Legionella is a waterborne bacterium that lives, harmlessly, in rivers and lakes almost everywhere. It only becomes a building problem when two things line up: water that sits warm and still long enough for the bacteria to multiply, and a way for that water to be breathed in as a fine mist. Break either link and the risk drops sharply. That single idea will carry you through most of what follows.
If you have just been handed responsibility for a building’s water — a new estates role, a landlord duty, a facilities handover — you do not need a microbiology degree. You need to know where warm water stagnates, where it can turn into aerosol, and who might inhale it. Everything else is detail hung on that frame.
The bacterium, in plain terms
There are many species of Legionella, but Legionella pneumophila causes most human illness. The bacteria thrive inside the slimy film — biofilm — that coats the inside of pipes, tanks and shower hoses, and they feed on the scale, sludge and rust that collect in neglected systems.
Temperature is the lever you can actually pull. As a general rule reflected in HSE guidance, Legionella multiplies fastest in roughly the 20–45°C band, sits dormant below about 20°C, and is progressively killed above about 60°C [1]. That is why “keep hot water hot and cold water cold” is not a slogan — it is the control. Tepid is the danger zone, and tepid is exactly what you get in a long pipe run, an oversized cold tank in a warm plant room, or a hot return that has lost its heat by the time it reaches a far outlet.
Inhalation is the other half. Drinking contaminated water does not give you Legionnaires’ disease; breathing in contaminated droplets can. So the high-risk fittings are the ones that make mist — showers, spray taps, cooling towers, spa pools, some hoses and washers. A tap that fills a bucket is far less interesting than a shower nobody has used in three weeks.
Why this is a workplace issue, not just a health fact
Legionnaires’ disease is a serious form of pneumonia. The NHS describes it as a lung infection caught from inhaling tiny droplets of contaminated water, more likely to be severe in older people, smokers, and those with weakened immune systems or existing lung conditions [2]. There is also a milder, flu-like illness called Pontiac fever; the CDC distinguishes the two and notes that infection follows inhaling mist rather than spreading person to person [3].
For a duty holder, the useful takeaway is narrow and practical: your job is to stop the bacteria multiplying and stop the aerosol reaching people. Diagnosis and treatment belong to clinicians. Exposure prevention belongs to you.
Myth vs reality
The fastest way to get Legionella wrong is to trust the wrong instinct. A few that catch people out:
| What people assume | What’s actually true |
|---|---|
| Clear, clean-looking water is safe water | The bacteria live in biofilm on pipe walls and inside fittings — invisible from the outlet. Clarity tells you almost nothing |
| One clean lab sample means the system is fine | A negative sample describes one outlet at one moment. It is evidence, not a clean bill of health for the whole system |
| It’s the water company’s responsibility | Mains water arrives compliant. What happens to it inside your building — storage, temperature, stagnation — is the duty holder’s responsibility |
| Only big or old buildings are at risk | A single low-use shower in a new building can be a higher risk than a busy plant room. Use pattern matters more than age or size |
| Our contractor handles all of it | A contractor carries out tasks. Accountability, oversight and the records that prove control stay with you |
The one thing beginners get wrong
New duty holders tend to fixate on testing. Sampling feels like proof. But a lab result is a snapshot, and a clean snapshot from a system you do not actually control is false comfort. The real work is unglamorous: keeping hot water hot, cold water cold, water moving, and tanks and fittings clean — then keeping records that show you did. Control first, verification second. If you only remember one ordering, make it that one.
Where to start this week
You do not need a programme on day one. You need a walk-round and a notebook.
- Find the cold water storage tank and feel whether it is genuinely cold; note if it sits somewhere warm.
- Check the hot water leaves the cylinder or calorifier hot and still arrives hot at the furthest tap.
- List every shower, spray tap and hose, and ask which ones rarely get used.
- Ask the obvious human question: who would be standing in the mist if any of these went wrong?
That list is the seed of a proper risk assessment, which is where the formal duties begin — see UK Legionella compliance 101 for who is legally on the hook and why. If you want the temperature side in detail, Temperature control basics for Legionella prevention covers the numbers and how to hit them.
FAQ
Is Legionella the same thing as Legionnaires’ disease?
No. Legionella is the bacterium; Legionnaires’ disease is the severe lung infection some people develop after inhaling contaminated droplets. The same bacterium can also cause the milder Pontiac fever [3].
Can you see or smell Legionella in the water?
No. It lives in biofilm and sediment inside the system and gives no taste, smell or colour. That is precisely why control depends on managing conditions — temperature, movement, cleanliness — rather than on how the water looks.
What temperature does Legionella grow at?
Guidance generally puts the active growth range at about 20–45°C, with the bacteria dormant below 20°C and killed off above around 60°C [1]. Confirm the exact figures and how they apply to your system through your risk assessment.
Is my building too small to worry about it?
Probably not. Risk follows stagnant warm water and aerosol-producing fittings, not floor area. A small site with an unused shower can carry real risk; the duties under UK law apply regardless of size.
Sources
[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems — Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] NHS, “Legionnaires’ disease”. https://www.nhs.uk/conditions/legionnaires-disease/ [3] CDC, “About Legionnaires’ Disease”. https://www.cdc.gov/legionella/about/index.html