In August last year, New York faced one of its largest outbreaks of Legionnaires’ disease ever seen. More than 100 individuals were hospitalized and several lives were lost. Following an investigation, the outbreak was traced to contaminated water in 12 cooling towers across 10 buildings—an infrastructure-wide failure that allowed Legionella bacteria to become aerosolized and spread rapidly across the Harlem and Bronx communities.
But while the media attention surrounding the outbreak has since diminished, it is critical that the underlying causes are not overlooked.
A Rapidly Rising Risk
The reality, after all, is that this incident is not an isolated event, but part of a broader and increasingly frequent pattern observed across the U.S. All across the country, aging infrastructure, warmer temperatures, and inconsistent maintenance practices are combining to create ideal conditions for Legionella growth. Modern buildings, with their more complex plumbing arrangements, can inadvertently encourage stagnation and temperature fluctuations if not carefully designed and maintained.
To this end, it may be easy to see why Legionnaires’ disease—a potentially fatal lung infection caused by inhaling droplets of water containing Legionella bacteria—is now one of the fastest-growing waterborne diseases in the United States. According to the most recent studies, cases have increased by more than 1,000% over the past two decades.
More worryingly still, experts suggest that this may not be the full picture. This is because symptoms frequently resemble pneumonia or flu, which can lead to misdiagnosis. Also, many cases go untested or unreported, particularly among vulnerable populations such as older adults, hospital patients and those with compromised immune systems. As a result, the consensus is that outbreaks may be far more widespread than public data suggests.
Thus, while the New York crisis may appear to be behind us, it raises all sorts of public health questions—namely, how can we address this growing risk within the built environment?
Dangerous Delayed Detection
Of course, most building operators will be well acquainted with the widely-adopted ASHRAE Standard 188 developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), which provides a framework to implement comprehensive water management programs that proactively control risks related to the growth and transmission of Legionella in building water systems, including regular monitoring and maintenance. Many states and cities also stipulate their own specific standards, especially in jurisdictions with recent outbreaks or stricter health requirements.
From our experience, though, there is scope to do much more to raise the bar on safety through a more robust and proactive approach to water safety.
For example, one of the most critical weaknesses exposed by outbreaks like New York’s is the lag between contamination and confirmation. Traditional Legionella testing methods rely on laboratory culturing, which takes up to 10 days to deliver results. Yet Legionella can double in a day. In a large building or cooling system, that delay can mean the difference between early intervention and widespread exposure. Water samples can also get damaged in the transportation process, potentially killing the Legionella inside and leading to a false negative result. In addition, culture-based testing cannot detect viable but non-culturable (VBNC) Legionella, which, though dormant, can resuscitate and cause an infection.
Importantly too, buildings will often continue to operate as normal during the waiting period, potentially dispersing contaminated aerosols into the surrounding environment. As a result, conventional testing methods often confirm the presence of Legionella only after exposure has already occurred—by which point the opportunity for prevention has been missed.
Proactive Prevention
The good news is adequately controlling Legionella risk can be made manageable through a combination of real-time detection and proactive water system management.
First, faster detection is essential. Modern on-site tests using lateral flow technology are able to deliver results in around 25 minutes, detect both culturable and non-culturable bacteria, and allow testing at the exact point of use. By incorporating these tools into routine maintenance, building operators can respond immediately to contamination, significantly reducing exposure risk. Beyond immediate risk mitigation, these tests also provide valuable data to inform ongoing water system risk assessments and compliance reviews.
Second, water management plans must evolve from static compliance documents into dynamic, living systems. This should incorporate continuous monitoring of temperature, disinfectant levels and flow—combined with rapid on-site testing—to create a real-time feedback loop that drives ongoing risk reduction and system optimization.
Finally, education remains critical. As Legionella risk evolves, those building operators and their teams who stay up to date with the latest guidance, testing innovations, and real-world risk factors will be far better equipped to identify vulnerabilities long before they escalate. Regular training ensures they understand how Legionella behaves, where it hides, and how system design, temperature control, and maintenance practices influence bacterial growth.
Legionella Learnings
If the recent New York outbreak has taught us anything, it’s how quickly things can go wrong when vigilance slips. In this way, we cannot afford to view this incident as a one-off issue. Legionella is a risk that evolves with climate, building use and water system complexity, and it requires ongoing attention. Building operators, who are responsible for managing and shaping the buildings we use, have a lead role to play in preventing future outbreaks.
