Legionella does not arrive in a building as a problem. It arrives as a few cells in the incoming main, far too dilute to matter. Whether those cells stay irrelevant or grow into a colonised system is decided by the conditions they meet inside your pipework — not by the bacteria, which do not change, but by the environment you give them.
That is the useful frame for anyone managing a water system. You are not trying to sterilise water. You are trying to deny the organism the conditions it needs to multiply and the route it needs to reach a person. A short list of environmental factors governs the first half of that, and on a real UK building they are easy to name and mostly within your control.
The conditions that decide whether it multiplies
Five factors do most of the work. They are worth understanding separately before looking at how they combine.
Temperature is the master control. Legionella sits dormant in genuinely cold water and is progressively killed in genuinely hot water; the danger is the tepid middle. HSE guidance puts active multiplication in roughly the 20-45 C range, which is why “keep hot water hot and cold water cold” is the whole strategy reduced to a sentence [1][3]. Tepid is exactly what you get in cold water that has warmed in a plant room, or hot water that has lost its heat by the time it reaches a far tap.
Stagnation is the quiet accelerant. Still water lets bacterial numbers and biofilm build, lets temperature drift toward room ambient, and lets any disinfectant fade. Dead legs, capped-but-connected branches, oversized storage and low-use outlets are where this happens [2]. Stagnation is such a common root cause that it gets its own treatment in.
Nutrients feed the colony. Scale, sediment, rust and organic debris give the biofilm community something to live on. An old galvanised run, a tank with grit in the bottom, or a strainer full of debris is a richer environment than clean copper.
Disinfectant residual is the chemical brake. Where a treatment is dosed, a healthy residual suppresses growth — but it is consumed by biofilm and decays along the pipe, so the level at the plant says little about the level at the end of a long, stagnant branch.
Biofilm ties the others together. It is the slimy film on every wetted surface, and it is both a food source and a physical shelter that protects the cells inside it from heat and chemicals. CDC lists biofilm, warm temperatures, slow or no flow, and low disinfectant as the core conditions that let Legionella grow [4].
| Factor | What it gives the bacteria | Where it shows up on a UK system |
|---|---|---|
| Temperature in the 20-45 C band | the window to multiply | warmed cold storage, cooled hot returns, far outlets |
| Stagnation | time, plus falling temperature and residual | dead legs, low-use taps, oversized tanks |
| Nutrients | something to feed on | scale, sediment, rust, old galvanised pipe |
| Low disinfectant | removes the chemical brake | ends of long runs, stagnant branches |
| Biofilm | food and a shelter from controls | every wetted surface, worst where flow is low |
The reservoir most checks walk past
Biofilm deserves a second look, because it is where the textbook picture and site reality part company. Inside that film, Legionella also lives within free-living amoebae — single-celled organisms the bacteria infect and multiply inside. Those hosts shelter the bacteria from the very things you use to control them.
That has a practical consequence many sites learn the hard way. A system can be shocked with heat or chemical, sampled days later, and come back clean — then rebound weeks afterwards. The protected population inside biofilm and amoebae survived the dose and recolonised the cleared surfaces. A single negative result after remedial work is reassurance, not proof the problem has gone, which is part of why interpreting counts takes more than reading the number.
What the single-factor view misses
Treat these factors one at a time and you will badly misjudge your own building. The thing experienced water-safety people internalise is that the factors multiply rather than add.
A far outlet that is tepid is one issue. A far outlet that is tepid and rarely used and fed through scaled pipe past the point where the residual has died is not four small issues — it is a single compounded hotspot, and it is where a real colonisation tends to start. Risk concentrates; it does not spread evenly across the system.
That changes where you look. Control rarely fails at the calorifier, which is hot, or at the incoming main, which is cold and treated. It fails at the place furthest from both — where the heat has bled off and the disinfectant has decayed — and that is usually the part of the system your monitoring visits least. Your worst environment and your thinnest evidence tend to sit at the same outlet.
It also reframes disinfectant. A good reading at the dosing point tells you the brake works there. It says nothing about a stagnant branch where the residual has been consumed by biofilm, so the level worth checking is the one at the outlet you worry about, not the one nearest the dosing pump — a theme picked up in on disinfectant limits.
Reading these factors on your own system
None of this is a set of numbers to copy into a procedure. The temperature bands, residual levels and turnover that actually control a given building depend on its plant, its occupants and the strategy a competent risk assessment sets — a hospital ward, a leisure centre and a half-empty office wing read the same science very differently. Use the factors above as the questions to put to your own system, and let the assessment decide what “controlled” means for each one and what evidence would prove it. Where you need a defensible figure for a specific limit, confirm it against current HSE guidance rather than a rule of thumb.
Where to take this next week
Pick the three outlets on your site that are both furthest from the plant and least used. Walk to each one. Feel whether the cold runs genuinely cold and the hot genuinely hot at the point of use, not at the cylinder. Ask when it was last drawn off. If you dose the water, check the residual there rather than trusting the plant-room reading. You are looking for the places where two or more of these factors stack — that short list is where your control scheme earns its money, and where the next revision of your written scheme should put its attention [2].
FAQ
Does cold water kill Legionella?
No. Genuinely cold water keeps the bacteria dormant and stops them multiplying, but it does not reliably kill them — warm the water back into the growth band and the surviving cells become active again. Cold is a brake, not a cure, which is why cold storage that drifts warm is a recognised risk [1][4].
Our chlorine reading is fine at the tank — is the whole system protected?
Not necessarily. Disinfectant is consumed by biofilm and decays as water travels and sits, so a healthy residual at the dosing point can fall away to little or nothing at a stagnant far branch. The reading that matters for control is the one taken at the conditions you are worried about, not at the easiest tap to reach [4].
Why did our Legionella counts come back after a disinfection?
Because the population sheltered in biofilm and inside amoebae usually survives a one-off dose and recolonises the surfaces afterwards. Rebound is common, and it is why control depends on sustained temperature, movement and cleanliness rather than periodic disinfection alone — and why one clean sample does not close the matter [5].
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, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [4] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [5] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm