What Is Legionella?
Legionella pneumophila is a bacterium that causes Legionnaires disease, a severe and potentially fatal form of pneumonia. It thrives in warm water systems (20-45 degrees C) and is transmitted through inhaling water droplets (aerosols), not through drinking water. Building water systems are a primary source of outbreaks.
High-Risk Water Systems in Buildings
| System | Risk | Aerosol Source | Control |
|---|---|---|---|
| Cooling towers | Very High | Drift/spray | Chemical treatment, testing |
| Hot water systems | High | Showers, taps | Temperature control (60C storage, 50C distribution) |
| Cold water tanks | Medium-High | Taps if water warms above 20C | Insulation, turnover |
| Spa pools / fountains | High | Water jets, spray | Disinfection, filtration |
| Fire sprinkler wet systems | Low-Medium | Only on activation | Annual flushing |
| Dead legs / unused outlets | High | Stagnant water | Remove dead legs, weekly flush |
Temperature Control — The Primary Defence
| Parameter | Target | Action If Failed |
|---|---|---|
| Hot water storage (calorifier) | 60C minimum | Investigate and restore immediately |
| Hot water distribution | 50C at furthest outlet within 1 min | Flush, check circulation pump |
| Hot water return | 50C minimum | Check recirculation pump |
| Cold water storage | Below 20C | Check insulation, location |
| Cold water outlets | Below 20C within 2 min | Investigate warm-up source |
| Danger zone | 20-45C is Legionella growth zone | No system should operate here |
Monitoring Programme
| Action | Frequency | Who | Record |
|---|---|---|---|
| Hot water temp (sentinel outlets) | Monthly | FM/Water team | Temperature log |
| Cold water temp (sentinel outlets) | Monthly | FM/Water team | Temperature log |
| Calorifier temperature | Monthly | FM/Water team | Temperature log |
| Cooling tower dip slide | Weekly | Water treatment co. | Test certificate |
| Cooling tower lab test | Quarterly | Accredited lab | Lab certificate |
| Infrequent outlet flushing | Weekly minimum | FM team | Flushing log |
| Dead leg audit | Annually | Water hygiene consultant | Audit report |
| Full Legionella risk assessment | Every 2 years | Qualified assessor | Risk assessment report |
Cooling Tower Management
Cooling towers are the highest-risk Legionella source in buildings. They create aerosols that can travel several kilometres.
- Registration: Many jurisdictions require cooling tower registration with health authorities.
- Chemical treatment: Continuous biocide dosing, corrosion and scale inhibitors.
- Blowdown: Regular blowdown to maintain water quality (conductivity control).
- Cleaning: Annual shutdown, drain, clean, and disinfect. Inspect fill pack, drift eliminators, and basin.
- Drift eliminators: Must be in place and in good condition, reducing aerosol emission by 99.9%.
- Log book: Daily checks, weekly dip slides, monthly water chemistry, quarterly lab tests.
What to Do If Legionella Is Detected
| Count (CFU/L) | Action | Timeline |
|---|---|---|
| Below 100 | Satisfactory, continue routine monitoring | — |
| 100-1,000 | Review controls, retest in 2-4 weeks | Immediate review |
| Over 1,000 | Immediate investigation and corrective action | Same day |
| Over 10,000 | Emergency shutdown, full disinfection, notify health authority | Immediate |
References
Insights & Guidance
- Legionella thrives at 20-45C. Hot water must be stored at 60C minimum and distributed at 50C minimum. Cold water must stay below 20C.
- Cooling towers are the highest-risk source. Weekly dip slides, quarterly lab tests, continuous chemical treatment required.
- Dead legs and infrequently used outlets must be flushed weekly.
- Legionella count over 1,000 CFU/L requires immediate investigation. Over 10,000 CFU/L requires emergency shutdown.
- Full Legionella risk assessment every 2 years by a qualified assessor.
Legionnaires disease has a fatality rate of 5-10% in healthy people and up to 25% in immunocompromised individuals. Building owners and managers have a legal duty to manage Legionella risk. A single outbreak can cause multiple deaths and criminal prosecution.
- Temperature drift — calorifier thermostat failure allows storage temperature to drop into the growth zone.
- Dead legs — removed fixtures leave stagnant pipe sections that become Legionella incubators.
- Cooling tower neglect — skipped treatment allows bacterial colonies to establish and aerosolise.
- Legionella risk assessment (within last 2 years)
- Monthly temperature monitoring logs
- Weekly flushing log for infrequently used outlets
- Cooling tower treatment records and lab certificates
- Is the calorifier temperature at 60C or above?
- Are there dead legs or capped-off outlets?
- Is the cooling tower drift eliminator in good condition?
- When was the last Legionella risk assessment?
- What are the current hot and cold water temperatures at sentinel outlets?
- Are all infrequently used outlets on a flushing schedule?
- Risk assessment — qualified Legionella risk assessor.
- Water treatment — specialist water treatment company.
- Outbreak response — public health authority.