What happens if legionella is found in a water system?

Finding legionella in a water system sets off a defined process: interpret the result against its count and location, confirm it with a repeat sample, review the control scheme to find out why it happened, take remedial action proportionate to the level found, and record every step. It is a management problem with a known procedure, not an automatic crisis — but it is also not a result that can be filed and forgotten.

Step one: understand what the result actually says

A laboratory result is a measurement of one volume of water, from one outlet, at one moment. It tells you legionella was culturable at that point, at that count — reported in colony-forming units per litre. It does not tell you whether the whole system is colonised, and it does not tell you anyone has been harmed. The count drives the response: the action levels in HSG274 band results so that low counts call for review and resampling while high counts call for immediate remedial action. Those bands, and what each triggers, are set out in our companion page on what to do if legionella is detected and in legionella sampling and action levels; this page is about the process those results set in motion.

Step two: confirm it before you rebuild anything

Sampling has failure modes. A poorly taken sample — a dirty tap, a flushed outlet sampled too late, a bottle contaminated in handling — can produce a result that says more about the sampling than the system. For an unexpected low-level positive, the standard next move is a repeat sample, taken correctly, often alongside temperature checks at the outlet. A confirmed result earns the full response; an unconfirmed one earns better sampling. This is also why the first response to a positive is usually calm and procedural rather than dramatic.

Step three: review the control scheme, because the system just told you something

A positive sample is information about the control scheme: something in it is not working. The review asks where the conditions for growth could exist — water held between 20°C and 45°C, stagnation, scale and sediment. The usual findings are a dead leg nobody knew about, an outlet that fell off the flushing schedule, a calorifier running cool at the base, or a cold feed warming in a loft. The written scheme of control is checked against reality: were the scheduled tasks actually being done, and were they the right tasks?

Step four: remediate in proportion, then verify

The remedial action matches the band: at low counts, corrected controls and monitoring; at high counts, taking the outlet out of use and disinfecting the affected part of the system, by thermal disinfection or chlorination. Whatever is done, the process is not finished until a follow-up sample — taken after the treatment has had time to work — confirms the count has come back down. Disinfection without the step-three investigation is the classic treadmill: the bacteria return because the dead leg or the cool cylinder base is still there. The remediation and the root-cause fix are one job, not two.

Step five: record it all, and know what does not need reporting

The paperwork is not bureaucracy here — it is the difference between an incident and evidence of control. Record the result, the interpretation, the resample, the reasoning, the action taken, and the follow-up result, kept with the rest of the log book for at least five years. As for external reporting: a positive water sample is not in itself notifiable to the HSE. The duties that involve outside bodies attach to illness, not to laboratory results — a work-attributed case of Legionnaires' disease may engage RIDDOR, and an outbreak brings in the health protection team. If someone may already have become ill, the situation changes category and should be escalated immediately through the responsible person, as described in the responsible person guidance.

How to need this process less often

Most positive results trace back to the same short list of conditions, which means most are preventable with the routine controls: hot stored at 60°C and delivered above 50°C, cold kept below 20°C, little-used outlets flushed weekly, tanks and calorifiers inspected, and the whole thing written down. The monitoring schedule sets out that routine, and the risk assessment tool structures the assessment that decides where your system's weak points are before a laboratory does.