Legionella Prevention: The Complete Guide (Ireland)

What is Legionella?

Legionella is a bacterium that can cause Legionnaires' disease, a serious form of pneumonia. It is commonly found in natural water sources and in building water systems, and it grows in water held between roughly 20°C and 45°C. Infection occurs when someone inhales tiny contaminated water droplets (aerosol) — so any system that stores, warms or sprays water can present a risk if it is not controlled.

What Causes Legionella?

There are several common causes behind a Legionella problem in a water system:

• Stagnant water — water that has been sitting with no movement for a long period gives the bacteria the still conditions it needs to multiply.
• Inhaling contaminated droplets — infection is caused by breathing in small water droplets that carry the bacteria.
• Construction and groundwork — Legionella can survive in soil, so those working on construction sites face an additional route of exposure.
• Contaminated medical equipment — in rarer cases, hospital equipment that has not been properly cleaned can also harbour the bacteria.

Who is Most at Risk?

Anyone exposed to contaminated aerosol can be affected, but some people are considerably more vulnerable:

• Smokers — already have weaker lungs, making them more vulnerable to the bacteria.
• Older people — weaker immune systems mean a much higher chance of infection.
• Construction and plumbing workers — face a higher risk of exposure through their work environment.
• Hospital patients — contaminated medical equipment, if not properly cleaned, can be a source of infection.

This is why control is a legal duty, not a discretionary one.

How Legionella is Prevented and Controlled

Prevention comes down to removing the conditions Legionella needs to grow and spread:

• Temperature control — keep cold water cold and hot water hot, avoiding the 20–45°C range where the bacteria multiply.
• Avoid stagnation — keep water moving; remove dead legs and little-used outlets, and flush infrequently used ones.
• Clean and disinfect — regularly clean and disinfect tanks, cooling towers and systems where scale and biofilm collect.
• Treat the water — apply appropriate biocides and control measures for the system.
• Monitor and test — routinely check temperatures and control parameters, and sample for Legionella to confirm control is working.
• Act on results — respond immediately if monitoring or sampling shows control has slipped.

Legionella in Cooling Towers

Prevention in a cooling tower means continuous biocide and treatment dosing, routine Legionella sampling (typically quarterly), biannual cleaning and disinfection, a written scheme of control, and notification to the local authority. Because this is a specialist regime, we cover it in full on the cooling-tower pages — see Cooling Tower Water Treatment and Cooling Tower Compliance.

The Framework in Ireland

In Ireland, control of Legionella is guided by the HPSC National Guidelines for the Control of Legionellosis (2009) and by HSG274:2014 (formerly the L8 ACoP), under general health-and-safety-at-work duties. The responsible person must assess the risk, put a written scheme of control in place, and keep records. Halpin & Hayward — Ireland's water treatment specialist for over 60 years — helps duty holders become, and stay, compliant.

The Consequences of Legionella Infection

Legionella infection can range from minor flu-like symptoms to severe pneumonia. Common signs include headaches, loss of appetite, fever, and lethargy. In more serious cases it can lead to kidney failure, and some people are left with long-term health effects such as chronic lung disease. If left untreated within the first week, the infection will typically worsen — and depending on who is affected, it can be fatal. Anyone who thinks they may have been exposed to Legionella should seek medical attention as soon as possible.

How to Get Rid of Legionella After an Outbreak

Acting quickly after an outbreak is essential. The affected water system must first be located — this may mean testing water systems or any area suspected of harbouring the bacteria. Once the source is identified, the system must be cleaned and disinfected using appropriate chemicals. Raising the water temperature and maintaining proper pH levels also helps keep Legionella levels low or non-existent going forward.

Legionella Prevention FAQs

• At what temperature does Legionella grow?

  Legionella multiplies in water held between roughly 20°C and 45°C. Keeping cold water below and hot water above this range is a core control measure — which is why temperature monitoring is central to prevention.

• How is Legionella prevented in a building water system?

  By controlling temperature, avoiding stagnation and dead legs, cleaning and disinfecting tanks and systems, treating the water, and monitoring and testing to confirm control. High-risk systems need a formal written scheme.

• What should I do after a Legionella outbreak or detection?

  Locate the source, clean and disinfect the affected system with appropriate chemicals, correct the conditions that allowed growth (temperature, stagnation), and re-sample to confirm control. For cooling towers this may include emergency disinfection and regulatory notification — our team can respond.

• Is Legionella control a legal requirement in Ireland?

  Yes. Under health-and-safety law and HPSC 2009 / HSG274 guidance, the responsible person must assess and control Legionella risk in water systems and keep records. For cooling towers the duties are especially strict.