Cooling Towers and Legionella: Testing Requirements, Control Guidelines and What UK Operators Must Know

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Written By Carly Duffy

Cooling towers are one of the highest-risk water systems for Legionella bacteria in the UK. They have been linked to some of the country's most serious Legionnaires' disease outbreaks and remain a primary focus for the Health and Safety Executive (HSE). If you operate, manage, or are responsible for a cooling tower, understanding your legal duties is not optional, it is a matter of public safety.

This guide covers the key questions operators ask: how cooling towers cause Legionella, what the testing requirements are, what the relevant guidelines say, and what the data tells us about the scale of the risk.

How Do Cooling Towers Cause Legionella?

Cooling towers remove excess heat from industrial or commercial processes by passing warm water through a system where it is cooled by evaporation. The problem is that this process creates exactly the conditions Legionella bacteria need to multiply and spread.

There are three reasons cooling towers are particularly high risk:

Temperature. Cooling tower water routinely sits within the 20°C to 45°C range — the precise temperature band in which Legionella thrives. Unlike hot water systems, where high temperatures can be used to kill bacteria, temperature control alone is not sufficient for cooling towers.

Aerosol generation. Cooling towers produce large volumes of fine water droplets as part of their normal operation. These droplets — or aerosols — are the primary route by which Legionella enters the lungs and causes Legionnaires' disease. Aerosols can travel significant distances from the tower itself; studies have recorded transmission over one kilometre from the source.

Nutrients for bacterial growth. Cooling tower water can accumulate rust, sludge, scale, biofilm, and organic matter — all of which provide nutrients that allow Legionella to multiply rapidly. Without regular cleaning and chemical treatment, a cooling tower can become a breeding ground for bacteria in a very short time.

Evaporative cooling systems have been linked to more Legionnaires' disease cases than any other type of water system, according to HSE research. This is why they are subject to the most demanding maintenance and testing regime of any water system in the UK.

The Scale of the Risk: UK Cooling Tower and Legionella Statistics

Understanding the numbers puts the risk in context.

Cooling tower registrations in the UK

Under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992, all operators must register their cooling towers with their local authority. There are estimated to be around 5,800 notified cooling tower sites across the UK, of which approximately 2,900 fall under HSE enforcement jurisdiction. The remainder are enforced by local authorities. Every single one of these sites must have appropriate Legionella control measures in place.

Notable UK outbreaks linked to cooling towers

Some of the UK's most serious Legionella outbreaks have been traced to cooling towers:

  • Edinburgh, 2012 — Over 100 confirmed cases and three deaths, linked to a cooling tower in the city centre.
  • Stoke-on-Trent, 2012 — Two deaths and 21 affected, linked to a cooling tower at a retail warehouse.
  • Barrow-in-Furness, 2002 — Seven deaths and 172 confirmed cases, one of the largest UK outbreaks, traced to a hospital cooling tower.

These events underline why cooling tower compliance is treated so seriously by regulators, and why routine testing and maintenance cannot be treated as a box-ticking exercise.

What Are the Cooling Tower Legionella Testing Requirements?

Testing requirements for cooling towers in the UK are set out in HSG274 Part 1 (updated in 2024), the HSE's technical guidance specifically for evaporative cooling systems. The key requirements are:

Legionella microbiological sampling: at least quarterly

Routine Legionella sampling of cooling tower water must be carried out at a minimum of every three months (quarterly), using a UKAS-accredited laboratory. More frequent testing is required:

  • When the tower is first commissioned or recommissioned after a period of shutdown
  • If previous results have indicated elevated Legionella levels
  • Where risk assessment identifies a higher-risk system

Weekly dip-slide monitoring

In addition to quarterly microbiological sampling, HSG274 recommends that general bacterial counts (using dip slides) are carried out weekly. This provides an early warning that water quality is deteriorating between formal laboratory tests.

Regular chemical monitoring

Water chemistry, including pH, conductivity, and biocide concentration (where an oxidising biocide is used for control), should be monitored and recorded weekly. Water quality testing which may include Alkalinity, Hardness, Iron and chlorides are recommended to be conducted monthly. The April 2024 update to HSG274 Part 1 added specific guidance requiring that oxidising biocide test results be corrected for pH, as pH significantly affects the efficacy of some halogen-based treatments such as chlorine.

**Cleaning and disinfection:

The following timescales, though not prescriptive, can be considered typical for different situations:

  • At least every three months for a cooling system in a dirty environment (eg a tower that is prone to process or environmental contamination);
  • At least twice a year for an air conditioning comfort cooling system;
  • At least every 12 months for a ‘clean’ industrial application (and any others).
  • Following any period of shutdown, after maintenance work, or if microbiological results indicate a problem.

Temperature monitoring

Because temperature control alone cannot prevent Legionella growth in cooling towers (unlike hot water systems), temperature monitoring is not required on a routine basis but man need to be taken into consideration in the event that non-oxidising biocides are used to control bacterial growth within the system.

What Does the Law Require? Cooling Tower Guidelines Explained

Several pieces of legislation and guidance govern cooling tower Legionella control in the UK:

Notification of Cooling Towers and Evaporative Condensers Regulations 1992

Any premises with a wet cooling tower or evaporative condenser must notify their local authority before the system is installed or operated. Failure to register is a criminal offence. The local authority maintains a public register of all notified cooling towers in their area.

