How HVAC Systems Spread Legionnaires' Disease and How to Prevent It

Legionnaires’ disease isn’t something you catch from a sneeze or a handshake. It’s not passed person to person. Instead, it hides in the air you breathe-especially if your building’s HVAC system is poorly maintained. Every year in the UK, over 300 cases are reported, and many of them trace back to cooling towers, humidifiers, and air conditioning units that should’ve been cleaned months ago. This isn’t a rare accident. It’s a preventable public health failure.

What Exactly Is Legionnaires’ Disease?

Legionnaires’ disease is a severe form of pneumonia caused by inhaling tiny water droplets infected with Legionella bacteria. These bacteria live naturally in freshwater environments like lakes and streams. But they don’t become dangerous until they find their way into human-made water systems-especially warm ones.

The bacteria thrive between 20°C and 45°C. That’s the exact temperature range many building water systems operate in. Once they settle into pipes, tanks, or cooling towers, they multiply fast. And when those systems are connected to HVAC units, the bacteria get blown into the air as mist or aerosols. People nearby breathe them in. Their lungs can’t fight off the infection, and within days, they’re in hospital.

It’s not just the elderly or people with weak immune systems at risk. Healthy adults can get sick too. In 2023, a cluster of cases in Nottingham was linked to a hotel’s rooftop cooling tower. Five people were hospitalized. One died. The system had gone uncleaned for 18 months.

How HVAC Systems Become Legionella Havens

Not all HVAC systems are the same, but they all move air-and sometimes water-through buildings. The real danger comes from components that involve water:

• Cooling towers: These are the biggest culprits. They use water to cool large buildings. As water evaporates, it creates mist. If the water is contaminated, that mist carries Legionella into the air.
• Humidifiers: Used in offices, hospitals, and homes, these devices add moisture to dry air. If the water reservoir isn’t cleaned weekly, bacteria grow inside.
• Decorative fountains and misters: Even aesthetic features in lobbies or shopping centres can spread the bacteria if water sits stagnant.
• Hot and cold water systems: HVAC units that rely on water for temperature control can draw from building plumbing. If water sits in pipes longer than 24 hours, especially at 30-40°C, Legionella blooms.

It’s not the air itself that’s dirty. It’s the water inside the system. A single dirty cooling tower can contaminate air across multiple floors. And because these systems often run 24/7, the risk never stops.

Why Regular Maintenance Isn’t Optional

The UK’s Health and Safety Executive (HSE) requires building managers to assess and control Legionella risks under the Legionnaires’ disease: Approved Code of Practice and Guidance (L8). But compliance is patchy. Many landlords think, “If it’s working, why fix it?”

Here’s what actually needs to happen:

1. Water temperature control: Hot water must be stored at 60°C and delivered at 50°C. Cold water must stay below 20°C. If water sits in the danger zone (20-45°C), bacteria multiply.
2. Flushing stagnant lines: Unused outlets, like guest room taps or rarely used showers, need flushing at least once a week. Water that sits for days becomes a breeding ground.
3. Cleaning cooling towers: These need professional cleaning every three months, plus quarterly disinfection with biocides. Rust, algae, and scale inside the tower trap bacteria and protect them from chemicals.
4. Filter replacement: HVAC air filters don’t catch Legionella-they’re too small. But dirty filters reduce airflow, causing systems to overheat and create more condensation, which feeds bacterial growth.
5. Monitoring and testing: Water samples should be tested every six months in high-risk buildings (hospitals, hotels, care homes). Results must be logged. No logs? That’s a legal violation.

In 2024, a care home in Derby was fined £120,000 after a resident died from Legionnaires’ disease. The water system hadn’t been tested in over two years. The HSE report called it “a textbook example of negligence.”

Who’s Most at Risk?

Legionnaires’ disease doesn’t pick favorites-but some people are far more likely to get seriously ill:

• People over 50
• Smokers or former smokers
• People with chronic lung disease (COPD, emphysema)
• Those with weakened immune systems (cancer patients, transplant recipients)
• People on long-term steroid use

But here’s the twist: even healthy people can get sick. In 2022, a 32-year-old gym-goer in Birmingham developed pneumonia after using a misting system in a sauna. He recovered, but spent three weeks in hospital. The system had been running without cleaning since 2020.

The risk isn’t just in hospitals or hotels. Office buildings, schools, and apartment complexes with central HVAC systems are equally vulnerable. You don’t need to be old or sick to be exposed. You just need to be in the wrong place at the wrong time.

What You Can Do as a Tenant or Visitor

You can’t inspect a cooling tower. But you can ask questions-and act if something feels off.

• If you smell a musty, damp odor near vents or air outlets, speak up. That’s often a sign of stagnant water or biofilm buildup.
• Check if your building has a visible water management plan posted. Legally, it should be.
• Don’t ignore malfunctioning air conditioners. If the unit drips water or smells strange, report it immediately.
• When staying in hotels, run the shower for 2-3 minutes before use. This flushes out water that’s been sitting in pipes overnight.
• If you’re part of a building committee or tenant association, push for annual Legionella risk assessments. Don’t wait for someone to get sick.

There’s no law forcing landlords to tell you their HVAC system is clean. But you have the right to know if there’s a known risk. If they refuse to provide documentation, contact your local environmental health department. They can inspect the building.

What Happens When Systems Are Cleaned Right

It’s not just about avoiding lawsuits or fines. Proper maintenance saves lives-and money.

A hospital in Leeds cut its Legionella cases to zero after installing automated water temperature controls and monthly biocide treatments. They spent £8,000 on upgrades. The cost of one hospitalised patient? Over £15,000.

Another example: a supermarket chain in the Midlands retrofitted all 42 stores with self-cleaning humidifiers and real-time water sensors. They reduced maintenance costs by 30% and saw zero Legionella incidents in three years.

Good systems don’t just prevent disease. They improve air quality, reduce energy waste, and extend equipment life. Cleaning isn’t a cost-it’s an investment.

What’s Being Done in the UK?

The UK has some of the strictest Legionella regulations in Europe. But enforcement is inconsistent. Local councils handle inspections, but many are understaffed. The HSE focuses on high-risk sites-hospitals, care homes, large commercial buildings. Smaller properties slip through.

In 2025, the government announced a new pilot program to require all buildings over 5,000 sq ft to register their water systems with local authorities. It’s a start. But experts say the real fix is education. Property managers need training. Tenants need to know their rights.

Meanwhile, new technology is helping. Smart sensors now monitor water temperature and flow in real time. Some systems even auto-disinfect using UV light. These aren’t science fiction-they’re available now, and prices are dropping.

Final Thought: This Isn’t Just a Building Problem

Legionnaires’ disease is a silent killer. It doesn’t make headlines unless dozens get sick at once. But behind every case is a system that was ignored. A pump that wasn’t checked. A filter that wasn’t replaced. A manager who thought, “It’ll be fine.”

You can’t see Legionella. You can’t smell it until it’s too late. But you can demand accountability. Whether you’re a tenant, a property owner, or just someone who walks into a building every day-you have a role to play.

Ask. Check. Report. Don’t wait for someone else to act.