---
title: "Risk assessing cooling towers for Legionella"
source_url: https://legionella.io/articles/risk-assessing-cooling-towers-for-legionella/
canonical_url: https://legionella.io/articles/risk-assessing-cooling-towers-for-legionella/
pillar: "Legionella Risk Assessment"
summary: "How to risk assess a cooling tower for Legionella: the findings that should worry you, what each one points to, and how to rank the fix before you leave site."
primary_keyword: "cooling tower risk"
date_published: 2025-07-05
date_reviewed: 2026-06-26
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url."
---

# Risk assessing cooling towers for Legionella

A cooling tower is the one water system on most UK sites that can put Legionella into the open air and carry it across a car park. So an assessment that confirms there is a treatment contract, a logbook and a recent sample has checked the admin and skipped the question that matters: is this tower actually under control, and if not, where did it slip?

The more useful way to run a cooling tower risk assessment is to read the plant like a fault report. Stand in front of it, look hard at what it is doing, and work backwards from each worrying sign to its cause — then forwards to the fix and where it ranks against everything else. Paperwork tells you what should be happening. The plant tells you what is.

## Why a tower rewards a fault-finding mindset

Cooling towers and evaporative condensers earn their reputation. They hold large volumes of warm water, they run for the warmer half of the year, and their whole job is to throw water through moving air — which is exactly how a contaminated system makes the fine aerosol that spreads Legionella to people who never went near the plant [3]. HSE lists evaporative cooling among the systems most likely to create Legionella risk for that reason [5].

That is also why the guidance treats them on their own terms. HSG274 deals with evaporative cooling systems in a separate part from hot and cold water services, because the failure modes and the monitoring regime are different [2]. If you came to this from domestic hot and cold work — covered in [Assessing risk in hot and cold water systems](https://legionella.io/articles/assessing-risk-in-hot-and-cold-water-systems/) — park those instincts. A tower is not a calorifier with a fan.

There is one duty here that has nothing to do with how well the tower is run and everything to do with whether it is on the radar at all: wet cooling towers and evaporative condensers are notifiable, and the local authority has to be told they exist [4]. An assessment that never asks "is this device registered?" has a gap before it starts.

## Work from the most likely failure first

Resist the urge to open the binder first. Records describe a tower; they do not let you see one. Start outside the louvres and work in.

The first thing worth your attention is whether aerosol is leaving the tower and where it would go. Visible drift, a damp patch on the wall above the air intake, a plume you can see against a dark background — that is the exposure pathway made obvious. Next, look into the basin: sludge, scale, slime and debris tell you the cleaning interval or the water treatment regime is not holding the line. Then look at how the treatment is actually delivered — dosing equipment, chemical stock, bleed and conductivity control — because that is where day-to-day control quietly fails. Records come last, and you read them to confirm or contradict what your eyes already found.

Ordering it this way puts the highest-exposure, most-likely problems at the top, which is also the order in which you should be ranking the actions.

## From finding to action: a fault-finding flow

This is the heart of a tower survey. Each row starts with something you can observe, works back to the usual cause, names the check that settles it, and ends with the action and a sense of where it sits on the priority list. Treat it as a prompt, not a substitute for the system's own treatment specification.

| What you find | What it usually points to | The check that settles it | Action, and how to rank it |
| --- | --- | --- | --- |
| Visible mist or a plume past the louvres; damp surfaces above the intake | Drift eliminators missing, damaged, displaced or fouled — or the system is overflowing | Inspect the eliminators are present and intact; check water level and distribution | Immediate. Aerosol leaving the tower is the worst-case pathway; this goes to the top regardless of what the paperwork says |
| Sludge, scale, slime or debris in the basin | Cleaning interval too long, or the treatment regime is not keeping up | Check the last clean-and-disinfect date and the biocide and inhibitor records | High. Schedule a clean and disinfection, then ask *why* it built up — do not just clean and move on |
| Gaps in the dosing log, or a dosing pump found off or out of chemical | Automatic dosing fault, empty drum, or nobody owns the routine check | Examine the dosing kit, chemical stock and conductivity or bleed control | High. Treat as a control failure, not a one-off; confirm recovery through monitoring before you sign it off |
| Tower stood idle over a low season with no restart record | Seasonal standby brought back into service without commissioning checks | Look for a pre-start clean, disinfection and sample record | High. The system should not be accepted back in use until that is done and recorded |
| No evidence the device has been notified to the local authority | Notification overlooked at install or handover | Confirm with the duty holder, or with the local authority, whether it is registered | Resolve regardless of risk score; this is a standalone legal duty for wet towers and condensers [4] |

If the same weakness shows up in more than one row — say, both a fouled basin and a stalled dosing pump — stop treating them as separate defects. That is a management failure wearing two costumes, and the fix is ownership, not another clean.