ACOP L8 — Approved Code of Practice

ACOP L8 is the primary legal framework for controlling Legionella in all water systems. It has special legal status: if you are prosecuted for a Legionella-related offence, failure to follow L8 can be used as evidence of non-compliance. ACOP L8 requires dutyholders to:

  • Carry out and document a suitable and sufficient Legionella risk assessment
  • Appoint a competent Responsible Person for Legionella control
  • Implement and maintain a written scheme of control
  • Keep records of all monitoring, maintenance, and test results

HSG274 Part 1 — Technical Guidance for Evaporative Cooling Systems

Published alongside L8 and updated in 2024, HSG274 Part 1 is the detailed technical document for cooling towers. It provides guidance for operators on exactly how to manage the risk — from water treatment programmes to testing methodologies and disinfection protocols.

Together, L8 and HSG274 form the definitive guidance framework that all cooling tower operators in the UK are expected to follow.

What Causes Legionella in Cooling Towers to Grow Out of Control?

Even well-maintained cooling towers can develop Legionella problems if certain conditions are not managed. The most common causes of bacterial amplification are:

Stagnation. Sections of pipework or plant that are not regularly flushed — known as dead legs — allow water to sit without treatment, creating ideal conditions for Legionella to establish and multiply.

Inadequate biocide dosing. If biocide concentrations fall below effective levels — whether due to underdosing, equipment failure, or pH not being properly accounted for — Legionella can grow unchecked.

Biofilm formation. Biofilm, a layer of organic matter and bacteria that adheres to surfaces, can protect Legionella from biocide treatment. Regular cleaning and descaling are essential to prevent biofilm establishing.

Shutdown periods. When a cooling tower is taken offline and recommissioned without proper cleaning and disinfection, stagnant water with potentially high bacterial loads is reintroduced to the system.

Drift eliminator failure. Drift eliminators reduce the volume of water droplets released into the air by the tower. If they are damaged or poorly maintained, aerosol output, and therefore the risk of transmission, increases significantly.

Your Compliance Checklist for Cooling Tower Legionella Control

If you operate or manage a cooling tower, the following must be in place:

  • ✅ Tower registered with your local authority
  • ✅ Current legionella risk assessment on file (reviewed internally at least annually and a new document commissioned following changes in management or control)
  • ✅ Documented Management Structure including Duty Holder, Responsible Person and Deputy Responsible appointed for Legionella control
  • ✅ Written scheme of control (WSoC) in place
  • ✅ Quarterly (Minimum) Legionella microbiological sampling by a UKAS-accredited laboratory
  • ✅ Weekly dip-slide bacterial monitoring
  • ✅ Regular water chemistry monitoring (pH, conductivity, chlorides, hardness, alkalinity, oxidising biocide levels — pH-corrected)
  • ✅ Regular cleaning and disinfection depending on the cleanliness of the system
  • ✅ Records of all monitoring, maintenance, inspections and cleans kept and available for inspection for 5 years

Useful Resources and Further Reading

The key HSE documents for cooling tower Legionella compliance:

Talk to a Cooling Tower Legionella Specialist

Managing cooling tower Legionella compliance is not something to handle without specialist support. The testing and monitoring regime, chemical treatment, documentation, and risk assessment requirements are technical — and the consequences of getting it wrong are serious.

Enviroteam Services is a certified member of the LCA of Legionella control services for industrial and commercial clients across the UK. Our team has over 150 years of combined expertise in water treatment, water hygiene, and Legionella management.

We can help you with:

  • Cooling tower Legionella risk assessments
  • Quarterly microbiological sampling and laboratory testing
  • Water treatment programmes and chemical dosing
  • Cleaning, disinfection, and recommissioning services
  • Written schemes of control and compliance documentation
  • Legionella training courses for your Responsible Person

Get in touch today:

📞 +44 (0) 1698 642 643 ✉️ enquires@enviroteamservices.co.uk 🌐 www.enviroteamservices.co.uk/contact

Frequently Asked Questions

Do I need to register my cooling tower with the local authority? Yes. Under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992, all wet cooling towers and evaporative condensers must be registered with the local authority before installation. This is a legal requirement — failure to register is a criminal offence. Registration is free.

How often does a cooling tower need to be tested for Legionella? As a minimum, Legionella microbiological sampling must be carried out quarterly (every three months) using a UKAS-accredited laboratory. Weekly dip-slide monitoring for general bacterial counts is also required. More frequent testing may be needed during commissioning or if previous results have been elevated.

Who is responsible for cooling tower Legionella control? The dutyholder — typically the employer or the person in control of the premises — holds legal responsibility. They must appoint a competent Responsible Person to oversee day-to-day Legionella management. For complex systems like cooling towers, using a specialist water treatment company is strongly recommended.

Can Legionella aerosol from a cooling tower travel outside the building? Yes. Legionella aerosol from a cooling tower can travel considerable distances — research has documented transmission well over one kilometre from the source. This is why cooling tower outbreaks can affect large numbers of people, including members of the public, not just building occupants.

What should I do if my cooling tower Legionella test results are high? Elevated results must be acted on immediately. Depending on levels this may include resampling or taking the system offline, carrying out emergency disinfection, identifying and addressing the cause which may require a clean of the internal components, and retesting before returning the system to service. Your water treatment provider should guide this process.

Carly Duffy
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