---
title: "How quickly can stagnant water become a Legionella risk?"
source_url: https://legionella.io/articles/how-quickly-does-legionella-grow/
canonical_url: https://legionella.io/articles/how-quickly-does-legionella-grow/
pillar: "Legionella Basics & Science"
summary: "Stagnant water has no fixed countdown - warmth, not the calendar, drives Legionella growth. Why UK guidance flushes weekly, and how to time reopening safely."
primary_keyword: "how quickly does legionella grow in stagnant water"
date_published: 2026-05-10
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 quickly can stagnant water become a Legionella risk?

There is no fixed countdown. Warm, still water amplifies Legionella far faster than cold water, and the speed is set by temperature and what is already in the pipes — not by the calendar. UK guidance copes with that uncertainty using a practical proxy: treat roughly a week without flow as the point at which a little-used outlet needs flushing [1][3].

So if you are scheduling flushing or planning to reopen part of a building, the answer you can actually use is not a number of hours. It is a cadence. Keep water moving often enough that it never gets the warm, undisturbed stretch Legionella needs to build up.

## Why "how fast" has no single answer

How quickly Legionella grows in stagnant water depends far more on temperature than on the number of days that have passed. The bacterium multiplies most readily in roughly the 20–45°C range, sits largely dormant below about 20°C, and is progressively killed above around 60°C [2]. That one fact explains why timing varies so much.

A cold-water pipe that stays genuinely cold over a quiet weekend is in no hurry to grow anything. The same pipe running tepid — warmed by a hot pipe beside it, by a heated plant room, or simply by summer — sits in the growth band, and stagnation then lets the population climb [1]. Two identical week-long gaps can leave you in very different places, because warmth, not elapsed time, is doing the work.

What is happening inside the pipe during that idle stretch — Legionella feeding and multiplying within biofilm and the organisms living in it — is set out in [Legionella life cycle and growth conditions](https://legionella.io/articles/legionella-life-cycle-and-growth-conditions/). For scheduling purposes the short version is this: still water lets biofilm thicken, sediment settle and any disinfectant residual fade, and each of those changes speeds the next [1][3].

## Where the one-week rule comes from

UK practice has settled on about a week as the working threshold. Outlets not used at least weekly are widely treated as "little-used" and flushed on a weekly cycle to keep water turning over [1][3]. It is sensible, memorable, and easy to put on a schedule — which is exactly why it has stuck.

But the week is a management interval, not a biological deadline, and the gap between those two things is where buildings get caught out.

## What the one-week rule doesn't tell you

The weekly convention is useful precisely because it is simple. That simplicity hides three things that matter once you are the one setting the intervals.

**The week is a floor, not a guarantee.** It quietly assumes the stagnant water is reasonably cool. In a warm void — a plant room, a heated riser, a closed-up building baking through August — water reaches the growth band sooner and grows faster, so seven days can be generous rather than safe. Where you cannot keep stored and standing water cool, the honest response is to flush more often than weekly, not to treat a week as a ceiling you can lean on. Designing the warmth out is the better fix, and [Avoiding stagnation: design tips for consistent water temperatures](https://legionella.io/articles/avoiding-stagnation-design-tips-for-consistent-water-temperatures/) covers that side of it.

**Stagnation is local, not building-wide.** A fully occupied, busy building can still hide a fortnight-old dead leg behind a disused tap, a guest WC nobody flushes, or a capped branch off a live riser. Occupancy tells you the main runs are moving; it tells you nothing about the spur feeding one forgotten outlet. You cannot judge stagnation by how busy the place looks — only by which specific outlets genuinely get used. [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/) is mostly about these hidden pockets inside otherwise-active buildings.

**The hazard spikes at the moment of first use.** Bacteria accumulating in a stagnant pipe harm nobody until something turns that water into a fine spray to be breathed in. The dangerous instant is therefore not the quiet week — it is the first shower, spray tap or hose run afterwards. That is why flushing has to happen before people come back, and why a half-reopening, where one busy outlet drags week-old water through to neighbouring fittings, can be worse than leaving the whole section shut.

