Legionella comes from natural freshwater environments, where the bacteria usually exist in small numbers. The greater risk develops after Legionella bacteria enter a building water system and find conditions that help them multiply.
Warm water, low water movement, stored water, scale and biofilm can allow bacterial concentrations to increase. Showers, taps, spa pools and other equipment may then release contaminated droplets into the air. When a person inhales those droplets, they may develop Legionnaires’ disease, a potentially serious form of pneumonia.
Understanding this chain is important. Legionella’s natural origin is not normally the main problem. The risk comes from the way bacteria survive, multiply and spread through poorly controlled building water systems.
Where Is Legionella Found in the Natural Environment?
Legionella is a naturally occurring group of waterborne bacteria. It can be present in freshwater and damp environmental materials without necessarily presenting an immediate health risk. Problems arise when small numbers enter artificial water systems, multiply and become dispersed through aerosols that people can breathe into their lungs.
- Rivers and streams: Legionella may occur naturally in flowing freshwater. Concentrations are normally low because moving water, changing temperatures and environmental conditions are less favourable for significant bacterial growth.
- Lakes and reservoirs: Natural and man-made bodies of stored water may contain low bacterial numbers. They can also contribute water to treatment and distribution systems supplying buildings.
- Ponds and freshwater environments: Legionella bacteria can survive in ponds and similar aquatic habitats, often alongside microorganisms and free-living protozoa that provide protection and support survival.
- Soil, compost and potting mix: Legionella longbeachae is associated with soil and potting products rather than conventional plumbing systems, making careful handling of compost particularly important for vulnerable people.
- Hot springs and naturally warm water: Naturally heated water can provide favourable temperatures for microbial activity, although most recognised outbreaks are linked to artificial systems where conditions are easier to amplify.
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How Does Legionella Grow in Man-Made Water Systems?
After Legionella enters a purpose-built water system, it may remain at a low level or begin to multiply. The outcome depends on temperature, water movement, cleanliness, system design and maintenance.
Growth is more likely when:
- Water remains within the bacteria’s favourable temperature range.
- Outlets, tanks or sections of pipework are used infrequently.
- Sediment, corrosion and organic matter provide nutrients.
- Biofilm develops on internal surfaces.
- Disinfection and maintenance controls are inadequate.
Warm Water, Stagnation and the 20–45°C Growth Range
The Health and Safety Executive states that Legionella can multiply where water temperatures are between approximately 20°C and 45°C and nutrients are available. The bacteria are generally dormant below 20°C and do not survive at temperatures above 60°C.
Temperature alone does not determine risk. Water stagnation and slow movement give bacteria more time to colonise tanks, outlets and pipework. Dead legs, oversized storage vessels and little-used showers can all create pockets of stagnant water.
Risk can also rise after a property has been empty, during refurbishment or when rooms and facilities are closed for an extended period. Water that would normally move through the system remains stored, while disinfectant residuals may decline.
How Biofilm, Rust, Sludge and Scale Support Bacterial Growth
Biofilm is a layer of microorganisms and organic material that forms on wet internal surfaces. It can protect Legionella from temperature changes and disinfectants while providing access to nutrients.
Rust, sludge, sediment and limescale can further support bacterial growth. These deposits create rough surfaces, shelter microorganisms and make effective cleaning more difficult. Legionella may also survive inside amoebae and other protozoa found within biofilm.
A system may therefore appear clear at the outlet while still containing favourable conditions elsewhere. Smell, taste and colour cannot confirm whether Legionella is present. Assessment must consider the entire water system, not only visibly accessible taps and showers.
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What Are the Most Common Sources of Legionella?
The most important Legionella sources are systems that store or circulate water and create spray, mist or airborne droplets. Domestic properties may contain relatively simple plumbing, while workplaces, hotels, healthcare facilities and leisure premises often have larger, more complex systems with additional risk points.
