Opening a lab report and seeing "Legionella detected" is enough to make anyone's stomach drop. Take a breath. A positive result is a warning that control has slipped somewhere, not proof that anyone's going to get ill. What matters now is acting quickly and sensibly.
We're writing this in late June, when warm weather pushes cold water temperatures up and positive results tend to be more of a worry. Here's how to read the result, what the guidance expects you to do and how to get the system back under control.
First, what does the result actually say?
Legionella results from a UKAS-accredited lab are usually given in colony forming units per litre, written cfu/L. Most reports will also tell you:
- Which outlet each sample came from, and whether it was hot or cold water.
- The count, or "not detected" if it was below the lab's detection limit.
- The species or serogroup, where it's been identified. Legionella pneumophila serogroup 1 is the type most often linked with Legionnaires' disease, so it's taken particularly seriously.
Look at the pattern as well as the numbers. One positive outlet out of ten tells a different story from positives right across the building.
What HSG274 says to do
For hot and cold water systems, the HSE's technical guidance, HSG274 Part 2, gives a table of action levels. In summary:
- More than 100 and up to 1,000 cfu/L. If only one or two samples are positive, resample. If a similar count comes back, review the control measures and the risk assessment to find what needs fixing. If most samples are positive, the system may be colonised, albeit at a low level. Disinfection should be considered, and the control measures and risk assessment reviewed straight away.
- More than 1,000 cfu/L. Resample, and carry out an immediate review of the control measures and risk assessment to identify remedial actions, including possible disinfection of the system.
Those levels are for domestic-type hot and cold water systems. Cooling towers and other systems have their own guidance, and healthcare settings may follow tighter rules in their Water Safety Plan, so check what applies to you. Your assessor or the lab can help you read your own report against the right table.
Who needs to know?
Tell your responsible person for water safety straight away, and the duty holder if that's someone different. If you have a Water Safety Group, it should meet. Let your water hygiene contractor know too, if you use one.
A positive sample on its own isn't something you report to the HSE. Illness is a different matter. If you hear of anyone with a confirmed or suspected case of Legionnaires' disease who's used the building, get advice immediately. Public health and environmental health officers lead those investigations, and you'll need to co-operate fully. Don't disinfect or alter the system before talking to them, because they may want to take their own samples first.
Immediate steps to reduce the risk
While you work out the cause, it's sensible to reduce exposure. Depending on the result and who uses the building, that might mean:
- taking positive showers or outlets out of use, or fitting point-of-use filters to them
- checking hot water storage temperature and raising it to at least 60°C if it's dropped
- flushing affected outlets and any little-used ones nearby
- extra care where vulnerable people are involved, such as care home residents or patients
Finding the cause
A positive result is a symptom. The important bit is finding what's allowing the bacteria to grow. The usual suspects:
- Temperature. Hot water not reaching 50°C at outlets, or cold water creeping above 20°C.
- Stagnation. Little-used outlets that haven't been flushed, or dead legs. We covered those in our flushing guide.
- Dirty tanks. Sediment and biofilm in cold water storage. Our article on cold water tank inspections goes through what to look for.
- Scaled-up shower heads and hoses, or TMVs that haven't been serviced.
- Recent changes. New pipework, a closure or a change in how the building's used.
Your monitoring records are a big help here. If temperatures have been drifting for months, that's often your answer.
Disinfection and resampling
If disinfection is needed, it's usually done either thermally (raising hot water temperature and running it through every outlet) or chemically (dosing the system with a disinfectant such as chlorine, holding it for a set time and flushing it out). Which one suits depends on the system and its materials. It should be done by people who know what they're doing, as getting it wrong can damage pipework or simply not work.
Disinfection on its own doesn't fix anything, though. If you don't deal with the underlying cause, the bacteria come back. So the order is: find the cause, put it right, disinfect if needed, then resample to confirm. Further follow-up samples are often taken over the following weeks to make sure the result stays clear.
A typical scenario
Imagine a small hotel takes routine samples and gets a low positive from a shower in a top-floor room, with every other outlet clear. Looking back through the logbook, that room has been out of use for a refit, and nobody added it to the flushing list.
A sensible response would be to take the shower out of use, clean and disinfect the head and hose, flush the outlet, check temperatures on that floor, resample, and add the room to the weekly flushing list until it's back in service. If the resample was still positive, the next step would be a closer look at that part of the system, and possibly a wider disinfection.
Afterwards
Once results are back under control, update the risk assessment and written scheme with what you've learned. A positive result is one of the classic triggers for a review, as we explain in how often to review a Legionella risk assessment.
If you've had a positive result and want help, call us. We can take follow-up samples through our water sampling service, track down the cause and carry out the remedial works and disinfection needed to put it right.