In 1976, an invisible threat sickened over 200 people at a Philadelphia convention, and 34 of them died. The source was a mystery until investigators traced it to the hotel’s air-conditioning system. This is how the world learned about Legionnaires’ disease and the bacteria that causes it. While this happened decades ago, the same risk exists today. For anyone in charge of a multi-tenant or corporate building, understanding the danger of Legionella bacteria in the AC of a commercial building is not just important—it’s a critical part of ensuring public health and safety within your facility’s walls.
You may be thinking:
What in the world is Legionella?
If you own a corporate building or a multi-tenant building and you’re in charge of maintaining the building, then you should know about this.
Legionella is a bacteria which causes Legionellosis (also known as Legionnaires’ disease). If you don’t know, Legionnaires’ disease is a form of pneumonia which is characterized by chills, a high fever, coughing, aches, and other terrible symptoms you don’t want to experience. It was fairly recently discovered. In 1976 over 200 attendees of the American Legion convention in Philadelphia became sick–no one knew how or what they had. Of those who were sick, 34 people died from the disease. That’s where it was discovered and where it got its name.
What is Legionnaires’ Disease and Who is at Risk?
It’s one thing to know where the name came from, but it’s another to understand what the disease actually is and who it affects. The term “Legionellosis” covers two distinct illnesses caused by Legionella bacteria, and they have very different outcomes. While the bacteria is found naturally in freshwater environments like lakes and streams, it becomes a health concern when it grows and spreads in human-made water systems. Understanding the difference between the two illnesses it causes, how it travels, and who is most vulnerable is key to protecting yourself and the people in your buildings.
Types of Illness Caused by Legionella
When someone is exposed to Legionella bacteria, they can develop one of two conditions: the severe Legionnaires’ disease or the much milder Pontiac fever. Think of them as two sides of the same coin. Both are caused by the same bacteria, but the impact on the body is vastly different. One is a serious medical condition requiring immediate attention, while the other typically resolves on its own without causing major complications. Knowing the distinction helps clarify the potential risks associated with exposure.
Legionnaires’ Disease
This is the more severe illness and the one people are usually referring to when they talk about Legionella. Legionnaires’ disease is a serious type of pneumonia, or lung infection. According to the Centers for Disease Control and Prevention (CDC), symptoms can include cough, shortness of breath, fever, muscle aches, and headaches. It requires prompt treatment with antibiotics and can often lead to hospitalization. For vulnerable individuals, it can unfortunately be fatal, which is why prevention in building water systems is so critical.
Pontiac Fever
Pontiac fever is the milder infection caused by the same Legionella bacteria. The symptoms are more flu-like, including fever and muscle aches, but it does not lead to pneumonia. People with Pontiac fever usually get better on their own within a week without requiring specific medical treatment. While it’s certainly unpleasant, it doesn’t carry the same severe health risks as Legionnaires’ disease. It’s named after Pontiac, Michigan, where the first identified outbreak occurred in 1968.
How It Spreads (and How It Doesn’t)
The most common way people get sick is by breathing in small droplets of water in the air—a mist or vapor—that contain Legionella bacteria. It’s not spread from person to person. This is an important distinction because it means the source is always environmental. The good news for homeowners is that your standard home or car air conditioning unit is not a risk; these systems don’t use water to cool the air, so they don’t create the conditions for Legionella to grow and spread. The risk primarily lies with larger, more complex water systems. This includes sources like decorative fountains, hot tubs, and the large-scale cooling towers often used in commercial HVAC systems. These systems can create the aerosolized water droplets that transmit the bacteria. That’s why proper maintenance of commercial and industrial HVAC and water systems is not just about efficiency and comfort—it’s a critical public health measure.
High-Risk Individuals
It’s important to know that most healthy people who are exposed to Legionella bacteria do not get sick. Our immune systems are generally equipped to handle it. However, certain groups of people are at a much higher risk of developing Legionnaires’ disease if they are exposed. This vulnerability is usually tied to age, lifestyle habits, or pre-existing health conditions that compromise the body’s ability to fight off infection. According to the CDC, people at an increased risk include individuals aged 50 or older, current or former smokers, and those with chronic lung disease or weakened immune systems. People with underlying illnesses like diabetes, kidney failure, or liver failure are also more susceptible. For these individuals, exposure can be especially dangerous and may lead to severe complications. This underscores the responsibility that building owners and facility managers have to ensure their water systems are properly maintained through regular service programs to protect not only their tenants and employees but also the most vulnerable members of the community.
Now why is this important?
