Legionella are bacteria that live naturally in water. They become dangerous when a building's water system lets them multiply — warm water, stagnation, sludge and biofilm — and then throws them into the air as a fine mist that people breathe in. Cooling towers are the classic source because they are warm, wet and open to the air. In Hong Kong the EMSD treats below 10 cfu/mL as “undetected”, and a result at or above 1,000 cfu/mL triggers a statutory nuisance notice and emergency decontamination.
What Legionella actually is
Legionella is a genus of bacteria found naturally in rivers, lakes and soil, usually at harmless levels. The problem is not its existence — it is amplification. A building water system can turn a trace population into a large one.
Inhaling it in aerosol form can cause Legionnaires' disease, a serious pneumonia. It is not spread person to person; transmission is by breathing in contaminated droplets or mist.
The four conditions it needs
Legionella multiplies when a system offers it the following. Remove any one and you disrupt the cycle:
- Warmth. Roughly 20–45°C is the growth window. Cooling towers, calorifiers and warm dead-legs all sit in it.
- Stagnation. Still water in dead-legs, idle towers and rarely used outlets.
- Nutrients. Sludge, scale, rust, organic debris — all of it is food.
- Biofilm. The critical one. Biofilm shelters Legionella from biocide, and amoebae living within it act as hosts in which the bacteria replicate.
This is why “we dose biocide” is not by itself an answer. If biofilm and sludge are present, the biocide may never reach the organism. Cleaning is not optional — see cooling tower cleaning.
Why cooling towers are the classic source
A cooling tower is close to a purpose-built Legionella incubator: it holds warm water, collects airborne dirt, and — by design — generates aerosol. Drift escaping the tower can travel well beyond the building, which is why an outbreak is a public health matter and not just a maintenance issue.
Cooling towers are not the only risk. Hot and cold water services, calorifiers, spa pools and humidifiers all appear in the literature. But towers combine amplification and aerosolisation better than anything else in a building.
The Hong Kong numbers
The EMSD regulates fresh water cooling towers through its Code of Practice, and has been able to enter premises and sample at random since 2011. Two thresholds decide your position:
| Result | Status | What happens |
|---|---|---|
| < 10 cfu/mL | “Undetected” | The Code’s routine monitoring band. Continue treatment and monitoring; this result is not a guarantee that Legionella is absent. |
| 10 – 1,000 cfu/mL | Review programme | Investigate, immediate on-line disinfection and a resample; on EMSD's own sample, an advisory letter. |
| ≥ 1,000 cfu/mL | Action level | Statutory nuisance notice, emergency decontamination and re-testing. Non-compliance is an offence. |
EMSD publishes its sampling results periodically — in one December 2024 round, 89 samples were taken from cooling towers across 68 buildings, with one at or above the upper threshold. The sampling is real and it is ongoing; we track every published round in EMSD cooling tower sampling results.
What actually controls it
- Continuous treatment — biocide dosing matched to the system's actual pH (see effects of pH on various biocides, because this is where many programmes quietly fail).
- Cleaning and desludging — removes the food and the biofilm the biocide cannot penetrate.
- Temperature and stagnation control — no idle warm water sitting still.
- Testing — you cannot see Legionella. Only a laboratory test tells you where you stand.
- Records — see below.
Who carries the liability
The building owner and duty holder. Hiring a contractor does not transfer that responsibility. If work is not done properly, or the records are not there to show it was, the exposure stays with the owner.
What protects an owner is two things together: treatment that keeps the water safe, and documents that prove it. See compliance and Legionella control.
Frequently asked questions
What is a safe Legionella level in Hong Kong?
Below 10 cfu/mL is treated as “undetected” under the EMSD Code of Practice for Fresh Water Cooling Towers. A result at or above 1,000 cfu/mL triggers a statutory nuisance notice requiring emergency decontamination and re-testing.
How does Legionella spread?
By inhalation of contaminated aerosol — fine water droplets or mist from sources such as cooling towers, hot and cold water services, spa pools and humidifiers. It is not transmitted from person to person.
Why does Legionella grow in cooling towers?
Because a cooling tower provides everything the bacteria need at once: water in the 20–45°C growth range, sludge and biofilm as shelter and nutrient, and — by design — aerosol that carries it into the air.
References
- EMSD, Regulatory Control of Water Quality inside Cooling Towers. Electrical & Mechanical Services Department, HKSAR.
- EMSD, Code of Practice for Fresh Water Cooling Towers, Part 2: Operation and Maintenance (2023 edition).
- EMSD / Department of Health, Code of Practice for Prevention of Legionnaires' Disease (2021 edition).
- CDC, Controlling Legionella in Cooling Towers. US Centers for Disease Control and Prevention.
- HKSAR Government, EMSD announces latest sampling results for legionella at fresh water cooling towers, 8 January 2025.
Every job includes
- A free site visit and assessment
- A written quotation for your records
- A dated report after every visit
- Work that follows the EMSD Code of Practice
Want to know where your tower stands?
We will sample it, test it, and give you a documented result — plus a programme to manage the identified risks.