comes from the system being managed properly.

If you want to understand how sampling fits into compliance, you can explore Titan’s Legionella testing service page here:https://www.titanwater.co.uk/water-hygiene-services/legionella-testing-services

What happens if Legionella is detected?

A positive result does not always mean immediate panic. What matters is the level detected, the type of system, and the risk profile of the building.

In many cases, the next steps include:

• reviewing control measures and recent monitoring records

• targeted cleaning or disinfection

• increasing flushing or temperature control checks

• identifying dead legs or poor circulation

• carrying out corrective work where needed

If corrective work is required, this falls under remedial actions. For a clear overview of what that involves, you can read:https://www.titanwater.co.uk/post/understanding-legionella-remedial-works

Further reading if you manage building water safety

If you want to go deeper into practical facilities led Legionella control, these guides are worth reading:

• https://www.titanwater.co.uk/post/how-to-test-for-legionella-bacteria-in-water

• https://www.titanwater.co.uk/post/bs7592-2022-legionella-sampling-guidance

Final note for facilities managers

Legionnaires’ disease is serious, but it is also preventable. The key is not just reacting to test results, but maintaining consistent control of temperature, flow, cleanliness, and system condition.

If you are reviewing your current responsibilities and want to ensure your approach aligns with UK guidance, the best place to start is your risk assessment process:https://www.titanwater.co.uk/water-hygiene-services/legionella-risk-assessment-services