Obviously you don’t hear about epidemics of Legionnaires’ disease on the news, you probably didn’t even know what it was, so why does it matter, right? As HVAC professionals, we learn all about it because it’s essential knowledge in our industry. In 1976 the bacteria spread through the HVAC system of the hotel and infected over 200 people and killed over 30.
The bacteria that causes Legionnaires’ disease is commonly found in cooling towers or commercial-size heating and air systems.
It thrives in warm, stagnant water. And when people inhale the mist or vapor contaminated by the warm air, pushed through the building via the HVAC system, it easily spreads and causes sickness.
Conditions That Help Legionella Thrive
Legionella doesn’t just appear out of thin air. It needs a specific environment to grow and multiply, and unfortunately, large building water systems can create the perfect storm. Understanding these conditions is the first step in preventing the bacteria from taking hold in your facility. It’s not about luck; it’s about managing the environment within your HVAC and water systems. Three key factors create a welcoming home for Legionella: water temperature, the presence of biofilm, and the chemical balance of the water itself. By controlling these elements, you can significantly reduce the risk of contamination and keep your building’s occupants safe.
Specific Temperature Range
Like most living things, Legionella bacteria have a preferred temperature range where they can really flourish. They thrive in water temperatures between 77°F and 113°F. Think of it as their comfort zone. This temperature is often found in large, complex water systems like cooling towers, hot water tanks, and even decorative fountains, especially when they aren’t properly maintained. Stagnant water sitting in pipes within this temperature range is a major risk. Regular monitoring and maintenance of your commercial HVAC and water systems are critical to ensure water is either kept too hot or too cold for the bacteria to multiply, effectively shutting down their ideal breeding ground before it becomes a problem.
The Role of Biofilm
Another major factor is the presence of biofilm. The CDC describes biofilm as a slimy layer where germs can grow. It’s a collection of microorganisms that stick to each other and to surfaces in moist environments—like the inside of pipes, water tanks, and cooling tower basins. This slimy layer acts like a protective shield for Legionella, providing nutrients and protecting it from disinfectants that are meant to kill it. Once a biofilm is established, it’s much harder to eliminate the bacteria. This is why routine physical cleaning and professional servicing of your equipment are so important; simply treating the water isn’t always enough to break through this protective barrier.
Water Chemistry and Disinfectants
The chemistry of the water in your building’s systems also plays a huge part. Legionella bacteria are happiest in water with a pH between 6 and 8. More importantly, they can multiply rapidly when there aren’t enough disinfectants, like chlorine, to keep them in check. If disinfectant levels drop too low, the bacteria have a green light to grow. When this contaminated water is aerosolized into tiny droplets—through a cooling tower, a showerhead, or a faucet—the bacteria can become airborne. Inhaling these microscopic, contaminated droplets is how the infection spreads. Proper water treatment and management are essential to maintain a chemical balance that is hostile to Legionella.
Places where the bacteria, Legionella, are often found are in
- Cooling towers
- Large air conditioning (HVAC) systems
- Hot water tanks
- Hot water systems
- Ice making machines
- Humidifiers
- Whirlpool spas
In various studies, an average of as many as 40-60% of cooling towers tested positively for Legionella. Meaning it is a lot more common than we think!
Common Sources of Legionella in Buildings
So, where does this bacteria actually come from? The risk isn’t about the air itself, but rather about tiny water droplets suspended in it. Legionella needs a very specific environment to grow: stagnant water that has plenty of nutrients and sits at a temperature between 77°F and 113°F. When that contaminated water is aerosolized into a mist or vapor, it can be easily inhaled, leading to a serious infection. This is precisely why certain water systems, particularly those found in larger commercial or industrial buildings, demand careful and consistent professional maintenance to prevent the conditions that allow this bacteria to flourish and spread.
Commercial HVAC Systems vs. Home AC
The primary concern for Legionella growth is in large commercial HVAC systems, specifically those that use cooling towers. These towers work by releasing a fine mist of water to cool the building, and if the water inside is warm and stagnant, it becomes an ideal breeding ground. When the system runs, it can send aerosolized droplets containing the bacteria into the air, where they can be inhaled by people nearby. This is a stark contrast to your standard residential air conditioning unit. Home AC systems operate on a different principle and do not create the airborne water mist that can carry and spread Legionella, so you don’t need to worry about this risk from the AC in your house.
Other Building Water Systems
While HVAC systems are a major source, they aren’t the only place Legionella can thrive. Any system where water is kept warm and can become stagnant is a potential risk. This includes decorative fountains, hot tubs, large plumbing systems in hotels or hospitals, and even industrial ice machines. You might also find it in less obvious places, like the produce misters in a grocery store or in dental water lines. The common thread is always the presence of warm, standing water that can be turned into a vapor or mist that people can breathe in. This highlights why professional maintenance of all building water systems is so important for preventing bacterial growth and ensuring safety.