## Reopening: the moment to plan for

This is where the timing question gets real for most duty holders. A building, wing or flat that has stood empty has handed Legionella the two things it wants — warmth creeping into still water — for the entire closure. The lesson the pandemic drove home, when thousands of buildings sat part-empty for months, is set out in [Lockdown lessons: water stagnation during COVID-19](https://legionella.io/articles/lockdown-lessons-water-stagnation-during-covid-19/): you cannot simply switch the water back on as though nothing happened.

Before normal use resumes, the standard move is to flush every outlet thoroughly so that whatever built up during the quiet period is cleared rather than aerosolised onto the first person to use it — and, where the risk assessment calls for it, to disinfect first [3][4]. Flush in a way that limits spray, start at outlets nearest the incoming main and work outwards, and run each one long enough to draw genuinely fresh water through. The longer the void, the more cautious the restart should be.

## A note on what this is and isn't

The "roughly a week" figure here is a widely used management interval, not a measured safe limit for your particular pipes — laboratory growth rates do not translate cleanly into how fast your system turns risky. What the right flushing interval actually is for your building, and whether disinfection is needed before reopening, is a judgement for a competent person working from a site-specific risk assessment [5]. This is general background, not legal, medical or design advice. A care home, a holiday let and a seldom-used sports pavilion will each warrant different intervals, and only an assessment of your own system can set them.

## Common questions

### Is there really a "one-week" rule for stagnant water?
Sort of. No law says water is safe for seven days and unsafe on the eighth. The week comes from the common practice of flushing outlets that are not used at least weekly, so that water keeps turning over [1][3]. Treat it as a scheduling default your risk assessment can tighten — weekly in ordinary conditions, more often anywhere standing water runs warm.

### How long before water actually becomes unsafe to use?
There is no single figure, because it depends on temperature far more than on elapsed time [2]. Genuinely cold water can stand for a while with little change; warm, still water in the 20–45°C band can build up much faster. Rather than chase a deadline, assume any outlet left unused for around a week needs flushing before it is relied on [1][3].

### Does the building have to be empty for stagnation to be a problem?
No. Stagnation is about individual outlets and pipe runs, not whole buildings. A busy office can still have a disused tap, a rarely-let room's shower or a dead leg where water sits for weeks [3]. Judge it outlet by outlet — which ones genuinely get used — not by how occupied the building feels.

### After a long shutdown, can I just turn everything back on as normal?
Not safely. Water that has stood through a closure should be flushed through every outlet before people return, in a way that limits spray, and disinfected first where the risk assessment requires it [3][4]. The first use after stagnation is the moment bacteria can be aerosolised and inhaled, so the flush has to come before occupancy, not after.

## What to do next

List the outlets across your buildings that get used less than once a week — the spare WC, the end-of-corridor tap, the rarely-let room's shower, the emergency shower, the section currently mothballed. Those are your stagnation clocks. Put each on a weekly flush, tighten the interval anywhere the water runs warm, and write down the date every time, so a quiet week never quietly becomes an unrecorded month. If part of your site is reopening soon, build the flush-before-occupancy step into the plan now — before the first tap is turned on.

## Related reading

- [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/)
- [Avoiding stagnation: design tips for consistent water temperatures](https://legionella.io/articles/avoiding-stagnation-design-tips-for-consistent-water-temperatures/)
- [Legionella life cycle and growth conditions](https://legionella.io/articles/legionella-life-cycle-and-growth-conditions/)
- [Lockdown lessons: water stagnation during COVID-19](https://legionella.io/articles/lockdown-lessons-water-stagnation-during-covid-19/)

## Sources

[1] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[2] HSE, "Hot and cold water systems". https://www.hse.gov.uk/legionnaires/hot-and-cold.htm
[3] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm
[4] HSE, "Legionella and landlords' responsibilities". https://www.hse.gov.uk/legionnaires/legionella-landlords-responsibilities.htm
[5] 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