## Ranking the findings, and when to escalate

Once the findings are on paper, sort them into two honest piles. One is the immediate defect: the broken eliminator, the dead dosing pump, the tower running dirty right now. The other is the underlying weakness: no named owner for the daily check, a cleaning interval set by habit rather than by the assessment, monitoring that gets recorded but never reviewed. The first pile protects people this week. The second stops the first pile coming back.

Be clear about your own limits, too. Reading drift, basin condition and dosing records is well within a competent assessor's reach. Deciding whether a treatment regime is correct for that tower's water chemistry, or whether a microbiological result warrants disinfection, is water-treatment territory — escalate it to the specialist who holds the regime rather than guessing. Sampling supports those decisions but does not make them, and a clean result never closes a finding on its own; monitoring frequency follows the risk assessment, not the other way round [6]. The discipline of recording and acting on readings is its own subject, covered in [Monitoring water temperatures in a Legionella control programme](https://legionella.io/articles/monitoring-water-temperatures-in-a-legionella-control-programme/).

Frame the whole thing against the legal backdrop. Risk assessment of an evaporative cooling system is a duty under the Approved Code of Practice, not a discretionary survey, and BS 8580-1 sets out a recognised method for doing it [1][7]. An assessment that names the defect, names the owner and sets the review trigger is doing the job. One that produces a tidy report nobody on the maintenance team can act on is not.

## Where this guidance stops

Take this as a way to think on site, not as the numbers for your tower. The figures that actually govern control — biocide levels, bleed rates, cleaning and sampling intervals, the temperatures and conductivity to hold — come from the system's own risk assessment and treatment specification, and from people qualified to set them for that plant and that water supply. Anything here is a prompt for sharper questions, not a pass mark.

## FAQ

### Does a cooling tower need its own risk assessment, separate from the rest of the building?

Treat it as its own system. Evaporative cooling sits under a different part of HSG274 from hot and cold water services because the aerosol risk and the treatment regime are genuinely different [2]. It can live inside one site report, but the tower needs its own analysis of how it could fail and how that is controlled.

### What is the first thing to look at on a cooling tower survey?

Whether aerosol is escaping the tower, and where it would travel. Drift is the exposure pathway, so a damaged or missing drift eliminator outranks almost anything you will find in the basin or the logbook [3]. Cleanliness and treatment come straight after.

### Do I have to tell anyone the tower exists?

Yes. Wet cooling towers and evaporative condensers are notifiable devices, and the local authority must be told about them [4]. Confirming the device is registered is a fair thing to check as part of the assessment, separate from how well it is being run.

## Related reading

- [Legionella and the Health and Safety at Work Act](https://legionella.io/articles/legionella-and-the-health-and-safety-at-work-act/)
- [Monitoring water temperatures in a Legionella control programme](https://legionella.io/articles/monitoring-water-temperatures-in-a-legionella-control-programme/)
- [Assessing risk in hot and cold water systems](https://legionella.io/articles/assessing-risk-in-hot-and-cold-water-systems/)
- [Special considerations for Legionella risk in healthcare facilities](https://legionella.io/articles/special-considerations-for-legionella-risk-in-healthcare-facilities/)

## 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] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[3] CDC, "How Legionella Spreads". https://www.cdc.gov/legionella/causes/index.html
[4] HSE, "Other duties: RIDDOR and notification of cooling towers or evaporative condensers". https://www.hse.gov.uk/legionnaires/what-you-must-do/duties.htm
[5] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm
[6] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm
[7] BSI, "BS 8580-1:2019 - Risk assessments for Legionella control. Code of practice". https://knowledge.bsigroup.com/products/water-quality-risk-assessments-for-legionella-control-code-of-practice-1