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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.
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.
Legionella multiplies when a system offers it the following. Remove any one and you disrupt the cycle:
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.
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 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 safe range. A working programme holds you here. |
| 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.
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.
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.
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.
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.
We will sample it, test it, and give you a documented result — plus a programme that keeps it in the safe range.