What Causes Legionnaires Disease in Buildings?
- 6 hours ago
- 6 min read
Legionnaires’ disease rarely develops because of one major failure.
In most commercial buildings, the risk builds slowly over time through a combination of stagnant water, unstable temperatures, poor water turnover and overlooked maintenance issues.
For facilities managers, that is what makes Legionella control challenging. Problems are often hidden inside systems that appear to be operating normally. A little used shower, poorly insulated pipework, oversized cold water storage tanks or dormant sections of pipe can all create conditions where Legionella bacteria begin to multiply.
Buildings with complex water systems are particularly vulnerable because temperature consistency becomes harder to maintain across every outlet. Add fluctuating occupancy levels, hybrid working patterns and ageing infrastructure into the mix, and the risk profile can increase quickly.
Understanding what causes Legionnaires’ disease in buildings is essential for anyone responsible for compliance, occupant safety and water hygiene management.
This article explains where Legionella develops, why certain systems become high risk, and what facilities managers should focus on to reduce the likelihood of bacterial growth.
What Is Legionnaires’ Disease?
Legionnaires’ disease is a serious form of pneumonia caused by inhaling airborne water droplets containing Legionella bacteria. The bacteria occur naturally in low levels within water sources, but they become dangerous when they are allowed to multiply inside man made water systems.
Unlike many other illnesses, Legionnaires’ disease is not usually spread from person to person. Infection typically occurs when contaminated aerosols are breathed into the lungs through systems such as:
Showers
Cooling towers
Hot and cold water systems
Spa pools
Humidifiers
Decorative fountains
Misting systems
Buildings with large or complex water systems are often at greater risk because there are more opportunities for water stagnation, fluctuating temperatures and underused outlets.
If you want to understand the legal responsibilities around managing these risks, our guide to the current regulations explains the core control measures expected in commercial buildings.
The Main Causes of Legionnaires’ Disease in Buildings
Water Temperatures Between 20°C and 45°C
Temperature control sits at the centre of Legionella prevention.
Legionella bacteria multiply most rapidly between 20°C and 45°C. When water remains within this range for extended periods, microbiological growth becomes far more likely.
In commercial buildings, temperature problems are often caused by operational issues rather than outright equipment failure.
Common examples include:
Inadequate hot water storage temperatures
Incorrect thermostatic mixing valve settings
Poor pipe insulation
Oversized systems with low water turnover
Dead legs and dormant pipework
Failing circulation pumps
Calorifiers operating inefficiently
Poor return temperatures on secondary hot water systems
Even well maintained buildings can develop problems if sections of the system become underused.
Facilities managers should pay particular attention to sentinel outlets because these locations often provide early warning signs that temperatures are drifting outside control limits.
This is one reason why many organisations implement structured water hygiene monitoring programmes to identify failures before they escalate into compliance concerns.
Stagnant Water and Low Use Areas
Stagnation is one of the most common causes of Legionella growth in buildings.
When water sits unused inside pipework or storage systems, disinfectant residuals begin to decline. Temperatures also become less stable, creating favourable conditions for bacterial growth and biofilm formation.
This issue is now more common in commercial buildings because occupancy patterns have changed significantly in recent years.
Hybrid working arrangements, partially occupied office floors and flexible working hours often mean water demand is inconsistent throughout the week. Systems designed for full occupancy may now experience prolonged low flow conditions.
Facilities managers are increasingly finding that dormant outlets, little used washrooms and oversized distribution systems create hidden risks that were not originally anticipated during system design.
Areas commonly affected include:
Empty office spaces
Seasonal accommodation
Closed wards
Spare changing rooms
Vacant apartments
Redundant branches of pipework
Guest facilities with intermittent use
Since the pandemic, concerns around stagnant water systems have increased substantially across the facilities management sector.
Routine flushing programmes remain one of the simplest ways to improve water turnover and reduce stagnation risks.
You may also find our article on how to flush little used outlets useful if you are reviewing your current control measures.
Biofilm, Scale and Sediment
Legionella bacteria rarely exist on their own within building water systems.
In many cases, bacterial growth develops inside layers of biofilm, rust, scale and sediment that accumulate over time on internal surfaces.
Biofilm is particularly problematic because it protects microorganisms from disinfectants while providing nutrients that encourage further bacterial growth.
Older pipework systems are especially vulnerable where corrosion and internal deposits have been allowed to build up gradually.
