It Can Happen to Anyone
Recently, a friend of mine relayed an interesting story to me. Her husband was walking by a hospital when he felt a couple of droplets hit his face. It was a sunny day, but he looked up, expecting to spot at least a small rain cloud. Seeing none, he realized the water droplets had come from the two cooling towers on top of the facility. He thought nothing more of it than to mention it to his wife in passing, who then mentioned it to me.
As I read about the recent outbreak of Legionnaires’ Disease in New York City, I can’t help but wonder how many people passed by those 12 cooling towers in Harlem in late July without giving them a second thought. For most, the towers were invisible. Unfortunately, the world quickly became aware of them through the shock of an outbreak and the news that seven lives had been lost.
The Anatomy of an Outbreak
As of August 29, 2025, 114 individuals had been diagnosed with Legionnaires’ disease, 90 hospitalized, and seven have died. If you’re unfamiliar with the disease, Legionnaires’ causes pneumonia-like symptoms. It is deadly in about 10% of cases and is particularly dangerous for the elderly and immunocompromised. We wish the six individuals who remain hospitalized a speedy recovery!
In the last 20 years, cases of Legionnaires’ disease have steadily increased in the U.S., and cooling towers are a frequent source of Legionella, the bacterium responsible for the illness. Immediately, cooling towers in the vicinity of the outbreak were tested. Twelve cooling towers across 10 buildings, four of which were owned by the city, tested positive for Legionella. However, more advanced genetic testing of the bacteria has allowed investigators to narrow the source down to cooling towers at two sites—Harlem Hospital and a construction site.
In a similar case in the Bronx in 2015, 50 towers were tested, but again, only two were found to have the same strain as those infected. Genetic was then performed to narrow the source even further to a cooling tower on top of a South Bronx hotel. On a related note, the Bronx outbreak is still the city’s largest to date. In that case, 138 people were diagnosed and 16 died.
How Cooling Towers Contribute to Legionnaires’ Disease
Cooling towers are integral to the cooling systems frequently used by industrial, commercial, and public buildings like data centers, manufacturing facilities, hotels, and hospitals. Water is used by the building’s cooling systems, e.g., air conditioning, to absorb heat. The warm water is circulated through the tower, where it is exposed to air, causing some water to evaporate. The remaining cooled water is then recirculated back through the cooling system.
Legionella thrives in warm water with temperatures between 77°F and 113°F. If not properly treated and maintained, cooling towers provide an ideal environment for Legionella. Additionally, biofilms, scale, and sediment within the cooling systems can offer the bacteria further protection and the nutrients needed for growth.
Legionnaires’ disease does not spread from person to person. Rather, a person becomes infected by inhaling water droplets containing the bacteria. Cooling towers can contribute to an outbreak by dispersing contaminated water droplets into the surrounding air. (Both those like the droplets my friend’s husband felt when he passed by the hospital, and fine aerosols imperceptible to humans.) When these droplets are inhaled by the building’s occupants or even someone just passing by, the person can become infected. Healthy individuals can be asymptomatic, but the disease carries a mortality rate of roughly 10% among those who develop symptoms.
One additional reason why cooling towers may be a frequent source of Legionella is that some facilities managers forget to include them in their water management planning. I was reminded of this while reading a recent story about Legionella detections at Rose State College in Oklahoma. College officials jumped on it, flushing the tap water systems and draining the water tanks, inside and outside the building. Flushing tap water systems is good as Legionnaires’ can be contracted by aspirating water while drinking or through showers and other sources of aerosolized droplets. However, it’s unclear whether the tanks drained included any cooling towers used by the University. Hopefully, they were included, and additional testing and maintenance was conducted.
Watch: Introduction to Legionella Regulations and Testing
Is Quarterly Legionella Testing Enough?
Almost certainly, the law passed in New York City after the 2015 Bronx outbreak was intended to stop an outbreak such as this one. This law requires building owners to test cooling towers for Legionella quarterly and to remediate any growth detected.
The specifics of this outbreak have led any to question whether quarterly Legionella testing is frequent enough. The city itself is re-considering this frequency. The Hospital’s last Legionella test was in June, roughly a month before the first outbreak-related illness was reported. Officials blamed the most recent outbreak on the particularly warm summer. If you’ve spent any time in New York City in the summer, you know what it means to be able to “fry an egg on the sidewalk.” All New York City summers are hot.
Furthermore, it does not take long for Legionella to proliferate in a cooling tower or other water system. Studies have shown that, under the right conditions and temperatures, a colony containing millions of bacteria can form in less than two weeks. Proper maintenance, including monitoring disinfectant levels, might have prevented the outbreak. However, it’s clear from the number of cases involving cooling towers, the CDC’s guidelines are either inadequate or not always followed.
This is No Time to Chill
According to the CDC, more cases of Legionnaires’ disease are reported in the summer and fall. That may be because cooling towers are used less frequently in the winter months. However, some facilities, such as data centers and certain types of manufacturing, rely on cooling towers year-round to dissipate heat created by their operations. In addition, other sources of warm water, such as hot tubs, humidifiers, and decorative fountains, can become a source of the disease. A water management plan can help you identify potential Legionella hot spots and implement appropriate prevention and remediation practices.
For more details on Legionella test methods, visit our website. If you have questions or need an estimate for services, you can also reach out to us directly. If you suspect a possible outbreak, contact our Rapid Response Outbreak team at 412-281-5335.