LEGIONELLA
PREVENTION GUIDE
A practical reference for facility managers, engineers and compliance officers: risk factors, control measures, action levels and Italian regulatory requirements — all in one place.
What is Legionella?
A gram-negative bacterium found in freshwater environments, capable of causing severe pneumonia when inhaled as a respirable aerosol.
Legionella pneumophilaserogroup 1 is responsible for approximately 70–80% of Legionnaires' disease cases in Europe. The bacterium is naturally present in aquatic environments at low concentrations but proliferates in man-made water systems when temperature, stagnation and nutrient conditions allow.
In Italy, Legionella is a notifiable disease under the National Communicable Disease Surveillance System (SIMI). The Italian National Institute of Health (ISS) publishes annual epidemiological reports documenting approximately 1,500–2,000 notified cases per year — likely a significant underestimate due to under-diagnosis.
Six principal risk factors
Understanding which conditions favour Legionella proliferation is the starting point for any effective control plan.
Water temperature between 20 °C and 45 °C
Legionella bacteria grow optimally between 25 °C and 42 °C. Hot-water circuits that cool below 50 °C or cold-water circuits that warm above 20 °C create ideal proliferation conditions.
Stagnant water and dead-legs
Low or zero-flow sections of pipework allow biofilm to develop. Dead-legs — pipe runs ending in capped tees or unused outlets — are a primary reservoir.
Biofilm and scale deposits
Biofilm provides nutrients and protection for Legionella. Scale, corrosion products and sediment further reduce the efficacy of chemical disinfection.
Aerosol generation
Showers, cooling towers, spa jets, dental unit water lines and humidifiers generate respirable droplets (< 5 µm) that carry Legionella into the lower respiratory tract.
Complex or ageing water systems
Long distribution networks, old galvanised pipework, mixing valves set too low and underused outlets increase the difficulty of maintaining effective control.
Seasonal shutdowns and reopenings
Seasonal facilities — hotels, campsites, sports centres — are at elevated risk when reopening after periods of water stagnation. Flushing and sampling before reopening is mandatory.
Technical interventions that work
Effective Legionella control combines physical measures, chemical treatment and routine monitoring — no single measure is sufficient on its own.
Hot water ≥ 60 °C at the heater, ≥ 50 °C at all outlets
Maintaining hot water storage above 60 °C and distribution above 50 °C is the primary physical control. Cold water must be kept below 20 °C. Thermostatic mixing valves must be installed as close as possible to the point of use.
Hyperchlorination and continuous chlorination
Shock hyperchlorination (2–5 mg/L free chlorine for 1–2 hours) is the standard emergency disinfection procedure. Continuous chlorination (0.2–0.5 mg/L) is used as a preventive measure in complex systems. Documentation of dosing and contact time is mandatory.
Electrochemical ion dosing for large networks
Copper (0.2–0.8 mg/L) and silver (0.02–0.08 mg/L) ions penetrate biofilm and provide residual protection. Widely used in hospitals and large hotel chains. Requires regular calibration and water quality monitoring.
Point-of-entry UV treatment
UV irradiation at ≥ 40 mJ/cm² inactivates Legionella at the point of entry into the distribution system. It provides no residual protection and must be combined with temperature control or chemical treatment downstream.
Regular flushing of low-use outlets
Outlets used less than weekly must be flushed for at least two minutes to remove stagnant water. Flushing logs are required documentation for health authority audits.
Descaling, decommissioning and pipe replacement
Annual descaling of water heaters and showerheads, removal of true dead-legs and replacement of corroded or biofilm-colonised pipework are physical interventions that chemical disinfection cannot compensate for.
Action levels for Legionella in water
Thresholds defined by the Italian National Institute of Health (ISS) in the State-Regions Agreement 2015 guidelines. Applicable to samples analysed by the UNI EN ISO 11731:2017 culture method.
| Result (CFU/L) | Risk level | Required action |
|---|---|---|
| < 100 CFU/L | Acceptable | No immediate action required. Continue routine monitoring at scheduled frequency. |
| 100 – 999 CFU/L | Alert | Review water management controls. Carry out corrective maintenance (temperature checks, flushing, descaling). Re-sample within 4–6 weeks. |
| 1,000 – 9,999 CFU/L | Action required | Immediate remediation: shock hyperchlorination or equivalent. Mandatory notification to health authorities in healthcare and nursing-home settings. Re-sample to confirm efficacy. |
| ≥ 10,000 CFU/L | Critical | Implement facility shutdown protocol (risk-exposed areas). Notify competent health authority. Carry out emergency disinfection and structural investigation. Re-open only after negative re-sampling. |
Source: Accordo Stato-Regioni 7 maggio 2015 — Linee guida per la prevenzione ed il controllo della legionellosi (ISS). Thresholds apply to quantitative culture results; PCR-based methods require separate interpretation.
Legal framework →
Italian and EU regulations on Legionella prevention.
Testing services →
qualified sampling and risk assessment for your facility.
Technical glossary →
40+ Legionella terms defined in plain English.
Prevention: frequently asked questions
Technical answers on temperature thresholds, transmission routes, sampling frequency and disinfection efficacy.
At what temperature does Legionella grow?
Legionella pneumophila grows between approximately 20 °C and 50 °C, with optimal proliferation between 35 °C and 42 °C. Above 60 °C the bacteria are killed within minutes; below 20 °C growth is effectively inhibited. Italian guidelines require hot water to be stored at ≥ 60 °C and to reach every outlet at ≥ 50 °C.
How is Legionella transmitted?
The primary route is inhalation of contaminated aerosols — fine water droplets smaller than 5 µm that reach the alveoli. Sources include showers, cooling towers, spa jets, dental unit water lines and decorative fountains. Person-to-person transmission has not been documented.
What is the difference between Legionellosis and Legionnaires' disease?
Legionellosis refers to all clinical forms caused by Legionella bacteria. Legionnaires' disease is the severe pneumonic form, with a case-fatality rate of 5–15% in community settings and up to 30% in immunocompromised patients. Pontiac fever is a milder, self-limiting febrile illness also caused by Legionella but without pneumonia.
How often should water be sampled for Legionella?
Sampling frequency is determined by the risk assessment outcome. Italian guidelines specify: at least annually for lower-risk settings; bi-annually or quarterly for medium-risk; quarterly or monthly for high-risk settings such as hospitals and nursing homes. Sampling is always required before reopening after seasonal closure and after any remediation procedure.
Can hyperchlorination alone eliminate Legionella from a water system?
Hyperchlorination is effective against planktonic Legionella but has limited penetration into mature biofilm. It must be combined with physical interventions (descaling, temperature control, flushing) and followed by routine monitoring. It is not a permanent solution when root causes such as dead-legs or chronic low temperatures are not corrected.