Common contributing factors include:
Dirty cold water storage tanks
Corroded pipework
Poor turnover within storage vessels
Debris entering damaged tank lids or screens
Scale accumulation inside calorifiers
Infrequent inspection and cleaning schedules
Inconsistent chlorination levels
Facilities managers often underestimate how quickly sediment can accumulate inside neglected tanks and low flow sections of pipework.
This is one reason why regular water tank cleaning and disinfection remains an important part of long term Legionella control.
Aerosol Generation Within Water Systems
Legionnaires’ disease develops when contaminated water droplets become airborne and are inhaled. Systems that generate fine aerosols create a greater risk because droplets can travel through the air and reach deep into the lungs.
Potential aerosol generating sources include:
Showers
Spray taps
Cooling towers
Spa pools
Humidifiers
Pressure washers
Misting systems
This is why even relatively small plumbing systems can become high risk if conditions allow bacteria to multiply.
Which Buildings Are Most at Risk?
Healthcare and Care Environments
Hospitals and care homes are considered higher risk because occupants are more vulnerable to infection. Older people, smokers and individuals with weakened immune systems are more susceptible to severe illness.
Healthcare estates also tend to contain highly complex water systems with large storage volumes and multiple outlets.
Hotels, Leisure Centres and Residential Buildings
Buildings with showers, spa facilities and fluctuating occupancy levels often face increased Legionella risks.
Hotels and residential developments can experience low use periods in certain rooms or apartments, which creates opportunities for stagnation.
Leisure facilities also commonly operate systems that generate aerosols, including spa pools and hydrotherapy equipment.
Commercial Offices and Educational Buildings
Office buildings, universities and schools can develop risk when parts of the building become underused.
Hybrid working arrangements have increased this challenge in many commercial properties. Facilities managers are now often responsible for maintaining systems where occupancy levels change week by week.
If your building has already undergone assessment work, our article covering what facilities managers should do after a Legionella risk assessment explains how to prioritise remedial actions and ongoing control measures.
How Facilities Managers Can Reduce Legionella Risk
Carry Out Regular Legionella Risk Assessments
A Legionella risk assessment is the foundation of any water hygiene programme. It identifies areas where bacteria may develop and highlights the corrective actions required to reduce risk.
Under UK health and safety law, duty holders are expected to assess and control Legionella risks within building water systems.
A suitable assessment should review:
Water temperatures
Tank condition
Pipework layout
Dead legs
Little used outlets
Monitoring records
Existing control measures
Vulnerable occupants
You can explore our dedicated Legionella risk assessment services page for a more detailed breakdown of what a survey typically includes.
Maintain Ongoing Monitoring and Record Keeping
Risk assessments alone are not enough. Buildings require ongoing monitoring to confirm that control measures remain effective.
This usually includes:
Temperature checks
Sentinel outlet monitoring
Inspection schedules
Tank inspections
Flushing records
TMV servicing
Sampling and testing where appropriate
Maintaining accurate records is a key part of demonstrating compliance with ACOP L8 guidance.
Complete Remedial Works Promptly
Many Legionella problems are linked to ageing infrastructure and poorly designed systems.
Common remedial works include:
Dead leg removals
Pipework alterations
Tank upgrades
Valve replacements
Improved insulation
System balancing
TMV replacement
Leaving these issues unresolved often increases long term compliance costs.
Our guide to Legionella remedial works explains how these improvements help reduce risk across commercial water systems.
Final Thoughts
Legionnaires’ disease in buildings is usually linked to a combination of stagnant water, unstable temperatures, aerosol generation and inadequate system maintenance.
Most failures are not caused by dramatic breakdowns.
In reality, Legionella risks tend to develop quietly through inconsistent monitoring, low use outlets, ageing infrastructure and gradual loss of temperature control.
For facilities managers, the most effective approach is a proactive one built around regular risk assessments, planned monitoring, flushing regimes, cleaning schedules and timely remedial works.
Buildings with complex water systems require ongoing attention because risks change as occupancy patterns, water usage and infrastructure conditions evolve.
The organisations that manage Legionella risk successfully are usually the ones that treat water hygiene as an ongoing operational responsibility rather than a one off compliance exercise.
If you would like to explore the wider subject in more detail, visit our Water Hygiene Services section for further guidance on Legionella control, compliance responsibilities and commercial water system management.
You may also find our article on how to test for Legionella bacteria in water helpful if you are reviewing your current sampling strategy.
