Mayor Zohran Mamdani is vowing to release the addresses of all buildings whose water-cooling towers test positive for Legionella, the bacteria that cause Legionnaires’ disease, as the city investigates the source of an outbreak of severe pneumonia on Manhattan’s Upper East Side.

“When there’s a public health threat, New Yorkers deserve urgency and transparency from their government,” Mamdani said of the pledge Tuesday afternoon, ahead of an evening town hall on the Upper East Side, where at least a hundred people turned out to learn about the current outbreak and steps being taken by the city to contain it.

So far, 28 people have been diagnosed with Legionnaires’ and 21 have been hospitalized, according to the city health department. No deaths have been reported.

The cases have been identified in people who live in or have recently visited the Upper East Side ZIP codes of 10028, 10128 and 10075, an area stretching from the East River to Central Park.

Last year, city health officials investigating a similar cluster of Legionnaires’ cases in Central Harlem under former Mayor Eric Adams faced criticism for being slow to identify buildings with cooling towers that contained the bacteria. Seven people died in that outbreak.

Legionnaires’ disease is a severe form of pneumonia that can be contracted by inhaling water vapor containing Legionella bacteria.

The cluster of cases on the Upper East Side, which was first identified late last week, is likely linked to a cooling tower that’s sending mist contaminated with the bacteria into the atmosphere, according to the city health department.

Water ecologists at the city health department are currently collecting samples from all 160 cooling towers in the affected ZIP codes and sending them to labs for Legionella testing.

The city has tested 139 of the 160 cooling towers since beginning the process on Thursday and once the process is complete, the city will announce all of the addresses of the towers that tested positive, Dr. Alister Martin, the city health commissioner, told the attendees of the town hall, which was held in a Park Avenue church basement.

The owners of buildings whose water-cooling towers test positive are being ordered to clean and disinfect them to eliminate the bacteria. City health officials said they would follow up to make sure that process was completed.

An audience member at the town hall asked what happens if a building owner fails to do so.

“We’ve never had that happen,” said Corinne Schiff, the health department’s deputy commissioner for environmental health. “They always comply.”

Martin echoed her sentiment, adding, “This is a matter of life and death.”

Martin said he expected dozens of buildings’ cooling towers to test positive for Legionella in this first round of testing.

But he said that doesn’t mean those towers are necessarily the ones getting people sick. He noted that the initial screening — a PCR test that can be completed in 24 to 48 hours — will bring back positive results even when the bacteria are no longer alive.

A culture analysis that takes about two weeks to complete is required to identify live bacteria, Martin said. And actually matching the bacteria in the cooling towers to the strains that are getting people sick could take even longer, requiring whole-genome sequencing, he added.

In the meantime, city officials are urging anyone who lives in the affected ZIP codes or has visited them since late June to keep an eye out for symptoms of Legionnaires’, which can be treated with antibiotics if it’s caught early. Symptoms can include body aches, cough, trouble breathing, fever, fatigue and gastrointestinal issues, according to the health department.

City health officials told Upper East Siders at the town hall to contact their doctors if they start to feel sick, or to call 311 or visit an NYC Health and Hospitals facility if they don’t have a primary care doctor.

But one man in the crowd said the city should have dedicated locations where people can go to get tested.

Others said they appreciated the information the city provided, but were frustrated that there weren’t more concrete measures they could take to prevent getting sick.

In response to audience member questions, Martin said city health officials don’t believe a mask will be effective against Legionnaires’.

He also said people living in buildings with cooling towers aren’t necessarily at higher risk than those whose buildings don’t have them, since mist containing Legionella can travel through the air.

Chin Hu, who lives on the Upper East Side and describes herself as a “senior citizen,” said getting out of the city seems like the best move.

“ We have a house upstate,” Hu said, “so maybe this is the best time to go.”

This story has been updated with additional information.