Assessing Legionella Prevention Efforts
By Ed Rutkowski
June 10, 2026—Next month marks the 50th anniversary of the American Legion convention in Philadelphia that gave Legionnaires’ disease its name. For months following the July 1976 gathering at the Bellevue Stratford Hotel, experts in infectious disease sought to determine the cause of the severe pneumonia that ultimately sickened 221 people and killed 34, including some who never entered the hotel. Finally, in January 1977, CDC verified that a rod-shaped bacterium, later named Legionella pneumophila, was the culprit.
Last week at AIHA Connect 2026 in New Orleans, a pair of educational sessions held back-to-back on June 3 appraised current efforts to prevent Legionnaires’ disease through the lens of recent outbreaks in New York City.
“Despite 50 years of knowledge in this disease, cases continue to increase every year,” said Brian Shelton at the first of these sessions, which sought to explain why regulations in New York intended to reduce outbreaks have repeatedly failed since their adoption in 2015. Shelton is the president and CEO of PathCon Laboratories, which specializes in Legionella testing.
The New York regulations focus on cooling towers, which have been linked to outbreaks of disease around the world. But Shelton clarified that Legionella can thrive in other places, including showers, decorative fountains, whirlpool spas, cruise ships, and hospitals—basically, any structure that has a water source. Legionella grows in water systems, which are difficult to access. Chemical treatments act only on the surface of the biofilm that lines the insides of pipes and from which Legionella acquires nutrients. Testing is essential to manage the issue, Shelton said.
One of Shelton’s co-presenters, Megan Canright Racicot, the director of scientific operations at Forensic Analytical Consulting Services Inc., summarized the New York regulations. They require registration of cooling towers and site-specific water management plans based on ASHRAE 188, Legionellosis: Risk Management for Building Water Systems. Other legal requirements include weekly monitoring, quarterly inspections, and biannual cleaning and disinfection.
Despite these requirements, outbreaks and cases of Legionnaires’ disease continue to occur in the city. The New York City Department of Health and Mental Hygiene, also called NYC Health, estimates (PDF) that between 200 and 700 cases occurred each year from 2019 through 2022. The most recent outbreak, last August in Harlem, led to 114 confirmed cases, 90 hospitalizations, and seven deaths.
“Are these regulations working to control cases?” Canright Racicot asked.
Clearly, the answer is no. NYC Health’s efforts to track Legionnaires’ clusters amount to “one of the most sophisticated monitoring programs in the United States,” Canright Racicot said, yet they likely prolong outbreaks due to built-in delays. For example, when an outbreak occurs, it can take up to 13 days for the city to allow the facility to conduct sampling of its water system. If the results are positive for Legionella—an outcome that, given the nature of culture-based laboratory analysis, won’t be determined for up to two weeks after samples are submitted—the facility has an additional three weeks to develop a remediation plan. Another 41 days can pass before the city allows the facility to begin remediation. “Building occupants are at increased risk for exposure and disease” during this period, Canright Racicot said.
Following Canright Racicot’s presentation, Mona Shum, the principal industrial hygienist at Aura Health and Safety Corp., summarized the many challenges related to preventing outbreaks of Legionnaires’ disease. Due to its lengthy incubation period, the disease may be asymptomatic for weeks following exposure. If symptoms resolve in a few days, an infected person might not even visit a doctor. And even when the illness requires hospitalization, healthcare providers might not connect it to an exposure that occurred weeks earlier.
Considerations for OEHS professionals include the lack of an exposure limit for Legionella and the uncertainty of what constitutes an infective dose. Legionella is difficult to culture, often requiring 10 days to grow, and can be overgrown by other bacteria, Shum said. Regulations differ from place to place, and, as the experience in New York demonstrates, compliance does not necessarily result in prevention.
Robert DeMalo later echoed this observation in a session that focused on prevention and mitigation strategies. “The buildings we’ve built are really designed to grow Legionella, unfortunately,” DeMalo said. The combination of warm water and periods of nonuse that characterize many building water systems provide Legionella with an ideal environment. DeMalo, the vice president of Pace Analytical Services, said that Legionella colonize between 12 to 70 percent of all building water systems. Internal studies at Pace indicate that Legionella is present in approximately 50 percent of hospitals in the U.S. and as many as 40 percent of residential homes.
CDC data indicate that the incidence of Legionnaires’ disease in the U.S. rose from less than 0.5 cases per 100,000 population in 2000 to 3 per 100,000 in 2018. The rate has fallen since then, with the latest data, for 2023, showing an incidence of roughly 2.5 per 100,000, which is still high by historical standards. DeMalo said that studies show high prevalence of Legionella antibodies in hospital workers, an indication that they’re being exposed, even if they aren’t developing disease. And the fatality rate for healthcare-associated Legionella infections averages 25 percent, according to CDC, significantly higher than the 10 percent fatality rate in the general population.
At the end of his presentation, DeMalo clarified that the goal of testing water systems is “zero cases, not zero Legionella.” Achieving that goal likely requires much broader adoption of industrial hygiene best practices, recalling Shelton’s earlier observation that Legionella “is fundamentally an industrial hygiene challenge.” Referring to the problematic public health regulations in New York, he called for OEHS professionals to assume a larger role in the fight against Legionella.
“A lot has happened in the last 50 years—some of it good, some not so good,” Shelton said. “Let’s make the next 50 years good by taking leadership on this, because this is the group that can do it.”
Ed Rutkowski is editor in chief of The Synergist.
Resources
AIHA: “Featured Education in Legionella.”
AIHA: Recognition, Evaluation, and Control of Legionella in Building Water Systems, 2nd edition (May 2022).
AIHA: “Technical Framework: Legionella.”
CDC: “Clinical Features of Legionnaires' Disease and Pontiac Fever.”
CDC: “Legionellosis Surveillance and Trends.”
Emerging Infectious Diseases: “Legionnaires’ Disease Outbreak Caused by Endemic Strain of Legionella pneumophila, New York, New York, USA, 2015” (November 2017).
NYC Health: “Legionnaires’ Disease Cluster in Central Harlem” (August 2025).
NYC Health: “Legionnaires’ Disease in New York City: 2019–2022 Surveillance Report” (PDF, 2024).
Sociology Lens: “Legionnaires' Disease Outbreak in Philadelphia and the 1976 United States Presidential Election” (June 2024).
The Synergist: “Legionella Regulations in New York: How Much Progress Have We Made?” (June/July 2019).
The Synergist: “Why Legionnaires’ Disease Outbreaks Continue” (December 2025).