Can you get legionella from air conditioning?
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Key points
- Standard air conditioning is not the risk. Split units and most comfort cooling use a sealed refrigerant circuit and do not create a water aerosol.
- The risk is water-based cooling: evaporative cooling towers and condensers, and humidifiers or evaporative air coolers that store and spray water.
- Cooling towers have their own regime under HSE guidance HSG274 Part 1, and must be notified to the local authority.
- The UK's worst outbreak, Barrow-in-Furness 2002, came from a cooling system serving air conditioning, which is why the two are linked in people's minds.
- If you have any system that sprays or mists water, it belongs in the legionella risk assessment.
Most air conditioning carries little or no legionella risk, because a standard unit cools air with refrigerant, not water. The real risk sits with systems that cool by spraying or evaporating water: cooling towers, evaporative condensers and water-based humidifiers. Those are covered by HSE guidance HSG274 Part 1.
The confusion is understandable. The UK's deadliest outbreak, at Barrow-in-Furness in 2002, was traced to a cooling system serving an arts centre's air conditioning, and headlines ever since have tied legionella to air conditioning. The detail that matters is which part of that system caused it: not the refrigerant circuit that chilled the air, but the water-cooled plant that rejected the heat. This page separates the three technologies that get lumped together as air conditioning.
Three different things get called air conditioning
1. Refrigerant air conditioning (split units, most comfort cooling)
A domestic wall unit, an office split system or a car's air conditioning all work the same way: a sealed circuit of refrigerant absorbs heat from the indoor air and dumps it outside. Water plays no part in delivering the cooling. The unit does produce a small amount of condensate where humid room air meets the cold coil, but that drains away and is not sprayed into the room as a breathable mist. HSE legionella guidance does not treat these systems as a legionella risk, and there is no established pattern of domestic split units causing legionnaires' disease.
Legionella spreads when contaminated water is turned into a fine aerosol that is breathed deep into the lungs (see how legionella spreads). A refrigerant circuit does not do that, which is why the honest answer for home air conditioning is: not a recognised source.
2. Evaporative coolers and humidifiers (water tanks that mist)
Portable evaporative air coolers, and some humidifiers used in offices and industry, work differently: they hold a reservoir of water and release it as a mist or pass air over wet media. Now there is stored water and an aerosol, the two ingredients legionella needs, so these units need to be maintained: emptied, cleaned and descaled regularly, filled with fresh water, and included in the risk assessment where they are used at work. A hotel humidifier caused the Copthorne Hotel Cardiff outbreak recorded in our enforcement tracker, in which two guests died and five others fell ill between 1999 and 2000.
3. Evaporative cooling towers and condensers (the regulated systems)
Large air conditioning and industrial plant reject heat through a cooling tower or evaporative condenser: recirculating water is sprayed through an airstream so some of it evaporates and carries heat away. This is a large, warm, open water system that deliberately creates an aerosol and can drift on the wind beyond the site boundary. It is the highest-risk category of water system, and it is regulated accordingly:
- HSE guidance HSG274 Part 1 sets out the control regime: a written scheme, water treatment, routine monitoring and cleaning, covered in cooling tower legionella control.
- Under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992, every cooling tower and evaporative condenser must be notified in writing to the local authority. See do I need to register a cooling tower.
When the control regime fails, the consequences are public, not just internal. In the Riaar Plastics case (West Bromwich, 2020), five people, including members of the public, contracted legionnaires' disease near a factory whose cooling towers were in extremely poor condition; the company was fined £50,000 plus costs in 2023. The Faltec case in the North East followed a similar pattern: recirculating cooling water, five people infected including a local resident, and an £800,000 fine in 2018.
The question to ask of any cooling system is simple: does it store water and turn it into a mist? If yes, it belongs in the risk assessment and, for towers and condensers, under HSG274 Part 1 and local-authority notification. If it is a sealed refrigerant circuit, it does not.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Barrow 2002: why air conditioning got the blame
The Barrow-in-Furness outbreak of August 2002 remains the worst in UK records: seven people died and around 180 fell ill. The source was a poorly maintained cooling system serving the air conditioning at the council-run Forum 28 arts centre. Barrow Borough Council was convicted of breaching section 3(1) of the Health and Safety at Work etc. Act 1974 and fined £125,000 plus £90,000 costs in July 2006; it was also the first UK public body to face corporate manslaughter charges, which were dropped before the conviction (source: HSE account of the Barrow outbreak).
Barrow is why legionella and air conditioning are linked in the public mind, and the link is half right. The failed equipment was the water side of the cooling plant. The lesson for duty holders is not that air conditioning is dangerous; it is that any plant which sprays water, inside or outside the building, needs a written scheme, monitoring and records.
What this means in practice
- Home with a split air conditioner: no legionella control regime applies; keep the unit maintained per the manufacturer's instructions.
- Portable evaporative cooler or humidifier: empty, clean and descale it regularly, and never leave water standing in the tank for weeks.
- Workplace with any misting or spray equipment: it goes in the legionella risk assessment with a control regime.
- Cooling tower or evaporative condenser: full HSG274 Part 1 regime, notification to the local authority, and specialist water treatment support.
As with everything in legionella control, the record is the evidence: if a piece of equipment is water-based and sprays, write down what it is, how it is controlled, and every check you make on it.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
Can you get legionella from air conditioning?
From a standard refrigerant air conditioning unit, the risk is negligible, because these systems cool air with a sealed refrigerant circuit and do not spray water. The legionella risk sits with water-based cooling: evaporative cooling towers and condensers, and evaporative humidifiers or air coolers that hold a tank of water and release a mist. HSE guidance (HSG274 Part 1) covers those systems specifically.
Can home air conditioning cause legionnaires' disease?
A domestic split air conditioner is not a recognised legionella source: it contains no stored water that is turned into a breathable spray. Portable evaporative air coolers and humidifiers are different, because they hold a reservoir of water and emit a mist, so they need regular emptying, cleaning and descaling in line with the manufacturer's instructions.
Is a cooling tower the same as air conditioning?
No, but the two are often connected. Large buildings reject heat from their air conditioning through an evaporative cooling tower or condenser, which cools recirculating water by spraying it through an airstream. That spray is exactly the aerosol that carries legionella, which is why cooling towers have their own HSE guidance (HSG274 Part 1) and must be notified to the local authority.
Do I need to worry about legionella in my car air conditioning?
Car air conditioning uses a sealed refrigerant circuit like a domestic split unit, and is not a recognised legionella source. The musty smell people associate with car air conditioning comes from microbial growth on the damp evaporator surface, which is a maintenance nuisance rather than legionella. The documented vehicle-related legionella risks are elsewhere, for example screen-wash reservoirs in some studies, not the air conditioning circuit.
What was the Barrow legionella outbreak?
In August 2002 an outbreak in Barrow-in-Furness killed seven people and made around 180 ill. The source was a poorly maintained cooling system serving the air conditioning at the council-run Forum 28 arts centre. Barrow Borough Council was convicted of breaching section 3(1) of the Health and Safety at Work Act and fined £125,000 plus £90,000 costs in 2006 (source: HSE account of the outbreak). It remains the UK's worst legionella outbreak and the reason air conditioning is so often linked with legionella.
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