Harlem Hospital failed to follow its own guidelines before Legionnaires' outbreak

Harlem Hospital ignored its own commitment to conduct weekly rapid testing that could have prevented the spread of Legionnaires' disease ahead of last summer’s deadly outbreak, according to documents obtained by Gothamist.

The hospital’s monitoring system also showed signs as early as June that one of its cooling towers was susceptible to developing Legionella bacteria, according to public health experts who reviewed the documents.

NYC Health and Hospitals, the agency that oversees the facility, said hospital staff interpreted the readings as normal and that they were not required to do weekly rapid testing, despite it being part of the facility’s own maintenance plan that building owners are mandated to follow, according to New York City’s health code.

The Legionnaires’ outbreak spread quickly through Central Harlem. Seven people died and 90 were hospitalized. By the end of August, the city’s Department of Health and Mental Hygiene pinpointed the source as Harlem Hospital and an adjacent, city-owned construction site for a new public health lab. Both were positive for Legionella bacteria that matched genetic samples taken from people who were sickened.

The fact that each of the cooling towers connected with the outbreak was located on government buildings and properties raised concerns among several public health experts.

Both buildings have rooftop cooling towers, which operate as part of a building’s air conditioning system. Legionella can grow rapidly inside cooling towers in hot weather if they are not maintained properly. The bacteria spread by being blown into the air and inhaled by passersby.

Harlem Hospital was at the center of a previous outbreak in 2021 that sickened well over a dozen people. Health officials at the time found that the hospital had violated the city’s cooling tower regulations and ordered it to revise its maintenance plan.

The city’s health code requires building owners to develop and follow detailed protocols to prevent their cooling towers from spreading Legionella, and city regulators are supposed to ensure they are being followed when inspections are conducted.

The hospital’s new plan, obtained through a Freedom of Information Law request, shows that the facility committed to weekly, on-the-spot rapid testing for Legionella during the summer months when an outbreak is most likely to occur, which would have exceeded the city’s legal requirements to send test samples to a lab every 90 days.

But Health and Hospitals spokesperson Adam Shrier said Harlem Hospital did not conduct the additional testing and claimed it was not required to do so. The health department said that because the rapid testing goes beyond what’s required by city health code, Harlem Hospital had the flexibility to not conduct the tests.

Chris Boyd, a former health official who helped write the city’s cooling tower regulations and left the health department in 2017, said that contradicts city law, which states that “cooling tower systems must be maintained and operated in accordance with the maintenance program and plan.”

Boyd, who is now a consultant for a company specializing in water management and other public health issues, said every building must have a maintenance plan that meets the specific needs of its cooling towers.

Chantal Gomez, a health department spokesperson, issued a statement calling Boyd “misinformed.”

Michele Swanson, a microbiologist at the University of Michigan, said a regular program of rapid testing could have, within hours, detected that the presence of Legionella DNA was increasing in the cooling tower before the outbreak.

“If it’s monitored regularly and you see an increase in the DNA, that would be consistent with growth and risk,” Swanson said, adding that the results would have enabled hospital staff to act quickly.

Other environmental experts expressed concern that the hospital failed to follow its own established protocols specifically designed for its cooling towers after having been the source of a previous outbreak.

The city’s health department found no violations in its inspection of Harlem Hospital’s cooling tower management in the aftermath of the 2025 outbreak.

Boyd expressed skepticism of the cooling tower’s clean bill of health and said the fact that Harlem Hospital has been connected to two outbreaks within five years is especially alarming.

“One link could be dismissed as an isolated failure,” Boyd said. “Two links in roughly five years at the same institution… should be treated as a warning sign of continued systemic problems until proven otherwise.”

‘Proof that something failed’

The summer of 2025 was a scorcher for New York City. Blistering temperatures in late June gave way to a heat index of well over 100 degrees in late July — the kind of weather that allows Legionella to thrive.

On July 25, the health department announced that it was investigating a Legionnaires' “cluster” after five people were diagnosed in Central Harlem.

Health investigators responded to the outbreak by sampling and testing water from cooling towers from across five ZIP codes in the neighborhood.

Air conditioning systems in large buildings often circulate cold water throughout a structure to absorb heat. The warmed water is then pumped into a cooling tower, typically on the roof, where a fan cools the water and evaporates it. Small particles of water vapor are released in the process.

If the water remains stagnant or warm, or if there isn’t enough disinfectant in the system, Legionella bacteria can grow to dangerous levels and blow out with the water vapor. If inhaled, the bacteria can cause Legionnaires’ disease, a type of pneumonia that can be deadly for elderly or immunocompromised people.

