Legionella is in your water before you do anything wrong. It lives, in small and harmless numbers, in rivers, lakes and the ground water that ends up in the mains. The bacterium itself is not the hazard. What turns a background organism into a building problem is what your tanks, pipes and fittings do to it after the water arrives.
Nature supplies the seed; man-made systems are the amplifier. A river keeps Legionella cold, moving and short of food. A neglected hot water cylinder does the exact opposite. Understand that contrast and you already know where to point your attention on site.
What growth actually needs
Legionella growth is not automatic. The bacteria do not multiply just because water is present — a few conditions have to line up at once.
Warmth is the engine. Legionella multiplies fastest in roughly the 20–45°C band; below about 20°C it sits dormant, and above about 60°C it is progressively killed [1]. That single fact is why “keep hot water hot and cold water cold” is a control measure rather than a slogan [4].
Then it needs time and stillness — water left standing in a dead leg, a rarely-used outlet or an oversized storage tank long enough for numbers to climb. And it needs something to feed on: the scale, rust, sediment and slimy biofilm that coat the inside of real pipework. The CDC describes biofilm, suitable temperatures, stagnation and low disinfectant levels as the combination that lets Legionella thrive, and notes the bacterium occurs naturally in freshwater to begin with [2].
None of that harms anyone by itself. The last ingredient is an escape route as a fine, breathable mist — a shower, a spray tap, a cooling tower plume — and a person close enough to inhale it. Growth and exposure are two separate links in the chain, and you can break either one.
Where it grows: nature versus your building
The same organism behaves completely differently depending on the water it sits in. That is the heart of the natural-versus-man-made distinction.
| Where the water sits | Why Legionella can multiply | How it could reach people |
|---|---|---|
| Rivers, lakes and ground water (the natural source) | Usually too cold, too mobile and too low in nutrients for large numbers | Rarely an everyday inhalation route; matters mainly as the source that seeds mains and stored water |
| Cooling towers and evaporative condensers | Warm recirculated water, plenty of nutrients and constant contact with air | Fine drift can carry aerosol well beyond the plant itself |
| Hot and cold water systems — tanks, calorifiers, long pipe runs | Tepid storage, dead legs and scale create warm, still, nutrient-rich pockets | Showers and spray taps turn it into respirable mist at the outlet |
| Spa pools and hot tubs | Deliberately warm, aerated and heavily used water | Direct aerosol across the pool; managed under separate guidance (HSG282) [5] |
| Other spray sources — humidifiers, vehicle washes, fountains, misting units | Warm or ambient water left to stand, then atomised | Spray released straight into occupied or public space |
HSE groups cooling towers, hot and cold water systems and spa pools among the systems most likely to create a Legionella risk, precisely because each one offers warmth, standing water and a spray route in a single package [3].
The systems hiding on an ordinary site
Most UK buildings do not have a cooling tower, which is exactly why people relax — and that is the mistake. The everyday amplifier is the hot and cold water system everybody walks past.
Start with cold water storage. A tank is meant to keep water below the growth range, but a large tank in a warm plant room, a loft or a riser can drift up into the tepid zone, especially in summer. Cold that is not actually cold is one of the most common growth conditions on real sites.
Then the hot side. A calorifier that stratifies — hot at the top, lukewarm sludge at the bottom — or a circulating loop that has lost its heat by the time it reaches a far wing gives Legionella a warm home with a shower waiting at the end of it.
The quietest risk is the outlet nobody uses. A guest room between bookings, a shower in a refurbished wing, a tap in a decommissioned office: each holds a slug of warm, stagnant water feeding straight into an aerosol-producing fitting. Stagnation does more to grow Legionella than almost anything else, which is why redundant pipework and dead legs deserve attention long before another round of sampling — the case for designing out stagnation is set out in Neglected water systems: the danger of stagnation.
Where to put your attention
The useful move is to stop thinking about Legionella as something in “the water” and start thinking about where your particular system creates warmth, stillness and spray together. Those three overlapping circles are your priority list.
In practice that means the low-use shower beats the busy one, the oversized tank beats the right-sized one, and the long dead leg beats the live riser — every time. Temperature is the lever you can pull hardest, so if you fix one thing first, make it the places where hot is not hot enough and cold is not cold enough [4]. Temperature control basics for Legionella prevention covers the numbers and how to hit them.
A necessary caveat
Knowing where Legionella tends to grow tells you where to look — not what is true of your building. The temperatures, the turnover, the people exposed and the right controls all have to be judged on the actual system by a competent person, working to the HSE Approved Code of Practice L8 and the technical guidance in HSG274 [1][6]. Treat everything above as a way to ask sharper questions, not as a substitute for a site-specific risk assessment.
FAQ
Does Legionella come from the mains water supply?
In effect, the natural environment is the original source — Legionella lives at low levels in freshwater, so trace amounts can arrive in mains water. The water company delivers it within the expected standard; what matters is whether your storage and pipework then let those trace numbers multiply [2].
Can Legionella grow in a cold water system?
Yes, if the cold water is not genuinely cold. Once stored or distributed water drifts above about 20°C — a warm tank, a pipe run through a heated space, a long summer — the cold system can sit in the growth range just like a poorly run hot one [1].
Which water system grows Legionella fastest?
There is no single answer, but the worst combinations share the same traits: warm, standing water with nutrients and a spray outlet. Cooling towers are high-consequence because their drift travels; on most ordinary sites, though, the realistic culprit is a neglected, low-use part of the hot and cold water system.
Your next step
Walk the building with one question in mind: where does water sit warm and still, and what turns it into mist? Note every cold tank that might be warming, every hot outlet that runs lukewarm, and every shower or spray tap that rarely gets used. That short list is the raw material your risk assessment turns into controls — and it is a far better starting point than a generic checklist.
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] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [3] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [4] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [5] HSE, “Control of legionella and other infectious agents in spa-pool systems (HSG282)”. https://www.hse.gov.uk/pubns/books/hsg282.htm [6] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm