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Kerem IsmailLegionella bacteria grow in water between 20 and 45°C. People catch Legionnaires' disease by breathing in tiny droplets of contaminated water. Cooling towers and evaporative condensers are among the most common sources, because they keep warm water circulating and throw spray into the air. Air conditioning is only a risk if it uses water, through a wet cooling tower, an evaporative cooler or a humidifier. Here's what the law expects and who needs training.
What Legionella is and how people catch it
Legionella is a bacterium found naturally in rivers and ponds. It rarely causes illness there. The trouble starts in man-made water systems that keep it warm and give it something to feed on.
Legionnaires' disease is a potentially fatal form of pneumonia. Two milder illnesses, Pontiac fever and Lochgoilhead fever, come from the same bacteria. You catch them by inhaling droplets. It isn't known to spread from person to person.
The numbers are smaller than some old training material suggests. UKHSA recorded 463 confirmed cases in England and Wales in 2024, and 13 deaths. That's a fatality rate of 2.8%, down from 10.4% in 2020. Older people are hit hardest. A third of 2024's cases were aged 70 or over.
Why cooling towers carry the risk
HSE says a water system is a reasonably foreseeable Legionella risk if it has any of these:
- water between 20 and 45°C
- a way of creating or spreading breathable droplets
- water that's stored or recirculated
- rust, sludge, scale, biofilm or other matter the bacteria can feed on
A wet cooling tower ticks every box. It cools water by blowing air straight through it. The water is warm, it recirculates, and the air leaving the tower carries a fine spray. An evaporative condenser works the same way, spraying water over a coil.
That's why HSE gives these systems their own technical guidance. HSG274 Part 1 covers evaporative cooling from design and commissioning through water treatment, cleaning, disinfection and monitoring. HSE updated it in March 2024. One of the changes covers correcting biocide test results for the water's pH. If you rely on those tests, HSE says to check your latest results are pH corrected.
Is air conditioning a Legionella risk?
It depends on whether the system uses water.
- Air conditioning that rejects heat through a wet cooling tower or evaporative condenser carries the risk above.
- Hybrid or adiabatic coolers, which switch between dry and wet running, vary a lot. HSE says their risk profiles range widely, so each one needs assessing.
- Humidifiers and air washers built into air-handling systems are on HSE's list of other risk systems.
- A unit that doesn't hold or spray water doesn't give Legionella the conditions it needs.
Whatever the kit, HSE says every water system needs a risk assessment. For many, it'll show the risk is low and already under control.
What the law requires
Assess, control and record
The duties come from the Health and Safety at Work etc Act 1974 and the Control of Substances Hazardous to Health Regulations 2002 (COSHH). If you're an employer or you control premises, you must:
- identify and assess sources of Legionella risk
- prepare a scheme to prevent or control it, and put it in place
- appoint a competent person to take day-to-day responsibility, known as the responsible person
- keep records
HSE's Approved Code of Practice, L8, explains how. Its fourth edition was published in 2013. Once you employ five or more people, the significant findings have to be written down. Monitoring records should be kept for at least five years.
Notify your council about cooling towers
The Notification of Cooling Towers and Evaporative Condensers Regulations 1992 add a duty specific to these systems. Whoever controls the premises must notify the local authority in writing, on an HSE-approved form, before a cooling tower or evaporative condenser is on site.
Tell the council about any change to those details within a month. Tell them again when the device is taken out of use for good. You don't need to for maintenance or a seasonal shutdown. HSE's L8 says the main purpose is to help investigators trace outbreaks. HSE's page on these duties explains where to get the form.
Report cases at work
Under RIDDOR, employers must report legionellosis to HSE when a doctor notifies them of a case in an employee whose job involves work on or near cooling systems, or on water systems likely to be contaminated.
Who needs Legionella awareness training?
L8 is clear that training matters. It says inadequate management, lack of training and poor communication all contribute to outbreaks. It picks out two groups:
- The responsible person and anyone they appoint to assess risk or run controls. L8 says they must be properly trained to a technically competent level, with regular refreshers and records of all training.
- Anyone else whose work could expose people to Legionella. L8 says they need suitable information, instruction and training.
In practice, that covers facilities and estates managers, building services engineers, maintenance staff, plumbers, and HVAC technicians who work on cooling systems. On construction sites, it can include anyone setting up or maintaining temporary water systems.
Be honest about the limits. An awareness course is a starting point. Running the water treatment programme on a cooling tower needs specialist training and experience on top.
Our online Legionella awareness training course is aimed at facility managers, health and safety managers and maintenance staff. It covers how to identify Legionella risks, control measures, and the law and HSE guidance behind them. You can study on a phone, tablet or computer, and a pass earns an e-certificate that day.
Where to go from here
If you have a cooling tower or evaporative condenser, check it's notified to your council and that the written control scheme is current. If you don't know what's cooling your building, find out. Then make sure the people looking after your water systems are trained for the job they do.
Start with our Legionella awareness course, and browse our other health and safety courses. If you work on clean water networks, read our guide to the EUSR National Water Hygiene card.
Frequently asked questions
Can you get Legionnaires' disease from air conditioning?
Only from systems that use water. Air conditioning served by a wet cooling tower or evaporative condenser can spread contaminated spray if it isn't controlled. Humidifiers and air washers can too. A unit that doesn't hold or spray water doesn't give Legionella the warm, stored water and droplets it needs. Every water system still needs a risk assessment.
Do I have to register a cooling tower?
Yes. Under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992, whoever controls the premises must notify the local authority in writing, on an HSE-approved form. Report changes within a month, and tell the council when the device is permanently out of use. Some hybrid dry and wet coolers may not need notifying, but HSE says to check each one.
What temperature does Legionella grow at?
Legionella grows in water between 20 and 45°C, according to HSE. That's why hot water should be stored at 60°C or above and cold water kept below 20°C. Temperature control works alongside keeping water moving, keeping systems clean and, in cooling towers, treating the water with biocides.
Who is the responsible person for Legionella?
It's the competent person the employer or person in control of premises appoints to manage Legionella risk day to day. HSE says they need enough authority, competence, knowledge of the system and experience. It can be an employee or someone from outside the business, and a competent self-employed dutyholder can appoint themselves. The dutyholder keeps the legal duty either way.
How often do you need Legionella training?
The law doesn't set a date. L8 says people who run Legionella controls must get regular refresher training, and records of all initial and refresher training must be kept. Training should also be updated when the work or methods change significantly. Set the refresher cycle in your written control scheme so it doesn't drift.
Is Legionnaires' disease contagious?
No. HSE says Legionnaires' disease isn't known to spread from person to person. People catch it by breathing in small droplets of water that contain the bacteria, such as spray from a cooling tower, a shower or a spa pool. Anyone with flu-like symptoms who's worried they've been exposed should see a doctor.
Kerem Ismail is Managing Director of The Builders Academy and has been helping people secure work in the construction industry since 2016.
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