Legionella and water hygiene: understanding the risk assessment cycle

A common misconception is that legionella compliance is a single event — get the risk assessment done, file the certificate, move on. In practice the risk assessment is closer to the start of an ongoing obligation than the end of one. Water systems change condition over time, and a risk assessment that isn't backed by continued monitoring goes stale in a way that's easy to miss until something forces the question.

What the risk assessment actually covers

A proper water hygiene risk assessment looks at the water systems on site — where legionella bacteria could potentially grow and how people could be exposed — and sets out a scheme for controlling that risk. It's specific to the actual systems and building, not a generic document, which is exactly why the assessment needs updating whenever something material changes: new pipework, a change in usage pattern, a period where part of the building was unoccupied and water sat stagnant.

The monitoring cycle behind the paperwork

Behind the headline risk assessment sits a routine of ongoing checks — temperature monitoring at outlets, periodic checks of tanks and calorifiers, flushing of little-used outlets, and monitoring for stagnation risk generally. These are typically more frequent than the risk assessment review itself, and they're the part that actually keeps the risk under control day to day rather than just on the date the assessment was last reviewed.

• –Temperature checks at representative outlets, on a routine that reflects the system's actual risk profile
• –Tank and calorifier inspections, checking for debris, scale or condition issues that could support bacterial growth
• –Flushing of outlets that see little or no regular use, since stagnant water is a core risk factor
• –A documented review whenever the building, its occupancy, or its water system changes materially

Where the evidence trail tends to break down

The risk itself is rarely the problem for most well-run teams — it's demonstrating, afterward, that the routine checks actually happened on schedule. A monitoring regime that's genuinely being followed but recorded on a mix of paper logs, spreadsheets and someone's memory is very hard to evidence convincingly months later, which is the same underlying evidence problem that shows up across most statutory inspection categories, not just this one.

When a finding needs to become a job, not just a note

A monitoring check that finds something out of range — a temperature reading outside the expected band, visible contamination, a fault with a control measure — needs a clear path to becoming an actual piece of remedial work, tracked through to completion, not just a note added to the log. What happens after a defect is found is where a lot of otherwise-good monitoring regimes quietly fall down, because the finding gets recorded but never becomes a job with an owner and a deadline.

For a multi-building estate, water hygiene risk profiles genuinely differ site to site — building age, water system design and occupancy pattern all matter — so a single estate-wide assumption about frequency rarely holds up to scrutiny on every site equally.

Where the trail usually breaks

The assessment gets done properly and the monitoring gets done reasonably well. What breaks is the middle: a temperature reading taken and logged, falling outside the expected range, and nothing downstream happening because logging it felt like the action. Six months later an auditor reads a monitoring log that faithfully records an out-of-range result every month and no corresponding job, which is a worse document than no log at all — it proves the deviation was seen.

The same gap appears with little-used outlets. Flushing regimes assume somebody knows which outlets are underused, and that knowledge lives with whoever walks the building. When a room changes use or a wing is mothballed, the regime does not automatically notice.

Common mistakes

• –Logging an out-of-range reading without it generating an action
• –Treating the risk assessment as a document rather than something with a review trigger
• –Not updating the assessment when a building's use or occupancy changes
• –Flushing regimes that do not track which outlets have actually become underused
• –Holding monitoring records separately from the jobs they should have raised
• –Assuming a contractor's visit covers items that are actually outside their scope

Water hygiene obligations and recommended practice are set out in HSE's own guidance (ACOP L8 and HSG274) — this is a directional summary, and any specific interval or control measure should be confirmed against current guidance and your own competent risk assessor.