Legionella is a type of bacteria that lives in water and can cause serious lung infections when inhaled. In the UK, legionella in building water systems is a significant health and safety concern, particularly in commercial premises, healthcare facilities, and rental properties.
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What is legionella bacteria?
Legionella is a genus of bacteria that occurs naturally in environmental water sources like rivers, lakes, and reservoirs. There are over 60 species of legionella, but Legionella pneumophila is responsible for about 90% of human infections.
The bacteria become a health hazard when they:
- Multiply in artificial water systems to dangerous levels
- Become aerosolised (turned into tiny water droplets or mist)
- Are inhaled into the lungs
You cannot catch legionella by drinking contaminated water or from person-to-person contact. The only way to contract Legionnaires' disease is by breathing in water droplets containing the bacteria.
Legionnaires' disease and Pontiac fever
Legionella bacteria cause two distinct illnesses:
Legionnaires' disease
A severe form of pneumonia (lung infection) that can be fatal. Symptoms typically appear 2-10 days after exposure and include:
- High fever (above 38°C)
- Chills and shivering
- Persistent cough
- Shortness of breath
- Chest pain
- Headache
- Muscle aches
- Confusion or altered mental state
Fatality rate: Around 10% of cases are fatal, rising to 40-80% in vulnerable individuals without prompt treatment.
Pontiac fever
A milder, flu-like illness without pneumonia. Symptoms develop within hours to 2 days and include:
- Fever
- Headache
- Muscle aches
- Fatigue
Most people recover fully within 2-5 days without treatment. Pontiac fever is much less common and less well understood than Legionnaires' disease.
Legionnaires' disease is notifiable in the UK. Doctors must report suspected cases to Public Health England (or equivalent bodies in Scotland, Wales, and Northern Ireland) within 24 hours.
Where does legionella grow?
Legionella bacteria thrive in specific conditions commonly found in building water systems:
The legionella growth zone
Temperature: Legionella multiplies rapidly between 20°C and 45°C, with optimal growth at around 35-37°C (human body temperature).
- Below 20°C: Growth slows significantly (dormant but not killed)
- Above 60°C: Bacteria are killed within minutes
- Above 50°C: Growth is prevented, but bacteria survive
This is why temperature control is the primary defence against legionella.
Water conditions that promote growth
Stagnation:
- Water sitting unused in pipes, tanks, or equipment
- Infrequent use of taps, showers, or outlets
- Dead legs (unused pipe branches)
- Oversized water storage tanks
Nutrients:
- Biofilm (slime) on pipe surfaces
- Rust, sludge, and scale deposits
- Organic matter like algae
Environment:
- Warm water storage systems
- Complex pipework with multiple branches
- Poor water circulation
- Poorly maintained systems
Legionella doesn't grow in mains water supply because the water is constantly flowing and chlorinated. The risk emerges when water enters building systems where it can stagnate and warm up.
High-risk water systems
Certain water systems pose higher legionella risks:
Cooling towers and evaporative condensers
These systems deliberately spray water into the air, creating aerosols. They're one of the most common sources of legionella outbreaks and are heavily regulated in the UK.
Hot and cold water systems
- Hot water storage tanks (calorifiers)
- Water heaters and boilers
- Showers and taps (especially if rarely used)
- Large buildings with complex pipework
- Buildings with long pipe runs
Spa pools and hot tubs
The combination of warm water (typically 30-40°C) and water jets that create aerosols makes spa pools particularly high risk.
Other systems
- Humidifiers and air washers
- Emergency showers and eye wash stations
- Garden hoses and sprinkler systems
- Water features and fountains
- Vehicle washing equipment
- Dental equipment water lines
Legionella Control Schedule
Run taps and showers that are used less than once a week for 2 minutes
Check hot water is stored at 60°C+ and cold water below 20°C
Document temperature readings and control measure checks
Visual inspection of tanks, pipe insulation, and dead legs
Full review of legionella risk assessment and control measures
Laboratory testing if the risk assessment identifies it as necessary
Who is at risk?
Anyone can contract Legionnaires' disease, but certain groups face higher risk:
High-risk individuals
- People aged 50+ (risk increases with age)
- Smokers and heavy drinkers
- People with chronic lung disease (COPD, emphysema)
- People with weakened immune systems (cancer patients, HIV/AIDS, organ transplant recipients)
- People with chronic heart, liver, or kidney disease
- People with diabetes
High-risk settings
- Healthcare facilities (hospitals, nursing homes)
- Care homes for the elderly
- Hotels and accommodation
- Leisure centres with spa pools
- Buildings that have been unoccupied (water stagnation)
- Industrial sites with cooling towers
Barrow-in-Furness legionella outbreak (2002)
An outbreak of Legionnaires' disease in Barrow-in-Furness, Cumbria, resulted in 7 deaths and over 170 people becoming ill. The source was traced to a poorly maintained air conditioning cooling tower at a local authority building.
