New York City health officials are continuing to search for the source of a cluster of Legionnaires’ cases on the Upper East Side. Mayor Zohran Mamdani on Tuesday committed to publishing the locations of all the cooling towers that test positive for Legionella bacteria as soon as results become available.

For some in the community, the presence of cooling towers is a mystery. The devices often sit unseen on top of buildings and function as part of the air conditioning system. Hot summer weather makes them especially susceptible to Legionella bacteria that can then be blown out into the air where it sickens people.

A Gothamist analysis of cooling-tower registration data found 203 active towers within the three ZIP codes covering the Carnegie Hill and Yorkville neighborhoods where the cluster has occurred. The city’s health department says it’s only identified 160 cooling towers within the same area. An agency spokesperson did not respond to questions about the difference between the two numbers.

That data offers a glimpse into how building owners complied with cooling tower regulations before the outbreak was first identified and announced on July 2. As of May, city law requires owners to test for Legionella bacteria and report the results to the health department on a monthly basis. Prior to that, testing was required every three months.

Building owners also have to develop detailed maintenance plans to keep their cooling towers free of potentially deadly contamination.

Health officials say their goal is to inspect each of the city’s nearly 5,000 cooling towers at least once each year to ensure compliance, though they have struggled to do so in recent years due to a lack of inspectors.

According to public data, 55% of the 203 active cooling towers in the three ZIP codes had Legionella test results on file that had been reported by building owners in 2026, a compliance rate that is only slightly lower than the rate for cooling towers citywide.

About half of the cooling towers in the Upper East Side ZIP codes have gone over a year without a city inspection, about the same rate for all cooling towers across the city.

Janet Stout, a microbiologist and Legionella expert, said residents who see that their building’s cooling towers have not been inspected or tested for Legionella in a long time should contact their building owners and ask whether the testing has been done.

“Ask them why the record is incomplete,” she suggested. “Are you doing what you’re supposed to be doing?”

Residents currently living or working near one of the cooling towers that does not have reported test results or that has not recently been inspected are not at a greater risk for contracting Legionnaires’ disease, Stout said. Legionella bacteria spreads through water mist exiting the cooling towers. It can cause a type of pneumonia when it’s inhaled by passersby.

“The mist that comes off a cooling tower can travel a great distance,” Stout said. “So it’s not so much that your particular cooling tower is what you need to be concerned about.”

Health officials urged anyone who visited these neighborhoods in the past several weeks and is experiencing flu-like symptoms to seek immediate medical attention. Elderly and immunocompromised patients are especially at risk for serious illness.

Census data shows that Carnegie Hill has a significantly larger proportion of elderly residents compared to city averages. Citywide, about 15% of residents were aged 65 or older in 2020, the most recent year data is available. In Carnegie Hill, 29% were 65 or older.

As of late Thursday, cases of Legionnaires’ disease continued to climb in these neighborhoods with 46 people sickened and 22 hospitalized.

Cooling towers that test positive for Legionella will have to be drained, cleaned and disinfected. The rapid PCR tests that the city is using detects both living and dead bacteria, so a positive test may not indicate that the cooling tower is actively spreading the disease.

This story has been updated with additional information.