Common Building Water Sources of Legionella
Showers and Showerheads
Showers can produce fine, breathable water droplets. If warm water has remained stagnant in the pipework or shower hose, contaminated aerosols may be released when the outlet is used. Little-used showers require particular attention, including flushing, cleaning and temperature control.
Taps and Sink Faucets
Taps may become a source of exposure when contaminated water is dispersed through spray, splashing or aerated outlets. Infrequently used taps, flexible hoses and complex mixer fittings can retain water and support biofilm. Regular use and suitable maintenance help reduce stagnation.
Hot-Water Cylinders and Storage Tanks
Incorrectly sized or poorly controlled cylinders and tanks can store water at favourable temperatures. Sediment may collect at the bottom, while inadequate turnover creates stagnant zones. System design, insulation, temperature monitoring and routine inspection are therefore important controls.
Cooling Towers and Evaporative Condensers
Cooling towers and evaporative condensers recirculate water and can disperse aerosols over a wide area. Their scale and aerosol-generation potential make effective treatment, cleaning and monitoring essential. Ordinary dry air-conditioning units without a water system do not present the same risk.
Spa Pools, Hot Tubs and Water Features
Spa pools combine warm water, turbulence, organic contamination and aerosol production. Decorative fountains, humidifiers and indoor water features may create similar exposure routes when maintenance is inadequate. These systems require controls proportionate to their design, use and users.
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How Does Contaminated Water Cause Legionnaires’ Disease?
Legionnaires’ disease is usually contracted by inhaling tiny droplets containing viable bacteria. Exposure does not normally come from simply being near a river or lake, and the illness is not generally passed between people.
Inhaling Aerosols, Fine Mist and Contaminated Water Droplets
An aerosol is a suspension of very small water droplets in the air. Showers, taps, cooling equipment, hot tubs and humidifiers can create droplets small enough to be inhaled deeply into the lungs.
Aspiration is another possible route. This occurs when contaminated water accidentally enters the lungs rather than travelling normally into the stomach. It can be particularly relevant for people who have difficulty swallowing.
Exposure does not mean that illness will always develop. Risk is greater for older adults, smokers and people with weakened immune systems or chronic respiratory conditions. Legionnaires’ disease remains uncommon, but it can be severe and may require hospital treatment.
How Can You Reduce Legionella Risks in a Property?
Effective Legionella control focuses on preventing bacterial growth and limiting exposure. The appropriate measures depend on the property, occupants and water-system complexity. Landlords, property managers and duty holders should maintain proportionate controls and arrange competent assistance where risks cannot be confidently assessed.
- Arrange a competent risk assessment: A competent assessor can review vulnerable occupants, water temperatures, storage, aerosol-generating outlets and maintenance records, then recommend practical and proportionate control measures.
- Maintain safe water temperatures: Keep hot and cold water outside the favourable Legionella growth range wherever reasonably practicable, and check representative outlets to confirm that controls remain effective.
- Flush little-used outlets: Regular flushing replaces stagnant water with fresh water from the circulating system. Record flushing where it forms part of an established water-management procedure.
- Remove dead legs and stagnation: Redundant pipework, capped branches and oversized storage increase retention time. Remove unnecessary sections and improve water turnover where system design permits.
- Clean tanks, outlets and pipework: Control scale, sediment, corrosion and biofilm through suitable inspection, cleaning and maintenance. Showerheads and hoses may require periodic dismantling and descaling.
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Book a Legionella Risk Assessment with LegionellaRiskandPAT.com
At LegionellaRiskandPAT.com, we help property owners and duty holders maintain safe, fully compliant water systems without the hassle. Our qualified specialists conduct thorough assessments to identify hidden bacterial threats, evaluate system risks, and deliver clear, actionable reports tailored to your building.
Whether you manage a residential rental or a commercial facility, we make compliance simple and stress-free. Protect your occupants and ensure total peace of mind—book your professional Legionella risk assessment with our expert team today!