As a property manager or owner, you have the power to prevent this disease from thriving and spreading in your buildings.
The only way for a person to contract the disease is by ingesting or inhaling the contaminated water vapors or mists, which are spread through poorly maintained HVAC systems. So the main way to protect your building’s occupants from Legionnaires’ disease is by keeping up with your HVAC system’s preventative maintenance. Scheduling routine, preventative maintenance for your commercial air conditioning systems not only saves you money and extends the life of the system, but it also will save lives. 1 in every 10 cases of Legionnaires’ disease is fatal. Learn more about preventative maintenance for your commercial HVAC system by clicking here.
Key Prevention and Control Strategies
Preventing Legionella growth is all about proactive management of your building’s water systems. It’s not about a single quick fix, but rather a consistent, strategic approach to maintenance and control. According to the Occupational Safety and Health Administration (OSHA), a few key strategies can significantly reduce the risk of contamination. By focusing on temperature control, regular cleaning, and proper water treatment, you can create an environment where Legionella simply can’t thrive. This protects not only the health of your building’s occupants but also ensures your systems are running efficiently and safely, which is a core part of responsible property management.
Implement a Water Management Program
The most effective first step is to establish a formal water management program. This isn’t just a casual checklist; it’s a comprehensive plan that outlines how you’ll monitor and maintain all the water systems in your building, from the cooling tower to the ice machines. The goal is to identify areas where Legionella could grow and implement control measures to stop it before it starts. Developing this kind of program can feel overwhelming, which is why partnering with professionals is so valuable. A dedicated team can help you create and execute a plan tailored to your building’s specific needs, ensuring every detail is covered for long-term safety and compliance.
Control Water Temperature in Hot Water Systems
Legionella bacteria have a preferred temperature range for growth, typically between 68°F and 122°F. One of the most direct ways to prevent its spread is to keep your water temperatures outside of this danger zone. OSHA recommends maintaining hot water heaters at or above 140°F and ensuring the water delivered to all faucets is at least 122°F. This simple but critical step makes the environment too hot for the bacteria to multiply effectively. Regularly checking these temperatures should be a key part of your building’s maintenance routine, as it’s a powerful, front-line defense against contamination in your hot water systems.
Professional System Cleaning and Design
Over time, water systems can develop biofilm—a slimy layer of microorganisms—along with scale and sediment. This buildup provides the perfect nutrient source and hiding place for Legionella. That’s why regular, professional cleaning of all water system components is non-negotiable. Beyond just cleaning, the initial design of your plumbing and HVAC systems plays a huge role. A well-designed system minimizes areas where water can become stagnant, which is a major risk factor. If you’re concerned about your current setup, our experts at QRC can assess your industrial HVAC and water systems to identify and correct potential design flaws.
Cooling Tower Maintenance
Cooling towers are a common source of Legionella outbreaks, making their maintenance especially critical. Because they use evaporation to cool water, they can easily disperse contaminated water droplets into the air. To combat this, OSHA advises that cooling towers be thoroughly cleaned and disinfected at least twice a year. This process involves draining the system, physically cleaning all surfaces to remove sludge and biofilm, and then treating it with appropriate biocides. This is a specialized task that requires the right equipment and expertise to ensure it’s done safely and effectively, making it a job best left to trained professionals.
Plumbing System Considerations
Stagnant water is Legionella’s best friend. Plumbing systems with dead-end pipes, unused sections, or infrequent flow create ideal breeding grounds. Proper system design aims to eliminate these areas, ensuring water is constantly circulating. It’s also important to keep hot and cold water lines well-separated and insulated to maintain their respective temperatures and prevent them from entering the bacterial growth “danger zone.” If you’re planning a new build or a major renovation, consulting with an HVAC and plumbing expert can ensure your system is designed from the ground up to minimize these risks and promote a healthier building environment.
Water Treatment and Disinfection
Even with excellent cleaning and temperature control, a water treatment plan is an essential layer of protection. This involves using chemical disinfectants like chlorine, bromine, or other biocides to kill or inhibit the growth of Legionella and other harmful microbes. The right chemical and dosage depend entirely on your specific system, water chemistry, and usage. Handling these chemicals requires expertise to ensure they are effective without causing corrosion to your equipment or posing a risk to people. This is why water treatment should always be managed by professionals who understand the science behind it and can implement a safe, effective disinfection strategy.
