Legionella in Cooling Towers: Risks, Warning Signs and How to Prevent It

A cooling tower that looks perfectly normal can still be releasing a fine mist that carries Legionella bacteria into the air around your building. Most large outbreaks of Legionnaires’ disease worldwide have been traced back to poorly maintained cooling towers, and in warm climates such as Southeast Asia the conditions for growth are present all year round.

This guide explains what Legionella is, why cooling towers are such a risk, what test results mean, and the practical steps that keep your system and the people around it safe.

What Is Legionella?

Legionella is a group of bacteria found naturally in lakes, rivers and soil. At those low natural levels it rarely causes harm. The danger starts when it enters man-made water systems, multiplies in warm water, and is released as aerosols that people breathe in.

The bacteria were named after a 1976 outbreak at an American Legion convention in Philadelphia, where a then-unknown pneumonia affected many attendees. More than 50 species are now known; Legionella pneumophila causes most human infections.

Legionnaires’ Disease and Pontiac Fever

Infection with Legionella is called legionellosis, which takes two main forms:

• Legionnaires’ disease: a serious pneumonia with fever, cough, shortness of breath and muscle pain. It can be fatal, especially for vulnerable people.
• Pontiac fever: a milder, flu-like illness that usually clears up on its own within a few days.

People catch it by inhaling contaminated water droplets, not by drinking water normally, and it does not spread from person to person. The risk is highest for people over 50, smokers, and anyone with a weakened immune system or chronic heart, lung or kidney disease. That is why hospitals, hotels and nursing homes need to be especially careful.

Why Cooling Towers Are a Major Source

A cooling tower combines everything Legionella needs:

• Warm water: recirculating water typically sits between 25 °C and 40 °C, right in the bacteria’s growth range.
• Nutrients and shelter: scale, sludge, rust and biofilm feed and protect the bacteria.
• Host organisms: Legionella multiplies inside amoebae living in biofilm, which shields it from disinfectants.
• Aerosols: fans and drift release fine droplets that can travel hundreds of metres, sometimes several kilometres.
• Stagnant zones: dead legs, idle standby towers and poorly circulated basins allow rapid growth.

One badly maintained tower can expose a whole neighbourhood, which is why some of the largest recorded outbreaks, with hundreds of cases, came from a single cooling tower.

How Temperature Affects Legionella

Cooling-tower water cannot simply be heated or chilled out of the danger zone, so control depends on keeping the system clean and treated.

Legionella Test Results: What the Numbers Mean

Legionella is measured in colony-forming units per litre (CFU/L) from regular water samples. A widely used European guideline sets these action levels for cooling systems:

In short: up to 1,000 CFU/L means the system is under control; 1,000–10,000 CFU/L calls for immediate resampling and a review of treatment; above 10,000 CFU/L requires immediate disinfection and corrective action.

Healthcare facilities usually apply much stricter limits. Always follow your local regulations and your own risk assessment.

How to Prevent Legionella in Cooling Towers

1. Carry out a risk assessment and water management plan. Map the system, identify stagnant areas and spray sources, assign responsibilities, and review the plan at least every two years or whenever the system changes.
2. Remove what Legionella feeds on. Scale, sludge, corrosion products and biofilm give the bacteria food and protection. A clean system is far harder to colonise. Electrolysis Scale Treatment (EST) continuously removes hardness from the water and reduces scale and biofilm build-up without scale-inhibitor chemicals, taking away the habitat Legionella depends on.
3. Disinfect effectively. Maintain an appropriate biocide programme, such as chlorine, chlorine dioxide or other approved disinfectants, and check residual levels routinely. Disinfectants work far better on clean surfaces than on fouled ones.
4. Keep water moving. Avoid dead legs, circulate standby towers regularly, and do not leave systems stagnant during shutdowns.
5. Control drift. Keep drift eliminators intact and correctly installed to minimise aerosol release.
6. Clean and inspect regularly. Drain and clean basins and fill, and inspect for slime, algae and deposits at least twice a year.
7. Test and keep records. Sample for Legionella at regular intervals, trend the results, and keep written records. In many countries, poor records have led to heavy fines after outbreaks.

Why Biofilm Control Matters Most

Most Legionella in a cooling system does not float freely in the water. It lives inside biofilm on pipe walls, fill media and heat-exchanger tubes, where amoebae protect it from disinfectants. That explains why systems with high chemical dosing can still return positive Legionella results: the chemicals never reach the bacteria hiding in the slime.

Controlling scale and biofilm at the source helps your disinfection programme work as intended, and it improves heat transfer and energy efficiency at the same time.

Protect Your People and Your Reputation

A Legionella outbreak can mean serious illness, legal liability and lasting damage to a hospital’s, hotel’s or factory’s reputation. Prevention costs far less.

Water Treatment Asia helps hospitals, hotels, commercial buildings and industrial plants across Southeast Asia keep cooling towers clean with Elgressy Electrolysis Scale Treatment, reducing scale and biofilm that harbour Legionella while cutting chemical use and cleaning downtime. Contact our team for a free cooling-tower assessment.