Jeffrey Siegel, a civil engineering professor at the University of Toronto and an expert in air conditioning systems, said cooling towers create the ideal temperatures and conditions for Legionella bacteria. But proper testing, monitoring and disinfecting procedures can stop it from growing to dangerous levels.

“Legionella outbreaks shouldn’t happen,” Siegel said. “The fact that a Legionella outbreak happened is proof that something failed.”

In last summer’s outbreak, bacterial samples collected from seven patients eventually traced back to two cooling towers.

One of the towers sat on the roof of the Harlem Hospital’s 18-story Martin Luther King Jr. Pavilion. Records and satellite imagery show that it’s the hospital’s largest cooling tower, and separate from the one connected to the 2021 outbreak.

City law requires the health department to inspect cooling towers annually. Data shows that health officials inspected the MLK tower in October 2024 and found no violations.

Less than 300 feet away, at a construction site managed by the company Skanska, was another cooling tower that was also traced back to the outbreak. The project was overseen by the city’s Economic Development Corporation on a property owned by the city’s health department. In its inspection of the construction site’s cooling tower, the health department found no records of routine monitoring, disinfection or water treatment and issued violations, according to the agency’s report.

A representative for Skanska said the company could not comment on its role in the outbreak because it is currently facing litigation. It has previously said it worked with the health department to disinfect the cooling tower at the construction site.

Boyd, the former health official who helped write the city’s cooling tower regulations, said both Harlem Hospital and the health department-owned site should have been modeling “what rigorous compliance looks like.”

Health and Hospitals maintains that Harlem Hospital did model rigorous compliance.

In response to the outbreak at the time, Health and Hospitals' President and CEO Mitchell Katz said the hospital fully followed city laws and that its cooling tower maintenance program exceeded the requirements of city regulations, without elaborating on what those additional measures were.

Records show that the hospital conducted required lab tests for Legionella on March 20 and June 17. Both tests, which took weeks to produce results, came back negative. But because Legionella can grow quickly, environmental scientists say frequent, rapid testing is all the more important, especially in the summer months.

“Legionella likes warm water,” Siegel said. “That’s why it shows up in cooling towers.”

Building owners are required under the health code to develop and follow detailed cooling tower management plans, and Harlem Hospital’s contained a safeguard for detecting accelerated Legionella growth. Between June and September, staff were required to use weekly PCR tests, similar to the ones used during the COVID-19 pandemic, to detect the presence of Legionella bacteria in cooling towers.

PCR tests can frequently deliver false positives because they detect both living and dead bacteria. But experts said the quantitative PCR tests required in the hospital’s management plan would have provided data on just how much of the bacteria is present. If used across weeks, the hospital staff could have tracked any sudden surges in Legionella, living or dead, and conducted additional testing to ensure the bacteria wasn’t growing to dangerous levels.

But the PCR tests outlined in the hospital’s maintenance plan never happened.

Shrier, the Health and Hospitals spokesperson, said rapid PCR testing was a recommendation added by a consultant the agency hired to write a maintenance plan for its cooling towers and that it was not required to follow the protocol.

City law requires that building owners work with a licensed water specialist or engineer to design maintenance plans for cooling towers. And the city’s health code states that cooling towers “must be maintained and operated in accordance with [a building’s] maintenance program and plan.”

Records show Harlem Hospital hired an Illinois-based company called Nalco Water to draft its cooling tower maintenance plan. Information attached to the document released by Health and Hospitals indicated that it was being drafted as early as 2022 — while Harlem Hospital was under increased scrutiny by the health department following the 2021 outbreak.

Health officials required the hospital to hire an outside auditor to certify its maintenance practices, revise its cooling tower management protocols and create “a heightened monitoring plan to guide the building in sustaining Legionella control,” according to a health department report.

The agency released Harlem Hospital from the heightened monitoring plan in late 2023 after test results came back negative for Legionella bacteria.

The hospital’s active maintenance plan was finalized in January 2025, records show. Alessandro Leo, a representative for Nalco Water, did not answer questions about why the PCR testing was added and why the hospital did not follow it.

Little disinfecting power

As early as June of last year, hospital staff received a warning sign that its cooling tower could be in trouble. Data that is automatically collected by Harlem Hospital’s cooling tower system indicated that a disinfectant regularly added to the cooling tower system was not effectively working in the weeks leading up to the outbreak.

The city’s health code requires building owners to monitor “biocidal indicators” in their cooling towers, meaning they have to measure the concentration and effectiveness of chemical disinfectants. One way to track whether a disinfectant like chlorine is doing its job of killing Legionella bacteria is what’s known as “oxidation-reduction potential.”

