On Jul. 10, 2026, The NYC Health Department released, for the first time, a preliminary list of 31 buildings with cooling towers that tested positive for the presence of Legionella bacteria during an initial PCR (polymerase chain reaction) screening test as part of the ongoing investigation into an Upper East Side Legionnaires’ disease community cluster.

The list is being released to provide New Yorkers with timely information while the investigation continues. Every building identified has been ordered to clean and disinfect its cooling tower immediately out of an abundance of caution, and remediation is expected to be completed for all buildings on the list by tomorrow, July 11. Additional PCR testing results will be available over the weekend and more buildings may be added to the list.

A positive PCR test result does not confirm that a building is the source of the outbreak. PCR testing detects the presence of Legionella bacteria but cannot determine whether the bacteria are alive or dead. Only live Legionella bacteria can cause illness.

To determine whether live Legionella bacteria were present when the samples were collected, the NYC Health Department is conducting culture testing on every cooling tower sampled. Those results take up to two weeks and will help investigators determine whether any of the cooling towers had live Legionella growing at the time the sample was taken. Following new emergency measures announced in response to this community cluster, the Mamdani administration is requiring owners of buildings whose cooling towers have received a positive PCR screening result to clean and disinfect those cooling towers immediately, rather than waiting for confirmatory culture test results.

Because PCR testing cannot distinguish between live and dead Legionella bacteria, not every building that receives a positive screening result will ultimately be found to have live bacteria. But where live bacteria are present, immediate remediation can eliminate the risk of additional exposure while testing continues.

Cooling towers, which release water mist in the outside air, have been identified as the source of previous Legionnaires’ disease community clusters.

As part of this investigation, the Mamdani administration is requiring building owners to perform a full cleaning and disinfection immediately after a positive PCR screening test, a more aggressive strategy than requiring a boost of chemical disinfectant levels while awaiting a culture testing confirms the presence of live Legionella bacteria, a process that takes two weeks, to conduct a full cleaning.