First-time look at culture results from all culture-positive buildings in the Upper East Side Legionnaires’ outbreak. Results confirm prudence of Health Department’s proactive management once outbreak was discovered.
Sept. 21, 2026 — Through a Freedom of Information Law (FOIL) request, Remapping Debate has obtained the results of the testing of the 58 buildings determined by the Department of Health and Mental Hygiene (DOHMH) to have a culture positive for Legionnaires’ disease. (Jump to building-by-building results.)
DOHMH requires building owners to get cultures of their cooling towers performed by a solid-culture method approved by New York State. We’ll call this conventional testing. Pursuant to local legislation passed in 2025, these cultures have to be taken and analyzed monthly. DOHMH assigns “Levels” each corresponding to a range of colony forming units (CFUs) of Legionella per milliliter, and also corresponding to required actions that a building owner must take. A little confusingly, Level 1 has two branches: one is detection of Legionella at a low concentration; the other is Legionella not being detected at all.
The only Level that in routine circumstances requires a building owner to perform full remediation of a cooling tower, including a shock dose of a biocide (chlorine or bromine), draining, cleaning, and flushing, is Level 4. Only three of 58 buildings had conventional culture results that came into the Level 4 band. I spoke with two DOHMH deputy commissioners: Dr. Andrea Howard of DOHMH’s Division of Disease Control, and Corinne Schiff, of DOHMH’s Division of Environmental Health. To Schiff, a Level 4 is an indicator that a building’s Legionnaires’ control program (which has many elements beyond monthly testing) is not working: the building has “lost control.” Schiff agreed that the results were less alarming than they would have been if a greater percentage of the buildings had been at Level 4.
The full table of building results is below. Note that a building may have more than one Legionella isolate (quick lesson on varieties of Legionella here). We have categorized a building by its highest-concentration isolate. The concentration range for each level are shown both in the visualization on the next page and here. The significance and utility of the supplemental testing technique referenced in the third column — “Legiolert” — is discussed later in this article, but, for now, understand that neither New York City nor the manufacturer (which has not responded to my request to speak with a representative) assigns Levels to the values. As such, those results are understandable as binary: either at or above the limit of the test (which only is designed for identifying Legionella pneumophila), or below that limit. In addition, Legiolert provides a statistical estimate of the number of individual organisms, and is limited to Legionella pneumophila; conventional culturing counts colony-forming units of Legionella, which can be one or more individual organisms, and is not limited to Legionella pneumophila.
Three things are immediately apparent. First, that there were another 39 buildings that clocked in at Level 2 or 3. I wondered whether particular concentrations are tied to a specific level of risk, with higher levels automatically being a signal of greater risk. Howard said that there are not “good data that can help to answer that question.” (The question is complicated by the fact that cooling towers are required to have “drift eliminators” — a device designed to remove aerosols from cooling tower exhaust. A low-concentration cooling tower could have a poorly functioning drift eliminator that allows more Legionella-containing aerosols than a high-concentration cooling tower with a well-function drift eliminator. [Building-by-building look-up tool, next page; article continues on page 3.]
Before continuing, be sure to understand that nothing in this article has to do with not-yet-determined question: which cooling tower or towers were responsible for the cases of Legionnaires’ disease in Upper East Siders. To answer that question, there needs to be a genetic match between the material cultured from a cooling tower and Legionella cultured from a patient (cultures are available from only a “handful” of patients, according to testimony before the City Council’s Committee of Health from Deputy Commissioner Howard). The process of determining source or sources is ongoing.
The second thing that the culture results make clear is that, when DOHMH in July required full remediation of all buildings based on preliminary PCR testing available even before any culturing was done, the Department was requiring vastly more remediation than its rules call for in routine circumstances. Schiff said that once an outbreak or cluster has been identified, there is an active disease investigation ongoing, and the focus is on eliminating any potential source of exposure.
Third, there were a substantial number of buildings that DOHMH categorized as culture-positive even though the conventional culture method came back negative. These 16 buildings were positive on the “Legiolert” test, a liquid culture assay that estimates the number of organisms present (three buildings were negative on Legiolert and positive by conventional culturing). Both types of testing can have false positives and false negatives. Legiolert is not a test that New York State allows municipalities to rely on for required testing.
There is, however, clearly a subcategory of buildings where it detects Legionella and conventional culture does not (it’s also faster). DOHMH’s Schiff and Howard described the use of Legiolert as a supplemental tool during an active disease investigation: “Our goal,” Howard said, “is to detect and mitigate any potential source of exposure to Legionella bacteria because we want to eliminate the source and prevent further illness.”
I asked the deputy commissioners that, given the identification by Legiolert of culture-positive buildings that would have been seen as culture-negative by the conventional method, whether Legiolert should become part of the routine workflow of each building with a cooling tower, at some frequency, not necessarily monthly. I did not get a direct answer. A press spokesperson for DOHMH, present during the interview, said only that the Department had not excluded from consideration the possibility of using Legiolert in the way I described.
As far as drift eliminators — the reduce the aersols going out of the cooling tower gizmo — Schiff said that DOHMH does review building documentation regarding the condition of the devices, but could not state whether DOHMH inspectors themselves routinely examine the drift eliminators as part of a departmental inspection. The departmental spokesperson emphasized that maintenance of the drift eliminator is the obligation of the building owner, as is documentation that such maintenance has occurred, records which are reviewed by DOHMH. Explaining that there are access problems due to both location of drift eliminators and safety concerns (a drift eliminator should not be inspected while the cooling tower’s fans are spinning), he acknowledged that DOHMH direct inspection of drift eliminators is uncommon.
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A Sept. 1, 2026 press release from the office of New York City Mayor Zohran Mamdani stated that, in the coming years:
the Health Department will explore faster culture methods and novel testing approaches, including to reduce the time it takes to test and confirm positive results. This will allow the Public Health Lab to rapidly scale up testing during a Legionnaires’ disease cluster while providing additional capacity for other emerging issues. This requires additional staff and resources, which the Health Department aims to have in place before the summer of 2028.
In response to my inquiries, a DOHMH spokesperson said that the release was not intended to imply that there is some emerging issue or threat to public health that DOHMH needs additional capacity to prepare for, and that the amount of additional funding remains to be worked out. I called DOHMH’s attention to what Remapping Debate had previously reported from an interview with Mitchell Stripling, director of the New York City Preparedness and Recovery Institute, based at Columbia’s Mailman School of Public Health. From about 2013 to 2019, Stripling was assistant commissioner for agency preparedness and response at DOHMH. Stripling confirmed the difference between a department that has “just enough” staff and one that has a staffing level that provides a “buffter” to be able to surge response. The department’s spokesperson insisted that DOHMH was sufficiently staff to “handle any number of emerging threats” and to surge at crisis moments.
I wanted to see if New York City Council Speaker Julie Menin and Council Health Committee Chair Lynn Schulman took the same view. I made repeated inquiry to their offices, juxtaposing the Stripling and DOHMH views, including Stripling’s position that, “We know that building just-in-time systems makes those systems more fragile. And when those systems are life safety systems, that is really what will lead to more morbidity and mortality … If you don’t have this ability to prepare, if you don’t have systems that can surge when needed, these systems will break, and it will hurt people.” I got no response.
It would appear that neither side of City Hall believes that there is systemic underfunding of DOHMH to remedy.