--- title: "How long can Legionella survive in water?" source_url: https://legionella.io/articles/how-long-can-legionella-survive-in-water/ canonical_url: https://legionella.io/articles/how-long-can-legionella-survive-in-water/ pillar: "Legionella Basics & Science" summary: "Legionella doesn't just survive in water - in warm, still conditions it multiplies and can persist indefinitely in biofilm. What actually controls it." primary_keyword: "how long does Legionella survive" date_published: 2026-04-04 date_reviewed: 2026-06-26 author: "Legionella.io editorial team (REMOTE TECH LTD)" reviewed_against: "HSE L8 and HSG274 guidance" region: "United Kingdom" license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url." --- # How long can Legionella survive in water? In the right conditions, Legionella does more than survive in water — it multiplies, and it persists for as long as those conditions hold. In a warm, neglected system that means effectively indefinitely. There is no clean expiry date you can wait out [1]. That is the honest answer to a fair question. But the question itself points at the wrong worry. Legionella does not sit in your pipes counting down a timer. It waits, and wherever the water is warm and still it grows. So the version of the question worth asking is not *how long does it last*, but *does my system keep handing it the two things it needs* — warmth and stagnation. Get those two under control and the bacteria have nowhere to build up. Leave them unmanaged and "how long" becomes irrelevant, because the population keeps renewing itself. ## Cold water holds it, but doesn't clear it Cold does not kill Legionella. It only stops it growing. Below about 20°C the bacteria are dormant: present, alive, but not multiplying. Warm that same water back into the growth range and they pick up where they left off [2]. This is why UK guidance treats "cold" as genuinely cold — kept below 20°C at the outlet where practicable — rather than just cooler than the hot supply [2]. A cold tank sitting in a warm plant room, or a cold pipe run alongside a hot one, can drift up past 20°C and quietly become hospitable. So a long spell of cold storage is not a cleanse. It is a pause. ## The warm, still water where it actually thrives The danger range sits roughly between 20°C and 45°C, with multiplication fastest around the middle of that band [1]. This is the temperature of lukewarm pipework, an under-set calorifier, a tepid storage tank, or cold water that has warmed up in summer. It is also why understanding the link between temperature and growth matters more than memorising a single number — [Temperature and Legionella growth](https://legionella.io/articles/temperature-and-legionella-growth-understanding-the-relationship/) sets that relationship out in full. Warmth alone is not the whole picture. Legionella also needs something to feed on and somewhere to settle. Stagnant water lets sediment, rust and organic debris collect at the bottom of tanks and in the bends of little-used pipes, and that debris is exactly what the bacteria use as nutrients [3]. Warm plus still plus dirty is the combination that turns a trickle of bacteria into a problem. ## Heat is what ends it The reliable way to end Legionella in water is heat. Above roughly 60°C it dies within minutes, and the kill becomes near-instant higher still [2]. That is the principle behind hot-water storage temperatures and thermal disinfection. The exact figures and what each one does are worth knowing precisely, so rather than restate them here, [What temperature kills Legionella?](https://legionella.io/articles/what-temperature-kills-legionella/) lays out the kill temperatures and how to turn them into daily targets. The short point for now: cold suspends the bacteria, the middle band feeds them, and only proper heat actually removes them. ## Why a single flush doesn't reset the clock Here is where the "how long does it survive" framing really breaks down. Legionella rarely floats freely in clean water for long. It lives inside **biofilm** — the thin, slimy layer that forms on the inside of pipes, tanks, shower hoses and tap fittings — and within the scale and sediment that build up over time [3]. Biofilm is protective. It shields the bacteria from temperature swings and from water treatment, which is why a system can test clean in flowing water while still harbouring a reservoir in its lining. It gets one layer deeper. Inside that biofilm, Legionella often grows within single-celled organisms called amoebae, using them as a host that protects and nourishes it [4]. So the bacteria are frequently sheltered twice over — inside an amoeba, inside a biofilm, inside your pipework. This is why durability, not lifespan, is the real issue. A one-off hot flush or a brief temperature spike knocks back what is in the open water, but bacteria sheltering in biofilm and sediment can recolonise once conditions return to the growth band [3]. The clock never really started, so flushing once does not stop it. Control is a habit, not a single event. ## Out of the water: surfaces, scale and the air Legionella is a waterborne organism. It needs moisture to persist, and it does not last long on a surface that is genuinely dry. The catch is how rarely surfaces in a water system are genuinely dry. Damp scale around a tap, the wet inside of a shower head, the residue in a seldom-used trap — these