Can you smell legionella?
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Key points
- Legionella has no smell. Colonised water can smell completely normal, and musty water may contain no legionella at all.
- A musty or stagnant smell is still worth investigating, because the stagnation and biofilm that cause it are the same conditions legionella needs.
- The only reliable detection is a laboratory culture of a water sample, not any of your senses.
- Day-to-day assurance comes from temperature control and flushing, carried out on a schedule and written down, not from sniffing the taps.
No, you cannot smell legionella. The bacteria produce no odour at any concentration found in a water system, and water carrying high numbers of them can smell, taste and look completely normal. This is not a technicality: it is the reason legionella control is built on measurement and routine, not on anybody's senses.
The confusion arises because contaminated water can smell. A musty, earthy or stagnant odour from a tap usually comes from other micro-organisms, decaying organic matter, or water that has stood in a branch or a tank for a long time. Those causes are real warning signs, but they are signs about the general hygiene of the system, not about legionella specifically. Water can smell foul and be legionella-free, and water from a heavily colonised shower head can smell of nothing at all.
Why a smell is still worth chasing
Although the smell is not legionella, the conditions behind it often suit legionella perfectly. Stagnant water in a little-used outlet or a dead leg, a cold water tank that has warmed up in the loft, sediment in the base of a cylinder: each of these produces exactly the warm, still, nutrient-rich water in which legionella multiplies. Treat an unexplained smell as a symptom. Flush the outlet through to temperature, and if the smell persists, trace the source rather than masking it.
What you can check instead of your nose
The practical proxies for legionella risk are all measurable. Hot water should reach at least 50°C at outlets within one minute (55°C in healthcare), cold water should drop below 20°C within two minutes, and any outlet used less than about once a week should be on a weekly flushing regime. Visible scale, slime around outlets, or discoloured water are worth noting as condition indicators, but none of them prove or disprove legionella either. The legionella temperature checker shows how to take readings properly, and what causes legionella explains the conditions behind the numbers.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
How legionella is really detected
The only way to know whether legionella is present is to send a water sample to a laboratory for culture. The laboratory grows whatever legionella is in the sample and reports a count in colony forming units per litre, which is then read against the action levels in the guidance. Sampling is not routine for every building; it is used where the risk assessment calls for it, in higher-risk systems, and after a suspected problem. Legionella sampling and action levels explains when it applies and what the results mean.
You cannot smell, taste or see legionella, so do not try to. Control the conditions it needs, measure the temperatures, and write the checks down. The record, not the nose, is the evidence.
If you are responsible for a water system, the honest position is this: you will never detect legionella directly. What you can do is keep the system outside the 20°C to 45°C growth range, keep water moving, keep spray-producing outlets clean, and record each control in a log book. That routine, set out in the written scheme that comes from your risk assessment, is what manages a risk you cannot sense.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
Does water with legionella in it smell?
No. Legionella itself has no smell, and water carrying even high numbers of the bacteria can smell completely normal. A musty, earthy or stagnant smell in water comes from other micro-organisms and decaying organic matter, not from legionella. Smelly water is a warning sign about the general condition of the system, but fresh-smelling water tells you nothing about legionella one way or the other.
What does legionella smell like?
Nothing. Legionella bacteria are microscopic organisms with no odour that a person can detect, at any concentration found in a water system. There is no smell, taste or appearance associated with their presence. The only way to know whether legionella is in a water system is to have water samples cultured by a laboratory.
Why does my tap water smell musty?
A musty or earthy smell usually points to stagnation, biofilm or organic material somewhere in the system: a little-used outlet, a dead leg, a warm cold-water tank, or sediment in the base of a cylinder. Those are also the conditions that let legionella grow, so a smell is worth investigating as a symptom of poor water hygiene even though the smell itself is not legionella. Run the outlet through to temperature, and if the smell persists, find and deal with the source.
How can you tell if water has legionella?
You cannot tell by looking, smelling or tasting. The only reliable way is laboratory testing: a water sample is taken and cultured, and the lab reports the number of legionella colony forming units per litre. Routine control does not wait for test results, though. It keeps the system outside the conditions legionella needs, hot water hot, cold water cold, no stagnation, and records those checks so the control can be evidenced.
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