According to data released from Health and Hospitals, the cooling tower at Harlem Hospitals showed very low oxidation-reduction potentials through June and July, with only small spikes in the system’s efficacy during those months.

“That’s around the range you could see in drinking water,” said Abraham Cullom, director of water safety management at Pace Analytical Services. “This could indicate that there’s no biocide in the system.”

He said the readings differ significantly from what is typically seen in well managed cooling towers, which should have higher readings sustained over longer stretches of time. The oxidation-reduction potential charts for the hospital’s two other cooling towers showed much higher and stable readings through June and July.

Other factors, like the water's temperature, acidity or alkalinity, could also affect readings, but Cullom and other water management experts said the low levels were a sign that Harlem Hospital’s system was vulnerable to the kind of Legionella growth that was ultimately connected with the outbreak.

According to the hospital’s management plan, Harlem Hospital staff overseeing the cooling towers should have responded to the low readings on the MLK cooling tower, but Health and Hospitals did not provide any record of such a response.

Shier, the agency’s spokesperson, said hospital staff instead relied on a different reading known as “total residual oxidant,” or TRO, to monitor the cooling towers, which he said was a more direct measure of disinfection.

But additional data provided by Health and Hospitals on TRO levels showed that the concentration of disinfectant also fluctuated during the months of June and July. The hospital collected samples three to four times each week, according to the data. In more than a third of those samples, the disinfection levels in the system had dipped below the concentration required by the hospital’s maintenance plan. On four days, the concentration of disinfectant was at or near zero.

Records show the cooling tower system automatically injects disinfectant whenever levels get too low. But increases in concentration did not correspond to spikes in the oxidation-reduction potential. In other words, adding more disinfectant did not increase the disinfecting power of the water in the system.

The city’s health department said those readings may have indicated that the disinfectant was being consumed quickly and could have resulted in the potential for some Legionella growth between doses.

A familiar lapse

New York City has some of the most strict regulations in the world when it comes to cooling towers. The city enacted a set of regulations after a 2015 outbreak in the Bronx that killed 16 people. And yet, several outbreaks of varying sizes have occurred since then. And the city’s health department has struggled in recent years to enforce the law.

In the lead-up to the 2025 outbreak, the department hit a near-record low in the number of cooling tower inspections citywide, Gothamist found. Health officials attributed the decline to short-staffing.

The outbreak linked to Harlem Hospital in 2021 sickened 18 people. That year, the health department found that the hospital had no records of regularly monitoring its cooling towers for bacteria. According to city law, hospital staff should have logged every time they tested for bacteria or disinfected the cooling tower.

It would take another four years — and another Legionnaires' outbreak in Harlem linked to the hospital — for the health department to complete its final investigation report on the 2021 outbreak. It found that the hospital cooling tower connected to the 2021 outbreak had areas of stagnant water and low water flow in some parts of the equipment, which contributed to the growth of Legionella.

According to an October 2022 letter from health officials, Legionella bacteria was detected again in the cooling tower a year later. A test that year found that a closely related strain of the bacteria remained present in the tower.

Amy Pruden-Bagchi, a professor at Virginia Tech and an expert in water systems, said completely eradicating the bacteria can be nearly impossible. But proper disinfecting can keep it at safe quantities.

“It tends to hide out at low levels, maybe even below detection in nooks and crannies,” Pruden-Bagchi said. “Once it’s given the opportunity to rear its head, like the disinfectant goes too low, then it’ll essentially bloom.”

Tightening regulations

In the aftermath of the 2025 outbreak, the City Council passed legislation requiring more testing for the Legionella bacteria by building owners, increasing the frequency from every 90 days to every 30 days. The new rule goes into effect in May.

Some public health experts, however, are skeptical that more testing is the solution. They said building owners often test immediately following a deep clean, which generates negative test results even if the cooling tower had been rife with bacteria in the weeks leading up the test.

Boyd said daily and weekly monitoring is the most effective way to prevent the growth of bacteria and dangerous outbreaks. He said Harlem Hospital’s connection to two outbreaks epitomizes the need to focus on regular maintenance, rather than periodic tests. He called the 2021 outbreak a result of a "cascading series of failures.”

“The limited information released so far from the 2025 outbreak also appears to suggest that Harlem Hospital did not consistently follow its own written water management program,” Boyd said. “Adding another required test to a program that is not being reliably followed does not solve that problem.”

This story has been updated after a NYC Health and Hospitals spokesperson clarified Harlem Hospital's response to low “oxidation-reduction potential" readings in one of its cooling towers that was later linked to a Legionnaire's outbreak last summer.