hold enough moisture to keep bacteria viable far longer than a dry worktop would. Drying out a fitting helps, but a fitting that merely *looks* dry while staying damp inside does not. The bacteria also travel briefly in the air, but only as part of water. Showers, taps, cooling towers and spa pools throw out fine aerosol droplets, and Legionella rides those droplets long enough to be breathed in — which is how people are infected in the first place [5]. The risk is not the bacteria drifting around like dust; it is contaminated water being turned into a breathable mist at the outlet. ## The practical lesson: stagnation and reopening All of this lands on one everyday situation: water that stops moving. A building, wing or flat that sits empty gives Legionella exactly what it wants — still water slowly warming toward the growth band, with biofilm building undisturbed. The danger of letting systems sit idle is covered in [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/), and it is the single most common way an otherwise ordinary system becomes a real hazard. It is also why reopening matters so much. After a holiday let stands empty, a school breaks up, or part of a building is mothballed, the safe move before normal use resumes is to flush every outlet thoroughly — and, where the risk assessment calls for it, to disinfect — so that whatever has built up during the quiet period is cleared rather than aerosolised onto the first person to turn on a shower. ## Common questions ### Does Legionella eventually die off if I just leave the system alone? No — leaving a system alone usually makes things worse, not better. Stagnant water, gradual warming and undisturbed biofilm are the conditions Legionella grows in [1][3]. Numbers fall reliably through use, heat and treatment, not through neglect. Idle time is when populations build, which is exactly why empty buildings need flushing before reuse. ### How long can Legionella survive in a water tank or cylinder? Indefinitely, if the tank sits in the growth range and is not properly managed. A cold tank that warms past 20°C, or a hot cylinder running too cool, gives Legionella warmth, settled sediment to feed on and biofilm to shelter in [1][3]. Survival time is set by conditions, not by a fixed shelf life — keep the tank genuinely cold or genuinely hot and you deny it the middle ground. ### Can Legionella survive on dry surfaces or in the air? It needs moisture, so it does not persist for long on a surface that is truly dry, but damp scale, wet fittings and residue keep it viable far longer than a dry surface would. In the air it travels only inside fine water droplets — long enough to be inhaled from a shower or tap, but not as free-floating bacteria [5]. (Verify exact dry-surface survival times against current guidance before quoting a figure.) ### Does boiling or running the hot tap clear Legionella for good? Heat above about 60°C kills the bacteria in the open water within minutes [2], but it does not permanently clear a system on its own. Bacteria sheltering in biofilm and sediment can recolonise when the water cools back into the growth band [3], so temperature control and regular use have to be ongoing rather than one-off. ## A note on scope This is general background on how Legionella behaves, not a verdict on your building. Whether your tanks, calorifiers and outlets are actually in a safe state, and what to do about any that are not, is a judgement for a competent person working from a site-specific risk assessment. We do not provide legal, medical or design advice — a care home, a holiday let and a small office will each need different controls, and only an assessment of your own system can say which apply. ## What to do next Spend ten minutes finding your warm-and-still spots: a tank or pipe run that sits lukewarm, a tap or shower nobody uses, a section of the building that has been closed off. Those are where survival turns into growth. Where Legionella tends to take hold across system types is worth a look too — [Where Legionella grows](https://legionella.io/articles/where-legionella-grows-natural-and-man-made-water-systems/) maps the usual locations. Then set a simple routine for the ones you cannot remove: flush little-used outlets regularly, keep cold water cold and hot water hot, and write down each time you do it. A dated record is what turns "we flush sometimes" into proof that warm, stagnant water never got the chance to sit — which is the whole game. Start with one neglected outlet today. ## Related reading - [What temperature kills Legionella?](https://legionella.io/articles/what-temperature-kills-legionella/) - [Temperature and Legionella growth: understanding the relationship](https://legionella.io/articles/temperature-and-legionella-growth-understanding-the-relationship/) - [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/) - [Where Legionella grows: natural and man-made water systems](https://legionella.io/articles/where-legionella-grows-natural-and-man-made-water-systems/) ## 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] HSE, "Hot and cold water systems". https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [3] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm [4] CDC, "About Legionnaires' Disease". https://www.cdc.gov/legionella/about/index.html [5] CDC, "How Legionella Spreads". https://www.cdc.gov/legionella/causes/index.html