Image: An artistic rendering of Legionella pneumophila. Credited to Getty Images and Peddalanka Ramesh Babu.

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On July 8, 2025, the Middlesex-London Health Unit declared an outbreak of Legionnaires’ disease for the city of London, Ontario. On Aug. 6, after 70 confirmed cases and three fatalities, MLHU declared the outbreak over—only to redeclare it on Aug. 28, after 25 more cases emerged. A total of 107 confirmed cases, including 97 hospitalizations and four fatalities, were associated with this outbreak before it was declared over a second time on Dec. 1.

Only weeks after MLHU began looking into reports of Legionnaires’ disease, the New York City Department of Health and Mental Hygiene also opened an investigation. When the investigation was closed on Aug. 29, 114 cases of Legionnaires’ disease had been reported, 90 people had been hospitalized, and seven people had died.

CDC surveillance data shows that cases of Legionnaires’ disease have been increasing since the early 2000s, aside from a brief drop in 2020. It’s one of the few diseases in the U.S. to demonstrate an upward trend. “If there was a better prevention mentality, that could be changed,” said Brian Shelton, MPH. “The fact that it's going up shows that what's currently happening isn't working.”

To reverse this trend, public health departments in the U.S. and Canada must develop policies to prevent Legionnaires’ disease outbreaks and respond to outbreaks quickly when they occur. Occupational and environmental health and safety professionals should play a critical role in these efforts.

Prevention Begins with Education

Legionnaires’ disease is a serious, possibly fatal form of pneumonia. With the milder Pontiac fever, it belongs to the category of legionellosis, a family of respiratory illnesses caused by Legionella bacteria. Legionella flourishes in warm, stagnant water in environments such as cooling towers and plumbing. When the bacteria are aerosolized by shower heads, decorative fountains, or other devices, they may be inhaled by vulnerable people who then become ill.

Industrial hygienist Mona Shum, MSc, CIH, described how the biofilm in plumbing systems protects Legionella, making the bacteria difficult to control. Early in her career, Shum testified as an expert witness in a lawsuit relating to the deaths of three people from Legionnaires’ disease, which they had contracted after staying in a motel. She related how motel management had tried to control the Legionella by super-shocking the plumbing system with chlorine, yet another guest died from Legionnaires’ disease several years later. Management “didn’t really deal with the issues that were causing the original Legionella proliferation,” such as low water pressure and dead legs in the plumbing, Shum explained. “So of course you’re going to get those conditions again.”

“Better to try to prevent Legionella in the first place rather than have to deal with the outbreak and then try to control it afterwards,” she added. “Once it’s established in your system, trying to get rid of it is really difficult.”

However, legionellosis prevention is hampered by poor reporting. First, cases may not be linked because Legionella exposures often occur in settings where many people spend short periods before moving on, such as hotels and hospitals. Shum noted with regards to her expert witness testimony that the original three deaths likely wouldn’t have been connected if the patients hadn’t been colleagues. Second, symptoms of Legionnaires’ disease may not appear for up to two weeks after exposure, and patients don’t often undergo testing to determine the cause of their illness. Third, the range of environments in which Legionella can grow isn’t always recognized, especially in Canada. “People think it’s only to do with cooling towers,” Shum said. “In cold climates, where they don’t use cooling towers, then they’re like, ‘Ah well, we don’t have a problem here.’”

As a result, Legionella is underrecognized as a hazard in many jurisdictions. The London, Ontario, outbreak has led to greater awareness among Canadian professionals, “but I think there’s still a long way to go,” Shum said. “When I talk to other hygienists who work with me and work around me, they’re all very curious about Legionella and Legionnaires’ disease.”

Better Regulations, Better Response

However, the 2025 New York City outbreak shows that awareness of Legionnaires’ disease isn’t enough to prevent illness. Whereas the London, Ontario, outbreak caught officials off guard, New York City had already implemented regulations aimed at preventing Legionnaires’ disease, including a registration of cooling towers and requirements for testing multiple times per year. Yet an outbreak occurred nonetheless.

“Something is wrong, if we have some of the most rigorous regulations in the country, but we're not preventing it,” said Megan Canright Racicot, MPH, CIH. Canright and Shelton have worked with the New York City Health Department to respond to previous Legionnaires’ disease outbreaks, allowing them to observe the city’s regulations firsthand.

According to Shelton, part of the problem is that while many methods for Legionella testing meet the requirements of ISO 11731:2017, Water quality—Enumeration of Legionella, the standard doesn’t ensure that testing methods meet adequate thresholds for reliability. Furthermore, sufficiently qualified professionals, such as industrial hygienists, are often not involved in Legionella testing and Legionnaires’ disease prevention. “Industrial hygienists, of all groups, know the reliability has to be high for the tests they're doing,” Shelton said. “We're dealing with health. This is not something unimportant. This is something that should be going to specialists.”

When outbreaks of Legionnaires’ disease arise, quick response is crucial. While the Health Department provides extensive oversight for outbreak response, this oversight impedes the response process. Canright described how New York City regulations require the owner of a building found to be the source of Legionnaires’ disease to hire both a qualified consultant to assist in remediation and a second consultant to provide third-party review. Then the building owner and consultant must submit plans for remediation and assessment to the Health Department. Sampling and implementation of controls cannot begin until the Health Department has reviewed and approved the documents. Days or weeks pass as the submissions are being processed, while people are still being exposed to Legionella and getting sick.

“We understand New York's approach,” Canright said. “They want to make sure that the building owners and operators are doing the right thing, that the consultants who are involved have the right qualifications. That's all well and good, and we need to do that. But our experience was that we need more collaboration and more work with departments of health so that we can respond quickly.”

A More Collaborative Approach

Canright, Shelton, and Shum advocated for greater involvement by OEHS professionals in legionellosis prevention and response. As Shelton pointed out, OEHS professionals are already used to evaluating hazards. “Why not just add one more hazard to the list of what they're looking for?” he asked. “To me, to go in and not look at a hazard that can kill people, when they're doing evaluations, is a miss.”

Official oversight and standardized processes are necessary during an outbreak, Canright said, “but to that end, we need more collaboration with organizations like AIHA and experts who are involved in response.” Through more streamlined processes that place more trust in better qualified experts, health departments and OEHS professionals will not be at odds, “but instead work together to respond quickly in the best interest of prevention,” she continued.

Overall, a more effective approach to Legionella regulation will require OEHS professionals to cooperate with better educated public health officials, building owners, engineers, and others to implement layered controls. “It’s a collective effort to try to control Legionella, to try to prevent the first case of Legionnaires’ disease, to prevent an outbreak,” Shum said. “I hope people understand that we have to work with others.”

Megan Canright, Mona Shum, and Brian Shelton will present “Revisiting Legionella Regulations in New York: Recommendations from a Panel of Experts” on Wednesday, June 3rd, from 9:15 to 10:15 a.m. Central time. AIHA Connect 2026 will be held June 1–3 at the Ernest N. Morial Convention Center in New Orleans. To view the conference agenda or to register, visit aihaconnect.org.

Further Reading

AIHA: “Featured Education in Legionella.”

The Synergist: “Legionella Regulations in New York: How Much Progress Have We Made?” (June 2019).

The Synergist: "Why Legionnaire's Disease Outbreaks Continue" (December 2025).