- ✗Cooling tower not properly maintained or cleaned
- ✗No adequate legionella risk assessment
- ✗Poor understanding of legionella risks by duty holders
- ✗Lack of competent person to manage water systems
- ✗No routine monitoring or treatment programme
The council's architect was prosecuted and fined £15,000. The incident led to significant changes in UK legionella legislation and the introduction of mandatory registration for cooling towers.
Cooling towers require specialist expertise and rigorous maintenance. Even in public sector organisations, inadequate attention to legionella risks can have tragic consequences.
How does legionella spread?
Legionella spreads when contaminated water is turned into small droplets (aerosols) that can be breathed deep into the lungs.
Common sources of exposure:
Showers:
- The most common source in outbreaks
- Shower heads create fine mist that can be inhaled
- Infrequently used showers in hotels, care homes, or empty properties are particularly high risk
Cooling towers:
- Release water droplets into the air
- Can affect people up to several kilometres away
- Have caused major community outbreaks
Spa pools and hot tubs:
- Warm water temperature ideal for legionella growth
- Bubbles and jets create aerosols
- Both public and domestic spa pools have been linked to cases
Other aerosol sources:
- Car washes and pressure washers
- Water features and decorative fountains
- Humidifiers and misters
- Industrial spraying equipment
Legionella does not spread through drinking water, from person to person, or through water that doesn't become aerosolised. You cannot catch it from swimming pools (which are well-chlorinated) or from drinking tap water.
Prevention and control measures
Preventing legionella growth requires a systematic approach:
Temperature control (primary defence)
Hot water:
- Store at 60°C or higher
- Deliver at outlets at 50°C or higher (within 1 minute)
- Caution: Risk of scalding, especially for vulnerable users
Cold water:
- Store and distribute below 20°C
- Insulate cold water pipes to prevent warming
- Keep tanks in cool locations away from heat sources
Water turnover
- Minimise dead legs (unused pipe branches)
- Flush little-used outlets weekly
- Consider removing rarely used outlets
- Size water tanks appropriately to prevent stagnation
- Regular use is better than occasional flushing
System cleanliness
- Prevent biofilm, scale, and corrosion
- Clean and disinfect water tanks annually
- Descale shower heads and taps
- Remove debris from cold water tanks
- Keep lids on water tanks to prevent contamination
Design and maintenance
- Regular inspection and maintenance
- Keep systems simple with minimal pipework
- Insulate pipes correctly (hot stays hot, cold stays cold)
- Replace damaged or corroded components
- Drain down systems that won't be used for extended periods
Legionella Control: Domestic vs Commercial
Simple Domestic Systems
- •Mainly reliance on temperature control
- •Regular use prevents stagnation
- •Simple flushing routine for unused outlets
- •Annual boiler service covers hot water
- •Lower regulatory requirements
- •Risk assessment not always legally required
Commercial/Complex Systems
Recommended- •Formal legionella risk assessment required
- •Written control scheme and logbooks
- •Appointed responsible person
- •Regular temperature monitoring and recording
- •Professional cleaning and disinfection
- •May require water sampling and testing
Bottom line: Domestic properties with simple water systems and regular use have lower legionella risk. Commercial premises, landlords, care facilities, and any premises with complex water systems must implement formal control measures and maintain detailed records.
UK legal requirements
Health and Safety at Work etc. Act 1974
Employers and those in control of premises have a general duty to ensure health and safety, including managing legionella risks.
Control of Substances Hazardous to Health Regulations 2002 (COSHH)
Legionella bacteria are classified as a biological agent. COSHH requires duty holders to:
- Identify and assess the risk of exposure
- Prevent or control the risk
- Ensure control measures are used and maintained
- Monitor and maintain the effectiveness of control measures
- Keep records of the assessment and control measures
- Provide information, instruction, and training
L8 Approved Code of Practice
The HSE's L8 ACOP provides detailed guidance on managing legionella risks. Key requirements:
- Risk assessment: Identify and assess sources of risk
- Written control scheme: Document how you will manage risks
- Responsible person: Appoint someone to manage legionella control
- Monitoring: Regular checks and record-keeping
- Competence: Ensure those managing risks have adequate training and knowledge
Notification of Cooling Towers and Evaporative Condensers Regulations 1992
Requires all cooling towers and evaporative condensers to be notified to the local authority within one month of first use, and records kept for at least two years.
Landlords have specific duties under L8. Even for simple domestic rental properties with hot and cold water systems, landlords should carry out a legionella risk assessment and implement identified control measures.
