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KEY POINTS
Legionnaires’ disease, caused by inhaling water droplets carrying Legionella bacteria, is the country’s deadliest waterborne illness.
The bacteria flourish in warm, stagnant water inside building plumbing systems.
Managing Legionella risk is complex and is a shared responsibility among water utilities, building owners, and building designers.
At the end of July, a lethal infection, one borne in water but carried in the air, spread through Harlem.
At least 114 people in the New York City neighborhood reported symptoms ranging from fever and chills to severe coughs. Ninety people were hospitalized as the disease outbreak multiplied, and seven people died.
All had contracted Legionnaires’ disease, a respiratory illness that resembles pneumonia and is spread by inhaling water droplets that carry Legionella bacteria. These bacteria are found in rivers and lakes, but they flourish in warm, stagnant water inside building plumbing systems. The disease stems from malfunctions in the built environment.
The Harlem outbreak was one of many in recent months. A jail in Tazewell County, Illinois, released some non-violent prisoners in October to reduce crowding after an elderly inmate died from Legionnaires’ disease contracted at the facility. An outbreak in September in Marshall County, Iowa, sickened 74 people and killed two. The largest U.S. outbreak of 2024 occurred in Grand Rapids, Minnesota, where Legionella was found in buildings throughout the city.
Microbes, as a water contaminant, do not garner the headlines or the attention accorded to hot-button chemicals like PFAS or lead. Because of the immediate threat to life and wellbeing, water quality and public health experts say that is a mistake.
“Legionella is the most substantial public health concern we are addressing, and need to better address, related to drinking water,” said Chad Seidel, president of Corona Environmental Consulting, a firm focused on water quality and treatment. “Nothing else comes close.”
“Legionella is real and present,” Seidel continued. “It is killing people. We know the names of the people who have died because of it, and we need to do a better job with it. It’s that simple.”
The growing number of cases nationally is a testament to a fiendishly difficult public health challenge that, despite its deadly consequences – between 7 and 10 percent of cases are fatal – remains under the radar.
Legionella is the most substantial public health concern we are addressing, and need to better address, related to drinking water.
Chad Seidel, president of Corona Environmental Consulting
Found in the natural environment but growing to dangerous levels inside of pipes, Legionella is one of the most complex drinking water contaminants to control, said Kerry Hamilton, an Arizona State University professor who studies pathogens in the built environment. The chain of responsible parties is lengthy, and the potential problem spots are so vast as to be overwhelming.
“There’s just a lot of people involved in a lot of places where the bacteria can grow, and a lot of places where people can get exposed,” Hamilton said.
Faucets and showerheads can harbor and spread contaminated aerosols. These can be the source of individual cases, which are underreported and not captured in outbreak statistics. Decorative water fountains and hot tubs can also be the culprit. A small fountain in the lobby of a Mexican restaurant caused an outbreak two decades ago in Rapid City, South Dakota. At a state fair in western North Carolina, a hot tub display is the suspected cause of an incident in 2019. That outbreak sickened 136 people, sending 96 to the hospital and killing four.
People over age 50, smokers, and those with weakened immune systems are at highest risk.
Large outbreaks are frequently linked to equipment like rooftop cooling towers that are part of a building’s heating and cooling system and can disperse aerosols far and wide. A New York City Health Department investigation identified the likely origin of this summer’s Harlem disease cluster: cooling towers at Harlem Hospital and a neighboring construction site. In response, the City Council passed a bill to strengthen its cooling tower management regulations.
Facilities with complex plumbing systems are vulnerable. A sprawling hospital campus might have miles of water distribution pipes, said Christoph Lohr, vice president of technical services research at IAPMO, a professional association that develops plumbing industry codes. This pipe network, if not properly designed and managed, gives Legionella ample opportunity to take root and grow.
Utilities typically disinfect water before it is sent into the distribution system. But disinfection is not a cure-all – disinfectants lose effectiveness over time – and things like pipe breaks and pressure loss can open the door to contaminants entering the system.
“We can’t expect that the water systems are sterile,” Seidel said. “So something leads to Legionella getting into buildings. And then once the conditions are ideal, then they’ll proliferate. And so we’ve got to work on mitigating those conditions in buildings.”
There is much work to be done in that area, said Lohr of IAPMO.
Part of it is cultural. Plumbing, Lohr said, is viewed as “easy” by non-plumbing engineers at engineering firms. “That is part of what leads it to being an afterthought in building design,” he said.
It’s also a matter of awareness. Once a building is completed, the task of managing Legionella falls on the building owner. Operational and design decisions, even laudable ones like energy and water conservation, can raise the risk. Legionella grow in a certain temperature range. Turning down the water heater below 120 degrees F to save energy can trigger bacterial growth.
“There’s a tricky balancing act there,” Lohr said.
A Shared Responsibility
In 2017, the EPA completed one of its periodic reviews of drinking water contaminants, to determine whether any require revision. In that review, the agency decided that eight contaminants, including Legionella, could have stronger standards.
For the EPA, Legionella control falls under a rule governing microbes and the harmful chemicals that can develop when disinfectants interact with organic matter in water. The agency has pushed back the new rule’s timeline, now scheduled for release in summer 2027.
Seidel anticipates that the rule will set numeric targets for the amount of disinfectant that should persist in drinking water as it moves through the piped distribution system and into buildings. That is what an expert working group recommended to the EPA in a 2023 report.
Rules and regulations are not necessarily sufficient to prevent an outbreak. New York has some of the strictest requirements nationally for cooling towers. But the Harlem outbreak occurred this summer. Grand Rapids, for its part, did not violate any Safe Drinking Water Act microbial standards before its Legionella outbreak, even though the city did not disinfect its groundwater supply. Once a disinfectant was added, Legionnaires’ cases went away.
In the absence of a strong regulatory landscape for building owners what Lohr sees instead is “regulation by litigation.” People get sick, and they sue. Lawyers then turn to industry guidelines to determine whether the standard of care was met.
Thomas Bernier, a partner at the law firm DeHay and Elliston, is involved in Legionella lawsuits. He agrees that litigation is a motivator, more so for corporate owners who can afford the extra oversight and have a lot at stake financially.
“Getting shut down is a problem,” Bernier said. “Bad publicity is a problem. Big businesses are waking up to that.”
Still, Bernier has seen a rise in Legionella lawsuits against smaller entities, like AirBnB property owners. That cute woodsy cabin might also be hosting bacteria in the hot tub.
Managing Legionella risk is a balancing act, according to the experts interviewed for this story. There are trade-offs between energy conservation, water conservation, and water quality that building designers and managers need to be aware of.
“It’s not that every building is super risky,” Hamilton said. “It’s just that every building is different and might require slightly different management.”
Seidel, whose firm was the technical lead helping Grand Rapids after its Legionella outbreak last year, compares it to the cold-storage chain for milk. A dairy farmer produces the milk, which is then pasteurized and shipped to a grocery store, where it is placed in a refrigerated case. All along the route, the milk’s temperature is rigorously controlled. A grocery shopper who buys a gallon needs to maintain temperature control. Otherwise, the milk will spoil. They can’t let it sit on the counter for a week.
The same thinking and shared diligence should apply to Legionella management.
“Everybody that uses water along the way – from the source all the way to the tap, the water utility to the customer – has to play their role in mitigating those risks,” Seidel said.