Worker Safety During Maintenance
While protecting building occupants is the primary goal, we can’t forget about the safety of the maintenance workers who service these systems. When technicians perform tasks like cleaning cooling towers or servicing contaminated water systems, they can be directly exposed to high concentrations of Legionella bacteria. Following strict safety protocols isn’t just a matter of compliance; it’s about protecting the health and well-being of the people doing this essential work. This includes using the right personal protective equipment (PPE) and adhering to established safety guidelines to minimize the risk of inhaling contaminated aerosols during their work.
Personal Protective Equipment (PPE) for Staff
When there’s a potential for Legionella exposure, standard work gear isn’t enough. Technicians should wear appropriate PPE to protect themselves. OSHA specifically recommends that workers consider using a NIOSH-approved respirator, such as an N95 mask or a more protective model, to prevent inhaling contaminated water droplets. In addition to respiratory protection, gloves and eye protection are also crucial for preventing contact with contaminated water. At QRC, our technicians are fully equipped with the necessary PPE and are trained to use it correctly, ensuring their safety on every job site, whether it’s a routine check or a full system disinfection.
Employer Responsibilities and OSHA Guidelines
Providing PPE is only half the battle; employers are also responsible for ensuring their teams know how to use it. OSHA requires employers to train workers on the proper way to put on, use, take off, and dispose of all personal protective equipment. This training is a critical part of our safety culture. It ensures that every technician understands the risks and is confident in their ability to protect themselves. When you hire a professional service, you’re not just getting technical expertise—you’re getting a partner who prioritizes safety and adheres to the highest industry standards for worker protection.
Never miss an HVAC system maintenance appointment to keep your system clear of Legionella and protect the occupants of your buildings from a deadly disease.
Frequently Asked Questions
I’m a homeowner. Should I be worried about Legionella in my house’s air conditioner? You can breathe easy on this one. Standard residential air conditioning systems, including the one in your car, don’t pose a risk for Legionella. These units work by cooling the air directly and don’t use a water-based process that could grow and spread the bacteria in a mist. The concern is almost exclusively with larger, more complex water systems found in commercial, multi-tenant, or industrial buildings.
How would I even know if my building’s water system has Legionella? Unfortunately, you can’t see, smell, or taste Legionella bacteria in the water. The only way to identify a potential problem is through a professional risk assessment and water testing. This is a key part of a comprehensive water management program, where a trained technician evaluates your systems to find areas where the bacteria could thrive and then takes samples for lab analysis if needed.
Is routine HVAC maintenance enough to prevent Legionella, or is this a separate, special service? Consistent, high-quality maintenance of your commercial HVAC system is the absolute foundation of prevention. However, the most effective strategy is a dedicated water management program. This goes a step beyond a standard tune-up by specifically targeting the conditions Legionella loves, including water temperature, biofilm, and disinfectant levels in all of your building’s water systems, not just the cooling tower.
My commercial building is fairly new. Do I still need to worry about this? Yes, it’s still a valid concern. A building’s age isn’t the only factor. The design of the plumbing and HVAC systems plays a massive role. Even in a new building, design flaws like “dead legs” in pipes can create areas where water sits and stagnates, providing a perfect breeding ground for bacteria. Starting a proactive maintenance and water management plan from day one is the best way to prevent issues before they ever start.
What does a professional cleaning for a cooling tower actually involve? It’s a much more intensive process than a simple rinse. A professional service involves completely draining the system and then physically cleaning all surfaces to scrub away the slimy biofilm and sediment where bacteria hide. After the physical cleaning, the system is treated with specialized biocides to disinfect it thoroughly before being refilled. This is a complex job that requires specific safety equipment and expertise to ensure it’s done effectively and safely.
Key Takeaways
- Know Where the Real Risk Lies: Don’t worry about your home AC unit; the real concern for Legionella is in large-scale commercial water systems. Cooling towers, large hot water tanks, and even decorative fountains can create the perfect environment for bacteria to grow and spread through airborne water droplets.
- Disrupt the Bacteria’s Comfort Zone: Legionella needs two things to thrive: stagnant water between 77°F and 113°F and a protective layer of biofilm. Consistently managing water temperatures and physically cleaning systems to remove that slimy buildup are your most powerful tools for stopping growth before it starts.
- Make Prevention a Routine, Not an Afterthought: A formal water management program is your best defense. This means scheduling regular professional cleanings, maintaining correct water temperatures, and using proper water treatments. Treating these tasks as non-negotiable parts of your building’s upkeep is essential for ensuring occupant safety.