Legionella statistics in the UK
Case numbers
- Average annual cases: 300-400 confirmed cases in England and Wales
- Actual cases likely higher: Many mild cases go undiagnosed
- Fatality rate: Approximately 10% of reported cases
- Peak season: Late summer and early autumn (July to October)
- Gender: Men affected more than women (ratio approximately 2.5:1)
- Age: Over 80% of cases occur in people aged 50+
Recent trends
- Cases have been relatively stable over the past decade
- Hospital-acquired cases have decreased due to improved controls
- Community-acquired cases (from unknown sources) remain a challenge
- Travel-associated cases account for 15-20% of UK cases
Outbreak statistics
- Most cases are sporadic (isolated incidents)
- Outbreaks (2+ linked cases) occur several times per year
- Cooling towers remain a common source of larger outbreaks
- Healthcare facilities and hotels are frequent outbreak settings
Many legionella cases are classified as "community-acquired" with no identified source. This highlights the importance of proper water system management across all sectors, not just high-profile facilities.
Common misconceptions
"I don't have a cooling tower, so I don't need to worry"
Wrong. While cooling towers are high risk and heavily regulated, most legionella cases in the UK are linked to hot and cold water systems, particularly showers. Any premises with water systems should consider legionella risk.
"Legionella testing is required by law"
Not always. The law requires a risk assessment and control measures. Water sampling is only necessary if your risk assessment identifies it as needed to demonstrate control. Over-reliance on testing instead of proper control measures is a common mistake.
"Chlorination solves everything"
Not quite. While chlorination helps, it's not suitable for all systems. Hot and cold water systems in buildings typically rely on temperature control, not chemical treatment. Biofilm can also protect legionella from disinfectants.
"My water is always running, so there's no risk"
Partially true. Regular water turnover does reduce risk significantly, but it doesn't eliminate it. Temperature control and system cleanliness are still essential, even in well-used systems.
"Only old buildings have legionella problems"
False. New buildings can have legionella issues too, especially during and immediately after construction when water systems may be stagnant, or if systems are poorly designed with dead legs and oversized tanks.
When to review your legionella controls
Review your legionella risk assessment and control measures:
- At least every 2 years (good practice, or more frequently if the risk assessment indicates)
- After changes to the water system (new equipment, alterations, extensions)
- After changes to building use (occupancy changes, different activities)
- After a case of Legionnaires' disease linked to your premises
- If monitoring suggests controls are failing (temperatures out of range, positive test results)
- After periods of shutdown (building closures, COVID-19 lockdowns)
COVID-19 building closures and legionella risk
During the COVID-19 pandemic, many buildings were closed or partially occupied for extended periods. Stagnant water in unused systems created increased legionella risks.
- ✗Water systems left unused for weeks or months
- ✗No flushing routines maintained during closure
- ✗Inadequate planning for reopening buildings
- ✗Staff unaware of increased legionella risks
- ✗Insufficient flushing before re-occupancy
HSE issued specific guidance for building reopening. Many organisations had to implement intensive flushing programmes and remedial cleaning. Some buildings required professional disinfection before safe re-occupation.
Building closures, even temporary ones, require active legionella management. Simply locking the doors and walking away can create serious health risks when you return.
Frequently asked questions
Yes, although the risk is generally lower than in larger commercial buildings. Domestic cases can occur, particularly from poorly maintained hot water systems, infrequently used showers, or domestic spa pools. Landlords should carry out legionella risk assessments for rental properties.
You cannot see, smell, or taste legionella in water. The only way to detect it is through laboratory testing of water samples. However, routine testing isn't always necessary - proper control measures (temperature control, preventing stagnation, system cleanliness) are more important than testing.
Not necessarily. The law requires a risk assessment and control measures, not routine testing. Testing is only required if your risk assessment identifies it as necessary to verify that controls are working. Many simple systems never need testing if temperature control and flushing routines are properly maintained.
There's no fixed legal requirement. If your risk assessment indicates testing is needed, frequency depends on the system type and risk level. Typical ranges: high-risk systems quarterly, moderate-risk annually, low-risk every 2 years or less. Many simple systems don't require routine testing at all.
Yes, legionella bacteria are killed by sustained high temperatures. Water at 60°C will kill legionella within minutes, and boiling (100°C) kills it instantly. However, boiling water at the point of use doesn't solve the problem if bacteria are growing in your water system.
No. Legionnaires' disease cannot be transmitted from person to person. You can only catch it by breathing in water droplets containing the bacteria. This means infected individuals don't need to be isolated from others.
Someone competent - meaning they have adequate training, knowledge, and experience to understand legionella risks and control measures. For simple systems, this might be you (with appropriate training). For complex systems, use a qualified water hygiene specialist.
Look for qualifications from recognised bodies such as the Legionella Control Association (LCA), City & Guilds legionella qualifications, or consultants with demonstrable experience. There's no single required qualification, but membership of professional bodies demonstrates competence.
Next steps
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