Where does legionella come from?

Legionella comes from the natural environment. It is a normal inhabitant of fresh water, present at low levels in rivers, lakes and groundwater, and it reaches buildings in trace numbers through the incoming mains supply. The water arriving at your boundary is not the hazard; the hazard develops inside the building, where conditions let those trace numbers multiply.

This origin story matters for one practical reason: it tells you where the responsibility lies. Because legionella is ubiquitous, no water supplier can deliver water guaranteed free of it, and no duty holder can keep it out of a building. What can be controlled is whether the building gives it somewhere to grow. Legionella control is therefore not about keeping the organism out; it is about keeping the conditions wrong for it.

Why the mains is safe and the building is not

Water company treatment and the chlorine residual maintained through the distribution network keep environmental legionella at very low numbers, so mains water arrives safe. But chlorine decays. As water sits in a cold water storage tank, creeps down long pipe runs, or warms up in a loft or a service void, the residual that protected it in the main fades away. What is left is water with little or no disinfectant, sitting at whatever temperature your building gives it. If that temperature is between 20°C and 45°C, and the water is still, and there is scale, sediment or biofilm to feed and shelter growth, the trace numbers that arrived harmlessly become a colony.

Every element of that sentence is a control point. Temperature control keeps water out of the growth range. Flushing removes stagnation. Cleaning and descaling remove the nutrients and the habitat. The whole framework in HSG274 follows directly from understanding where the organism comes from and what it needs once it arrives.

The boundary of responsibility

The water company's duty ends at the boundary of your property. Everything beyond it, the storage tanks, the calorifier, the distribution pipework, the outlets, is the responsibility of whoever controls the premises. Under the Health and Safety at Work etc. Act 1974 and ACOP L8, that duty holder must assess the risk from their water system and control it. It is no answer to an inspector that the bacteria came in with the supply: they come in with everyone's supply. The question is always what your system did with them.

Routes in besides the mains

The mains is the usual route, but not the only one. Tanks that are not properly lidded and sealed can admit contaminated material directly. Construction and alteration work can introduce the organism or, just as effectively, create the stagnation that lets it establish: pipework filled for pressure testing and left for weeks, sections commissioned before the heating is balanced, dead legs left behind when fixtures are removed. A brand-new building can be colonised before occupation, which is why new and refurbished systems are flushed and disinfected before handover rather than assumed clean.

The practical response follows the origin. Assess the system you actually have, its tanks, its temperatures, its stagnant branches, using the legionella risk assessment tool or our guide to legionella in water, then run the controls the assessment identifies and record each one in a log book. The organism's origin is natural and unavoidable; the colony is a building management outcome, and that is what the records need to show you managed.