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Why so many Legionnaires' cases in NY? Environmental and human factors may play a role.
By
Caroline Lewis
Published Aug 10, 2025
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Michael H. / Getty Images
By
Caroline Lewis
Published Aug 10, 2025
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New York City’s ongoing outbreak of Legionnaires’ disease in Central Harlem, which has killed three people and sickened dozens more, is the latest in a series of similar clusters that have made headlines in recent years, usually in the summer months.
Legionnaires’ experts say a number of environmental and human factors are likely behind a recent rise in reported cases nationally and the concentration of cases in hotspots like New York. These include heightened awareness about Legionnaires’ — a severe form of pneumonia — as well as climate change and ongoing issues with buildings’ water systems. To get Legionnaires’, a person has to inhale mist or aspirate water containing a bacteria known as Legionella, which often grows in plumbing systems or water-cooling towers.
New York isn’t the only place in the country that has battled outbreaks, but the state has emerged as an outlier. The worst outbreak in New York City history occurred in the South Bronx in 2015, infecting more than 130 people and causing 16 deaths. That led both the city and state to implement what experts say are strong regulations for preventing the growth of Legionella in buildings' water systems.
But hundreds of cases are still reported by the city health department each year. Last year, 257 New Yorkers were diagnosed with Legionnaires’, according to preliminary numbers from the city health department. In 2018, the number topped 650. Outbreaks over the past decade have also resulted in additional deaths, including those of five people in an outbreak at a nursing home in 2022.
The number of Legionnaires’ cases reported annually in the United States has increased dramatically since 2000. New York and some other states in the Northeast have particularly high case rates, whereas Legionnaires’ is practically unheard of on the West Coast, according to data from the U.S. Centers for Disease Control and Prevention.
Within New York City, high-poverty neighborhoods where underlying health issues like asthma are generally higher are also disproportionately affected by Legionnaires’, city data shows.
Sickened in an earlier outbreak
When Anita Long got sick with Legionnaires’ disease in 2022, she had lived in the Bronx for more than 30 years.
“I loved my area,” said Long, now 68, who was once a member of Bronx Community Board 4 and a tenants’ rights activist.
But after complications from Legionnaires’ landed her in the hospital for 22 days, Long said, she didn’t want to go outside anymore for fear of getting sick again. Medical experts say infections typically occur when people inhale bacteria-laden water droplets or vapors when they are outside.
“ This was one of the reasons why I left and moved to Connecticut,” Long said, adding that she worried the owners of buildings in the area weren’t properly maintaining their cooling towers.
Increased awareness of the disease within the medical community is one factor likely contributing to more diagnoses in recent years, researchers said.
“Physicians have a higher ‘index of suspicion,’ so they think about Legionnaires’ disease when they see a patient with pneumonia,” said Janet Stout, an infectious disease microbiologist at the University of Pittsburgh and president of Special Pathogens Technology, a company that seeks to prevent Legionnaires’ disease.
In New York City, the health department helps keep Legionnaires’ on doctors’ radar. A notice sent to health care providers on July 1 reminded them that New York state “has a high burden of legionellosis, with increased incidence during the summer and early fall.”
Rising temperatures and other theories
Legionella bacteria also thrive in warm water and researchers said rising temperatures are likely contributing to the increase in cases in recent decades. Stout said parts of the country that have more humidity and less sunshine are particularly hospitable to Legionella, especially in the summertime.
Within New York state, there are also particular conditions that may help Legionella thrive, such as aging infrastructure and the high level of sediment in the water, which “ can be a food source” for the bacteria, said David Pierre, director of water safety programs at LiquiTech, a company that offers water treatment solutions with a focus on Legionella.
Researchers are still coming up with new theories as to why Legionnaires’ disease has become so much more common over the past couple of decades. A University of Albany study published last year suggested one culprit might be the successful effort in recent decades to lower the levels of the sulfur dioxide in the air, which is typically seen as a win for human and animal health.
Less sulfur dioxide makes the air less acidic, which allows Legionella bacteria to survive longer, said Fangqun Yu, a senior researcher at the University of Albany’s Atmospheric Sciences Research Center and one of the authors of the paper.
“ We think that air quality improvement is very good,” Yu clarified, but said this could be a potential negative side effect that has to be mitigated.
Stout said it was an interesting finding that needs to be studied further.
Experts said the population density in New York City means that when a water cooling tower emits Legionella into the air, more people are likely to be exposed to it. But not everyone who’s exposed gets sick and the demographics of the people living in the area matter.
Most of the people who get diagnosed with Legionnaires’ in New York City have at least one characteristic that puts them at higher risk for the disease, such as being over 50, being a smoker or having a chronic lung disease, according to the city health department.
Still a preventable disease
But despite all these factors, experts emphasized that Legionnaires’ disease is largely preventable if building owners take the proper steps to test for and mitigate Legionella. And, they said, if followed properly, New York City and state protocols should be effective.
“ New York State and New York City have really done far more than most states and cities to address prevention of Legionnaire's disease,” Stout said. “Yet, despite that, outbreaks still occur, mostly because of the human factor.”
New York state law requires hospitals and residential health care facilities to establish a sampling and management plan to control Legionella in their potable water systems.
Under a New York City law passed in 2015, all building owners must also register their water cooling towers with the city. They have to put in place a maintenance plan to prevent and control the growth of Legionella and inspect and test the towers at least every three months.
The city can also conduct its own inspections and those who violate the law could face fines or misdemeanor charges. City protocols for building owners are based on a national standard developed by the American Society of Heating, Refrigerating and Air Conditioning Engineers.
But public data shows city inspections of cooling towers have dropped off precipitously since the law was enacted. Asked about the decline last week, the city health department attributed it to a staffing shortage.
Pierre also said some building owners do “the bare minimum” to comply with the laws, which can amount to box-checking rather than proper maintenance.
City health officials say they have taken aggressive measures to stop the outbreak in Harlem. They said they have screened all water cooling towers in the affected area for Legionella and conducted remediation efforts at 11 buildings that tested positive.
The officials have not said definitively whether those positive tests are linked to the current outbreak.
Tagged
Health and Science
Legionnaires disease
new york city
Caroline Lewis
Caroline Lewis is on the health care beat for WNYC and Gothamist. She has covered COVID, a nurses' strike, the overdose crisis and New York’s marijuana legalization effort, and spent a year investigating patients' medical bills. She always wants to hear about how everyday New Yorkers experience the health system. Got a tip? Email [email protected].
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Gothamist is funded by sponsors and member donations
Gothamist is funded by sponsors and member donations
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