Air conditioners have been working overtime this hot summer, from those tiny window units to the massive AC towers that serve the tightly packed apartment buildings in major cities. And while they bring the relief of cool air, these contraptions also create the conditions for dangerous bacteria to multiply and spread.
One particularly nasty bacteria-borne illness is currently spreading in New York City using those enormous cooling units as its vector: Legionnaire’s disease. The bacterial pneumonia, which usually recurs each summer in the US’s largest city, has sickened more than 100 people and killed five in a growing outbreak.
If you don’t live in New York City or the Northeast, you may never have heard of Legionnaire’s, but this niche public health threat may not be niche for much longer.
Climate change is both helping to make Legionnaire’s disease more plentiful in the places where it already exists and creating the potential for it to move to new places where the population may not be accustomed to it. Cities in the Northeast and Midwest, where hotter weather meets older infrastructure, have reported more cases in recent years. Recently, Legionella bacteria was discovered in a nursing home’s water system in Dearborn, Michigan — one of the states, along with Ohio, Pennsylvania, Illinois, and Wisconsin, that have seen more activity in the past few years.
Anyone can contract Legionnaire’s disease by inhaling tiny drops containing the bacteria, and the symptoms — fever, headache, shortness of breath — appear within days. It can cause a severe lung infection, with a death rate of around 10 percent.
While healthier people often experience few symptoms, the more vulnerable — young children, the elderly, pregnant people, and those with compromised immune systems — face serious danger from the illness. Around 5,000 people die every year in the United States from Legionnaire’s disease, many of them living in low-income housing with outdated cooling equipment where the bacteria can more readily grow and spread.
Legionnaire’s disease is a microcosm of climate change’s impact on low-income communities. As warmer temperatures facilitate the spread of disease, the most socially vulnerable populations are going to pay the steepest price.
The collision of Legionnaire’s disease, climate change, and economic disparities
Legionnaire’s disease was first documented after an unusually aggressive pneumonia outbreak during an American Legion conference in Philadelphia in 1976. Soon, Centers for Disease Control and Prevention scientists confirmed the cause of the mysterious illness: a previously unknown bacteria that was accordingly named Legionella. Legionella, unfortunately, is everywhere — in streams, lakes, and water pipes across the country.
But usually, it occurs in such low concentrations and is so remote that it doesn’t pose a threat to humans. Usually.
Now, city health officials have found the bacteria in the large cooling tanks that serve massive apartment buildings across New York City, particularly in Harlem. Cooling tanks are ideal for Legionella to grow and spread. They’re filled with stagnant, warm water that is more hospitable to bacterial growth. Like an evaporative cooler, the systems convert warm stagnant water into cool air for apartment dwellers. They can spray mists laden with the bacteria into the open air, dispersing it across the surrounding air, where it can enter a person’s lungs when they inhale. According to the Environmental Protection Agency, 80 percent of Legionnaire’s cases are linked to potable water systems.
Summers have always been prime time for the spread of Legionnaire’s, and climate change only exacerbates all of these problems. For starters, the quality of natural water is degrading — creating more favorable conditions for Legionella, which loves acidic water and low oxygen levels, to multiply — as the planet warms. In the summer, the higher the temperatures and the more humid the air, the easier it is for the bacteria to reproduce in these cooling systems.
A decade ago, researchers warned that “climate change might increase the incidence of legionellosis through increased reliance on air conditioning systems,” especially the large units serving city housing buildings. The European Climate and Health Observatory advised in 2022 that higher temperatures and the increased rainfall associated with climate change could soon make Legionnaire’s disease a more widespread issue.
The EPA also expects the bacteria’s range to expand as global temperatures rise. Each heat wave and heavy rainstorm creates the hot and wet conditions that allow Legionella to thrive. Flooding can allow Legionella-tainted water to more easily penetrate a community’s water system, too.
And those who live in a low-income area are more likely to be at risk from this increased prevalence of Legionnaire’s disease. As the New York Times reported recently, poorly maintained cooling systems are more likely to see Legionella flourish and pump the bacteria into their neighborhoods — and low-income housing is more likely to be poorly maintained. Though NYC attempted to require more inspections for defective cooling systems after a 2015 outbreak of Legionnaire’s, inspections have fallen off in recent years, according to reporting from The Gothamist.
This dynamic isn’t unique to Legionnaire’s disease. As the EPA has observed, low-income people are more likely to face health consequences from climate change because they are more likely to live in places exposed to extreme weather, they are more likely to have health conditions that make them more vulnerable to infectious diseases and natural disasters — and they are more likely to live with the kind of aging infrastructure that allows Legionnaire’s to flourish.
What you can do to protect yourself
Legionnaire’s is a particularly disconcerting health threat: It rains down from the sky in tiny invisible droplets, packed with bacteria so potent it can kill one in 10 people whom it manages to infect through its telltale aggressive pneumonia. It is as deadly as tuberculosis, though Legionnaire’s disease usually kills much faster.
I asked some experts what people can do about Legionnaire’s disease. The most important advice was to be vigilant about any public health warnings if you reside in an area where Legionella has historically lived or has recently been detected, especially if you have certain preexisting conditions like diabetes, chronic lung disease, or autoimmune disorders, or if you’re over the age of 50. Follow the news and make sure you’re registered for alerts from your local public health officials.
People could consider investing in a high-quality water filter during an outbreak, the CDC says. If you use an evaporative cooler or your home is equipped with a swamp cooler, you can make sure you’re keeping them clean and using filtered water, if possible.
But the issue here is ultimately not your small humidifier. The problem resides much further up the water supply chain, in the massive community water systems where Legionella flourish. You can’t suddenly stop drinking water or breathing air, of course, even if there is a local outbreak. The CDC itself notes that point-of-use water filters only provide limited protection.
So, whenever you know Legionnaire’s disease is spreading nearby, you should be paying attention to your body, said Janet Stout, president and director of the Special Pathogens Laboratory, who has studied the disease for 30 years. See your doctor immediately if you’re experiencing fever, shortness of breath, or a cough. You could have a special test done for Legionnaire’s disease and potentially receive antibiotics, such as levofloxacin or azithromycin, that have proven effective against the bacteria. Most cases can be cured if they are caught early enough.
But this is a truly public health challenge; when bacteria are multiplying in community water systems and then spewing into the open air, anyone can be exposed. There is only so much that any one person’s own, individual effort can do to stop infections.
The main way to prevent Legionnaire’s disease, however, or at least significantly reduce its risk, is by staying on top of maintenance to make sure community water sources are kept clean, preventing the bacteria from proliferating. But that kind of maintenance requires vigilance and perhaps even public pressure. In New York City, for example, only 1,200 cooling towers were inspected in the first six months of 2025, a steep drop-off from 2017 when the city had checked more than 5,000 over the same period.
Landlords and public health authorities must take Legionnaire’s disease seriously. We are going to see more and more outbreaks as climate change continues to heat the planet.
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