Legionella in Water: Where It Lives, How It Grows and What the Action Levels Mean

Legionella is a waterborne bacterium found naturally in rivers and lakes that becomes a risk when it enters a building water system and multiplies. It grows between 20°C and 45°C in stagnant, scale-lined pipework, then spreads through inhaled spray from showers, taps and cooling towers.

Understanding legionella in water systems comes down to three things: where it settles, the conditions that let it multiply, and what the action levels of 100 and 1,000 CFU/litre actually signal. This page is the overview; the sampling and temperature pages hold the detail.

Where legionella lives in a building water system

Outdoors, legionella bacteria live in rivers, lakes and soil, usually at levels too low to make anyone ill. Small numbers arrive in almost every building through the mains supply. The risk only appears when the building's own pipework gives the bacteria somewhere warm and still to settle, feed and multiply.

The reservoirs that matter in a typical hot and cold water system are:

• Hot water cylinders and calorifiers, where water near the base can sit cooler than the stored temperature at the top.
• Cold water storage tanks that are warmed by their surroundings, poorly lidded or rarely turned over.
• Dead legs and blind ends, lengths of pipe serving a removed or little-used fitting that never see flow.
• Showerheads, spray taps and flexible hoses that hold warm water and break it into a fine aerosol.
• Thermostatic mixing valves, which blend hot and cold to a warm outlet temperature that sits inside the growth range close to the tap.
• Infrequently used outlets in spare rooms, void offices and end-of-line taps.

Underneath all of these sits biofilm, the thin slime layer on pipe and tank walls. Biofilm shelters legionella from heat and disinfectant, which is why a system can hold the bacteria for months before a sample ever picks it up.

What causes legionella to grow: temperature, stagnation and nutrients

Three conditions turn a trace of legionella into a colony: warmth, stillness and food.

Temperature is the strongest lever. HSE guidance is clear that legionella multiplies between 20°C and 45°C, stays dormant below 20°C and begins to die above 50°C. That is the reasoning behind the standard control regime: cold water kept below 20°C, hot water stored at 60°C and delivered to the outlet at 50°C, or 55°C in healthcare settings under HTM 04-01. Our temperature control and temperature chart pages set out the full figures.

Stagnation does the rest. Water that does not move cools into the danger band and lets biofilm thicken. Dead legs, oversized tanks and outlets that are rarely opened are the classic culprits.

Nutrients feed the colony. Scale, rust, sludge, sediment and organic matter all give legionella and the other organisms it lives alongside something to grow on.

There is no fixed clock for how long legionella takes to develop in water. Given warmth and stillness, numbers can climb over days to weeks, which is exactly why control has to be continuous rather than a single test.

HSG274 Part 2, HSE

Acceptable levels of legionella in water: what the numbers really mean

Search for "acceptable levels of legionella in water" and you are really asking whether there is a safe number. HSG274 Part 2 sets two figures for hot and cold water systems: 100 and 1,000 colony forming units per litre (CFU/litre). It matters that these are read correctly.

They are action levels, the points at which the guidance tells you to do something, not a pass mark below which the water is declared clean. A result under 100 CFU/litre does not certify the system as safe. It means the sample found the system under control on that day and that you should keep monitoring. This is why many specialists say plainly that there is no safe level of legionella; the bacterium is opportunistic, and a low count can climb once conditions change.

The table below summarises the response HSG274 Part 2 sets out. The detail of how and where to take samples lives on our sampling and action levels page.

How much legionella is dangerous, and who is most at risk

"How much legionella is dangerous" has no single answer, because infection depends on far more than the count in the water. It depends on how much aerosol the outlet produces, how fine the droplets are, how long someone breathes them in, and how susceptible that person is.

HSE identifies groups at higher risk, including people over 45, smokers and heavy drinkers, and anyone with chronic lung, heart or kidney disease, diabetes or a weakened immune system. The same water can be harmless to a healthy adult and dangerous to someone in one of these groups. That variation is why control targets the system rather than a single safe number, and it is why this page cannot tell you a system is safe. This is general information, not medical advice.

How legionella in water reaches your lungs

Legionella in water only becomes a health risk when it is breathed in. Infection happens when someone inhales, or aspirates, tiny aerosol droplets around 5 microns across that carry the bacteria deep into the lungs. Anything that turns contaminated water into a fine spray is a potential source: showers, spray taps, cooling towers, spa pools, humidifiers, garden hoses and vehicle washers.

Two points correct common confusion. Legionnaires' disease does not spread from person to person, and it is not usually caught by drinking water, since the risk is inhaling spray rather than swallowing. That reassurance only goes so far, because aspiration, where water goes down the wrong way into the lungs, can still carry bacteria across, so a heavily contaminated supply is never something to ignore. The same bacteria can also cause the milder, flu-like Pontiac fever, which clears up on its own but still signals that exposure has taken place.

Controlling legionella in your water system

Because legionella grows inside systems people rely on, UK law places a duty on those in control of premises to assess and manage the risk under the Health and Safety at Work etc. Act 1974 and the Approved Code of Practice ACOP L8. The practical response is a chain of recorded checks rather than a one-off fix.

• Start with a legionella risk assessment that maps where water sits, warms and sprays.
• Set out the controls in a written scheme of control.
• Monitor temperatures, and flush little-used outlets to keep water moving.
• Record every reading in a log book, and sample where the assessment calls for it.

Every one of these steps is about recording what you checked and when, so you can show a control regime is being followed. None of them proves the water is clean; they demonstrate that the risk is being managed. If legionella is found, our guide on what to do if legionella is detected sets out the next steps.

You can capture the whole regime in one place with our free legionella risk assessment template, which gives you a structure for logging each control as you go.