The same machines that have brought relief from sweltering summer temperatures are now at the center of a public health crisis in New York City, where a bacterial outbreak linked to air conditioning has killed three and sickened dozens more.

According to a report by USA Today, at least 67 people have contracted Legionnaires’ disease since 25 July in upper Manhattan, particularly in Harlem, where bacteria thriving in poorly maintained cooling towers of several apartment buildings were blown into the air through central air conditioning systems.

As the cooling towers pushed warm exhaust air into the sky, they inadvertently released Legionella bacteria — the airborne pathogens responsible for the outbreak — causing residents to unknowingly inhale the contaminated air.

So far, 24 individuals have been hospitalized and three have died, making it the largest Legionnaires’ outbreak New York City has recorded in the last 10 years. Officials say the number of cases is likely to grow.

Mayor Eric Adams announced in a video update that all cooling towers in the five affected ZIP codes have been tested, and those found contaminated have been disinfected. “We are treating this outbreak with the urgency it demands,” Adams said.

A growing threat in warming cities

Originally named after an outbreak at an American Legion convention in 1976, Legionnaires’ disease was once considered rare. But over the past two decades, cases have steadily increased nationwide. The U.S. Centers for Disease Control and Prevention (CDC) attributes this trend to aging infrastructure, increased urban density, and most notably, climate change.

USA Today reports that Legionnaires’ cases have risen ninefold since 2000. The disease flourishes in warm, stagnant water — an increasingly common condition in large cooling systems as summer heat intensifies and maintenance standards lag behind.

Health experts say the outbreak in Manhattan is a warning sign of what’s to come as cities continue to heat up.