When heat hits home: Climate change’s hidden and deadly toll
Since 2008, we have been living on an urban planet. That year, for the first time, more people worldwide lived in cities than in rural areas. We have also become an indoor species, spending as much as 90 percent of our time inside.
For the past two decades, my work has focused on how indoor environments shape health, from apartments in Boston to informal settlements around the world. Again and again, I have seen how housing quality, building materials, ventilation, and social conditions quietly but powerfully shape people’s exposure to environmental hazards. These factors, in turn, reflect upstream policies and decisions often made far from the people who live with their consequences.
Climate change is intensifying the risks. Its most visible effects include parched fields, flooded cities, and wildfires, but much of its toll on human health remains hidden inside homes, especially in crowded cities where many people lack climate control. Extreme heat offers a particularly stark window into how climate change is already being experienced in everyday life.
City leaders can reduce these risks by identifying the human and structural factors that compound them and by targeting interventions where they are needed most.
From Chicago to Paris and beyond
In 1995, a heatwave left a deadly toll in Chicago, exposing clear failures in city preparedness. But its deeper lessons were about risk factors hiding in plain sight. Living alone increased the risk of death by 130%. Lacking air conditioning more than tripled it, and living in a top-floor apartment raised the risk nearly fivefold. The city’s structural DNA compounded the danger. Asphalt, concrete, and dark surfaces absorbed heat, while limited tree cover reduced cooling. The built environment trapped people precisely where they lived.
The deaths were not driven by mysterious factors. They reflected the built and social conditions that continue to shape the toll of extreme heat we see in each year’s news cycle.
Even today, official mortality counts still underestimate the true toll. Death certificates rarely identify heat as the cause, and heat stroke represents only the most visible cases. More often, heat worsens cardiovascular disease, respiratory illness, diabetes, and other underlying conditions. Careful epidemiological analysis is needed to reveal the true number of excess deaths. (In fact, one reason the death toll in Chicago was on the evening news in real time was the morgue’s need to secure a fleet of very-visible refrigerated meat-packing trailers to accommodate the unprecedented overflow of bodies.)
The same pattern emerged during Europe’s 2003 heatwave. In Paris, the city’s iconic zinc roofs could reach 140o F (60o C) in direct sunlight, turning attic bedrooms into dangerous heat traps, especially for older people living alone. Nearly 15,000 people died across the country. This is not a historical footnote. Early estimates suggest heatwave in Europe this past June was responsible for between 10,000 and 20,000 excess deaths.
Similar scenarios exist across the globe. Parisian roofs share some physics with the sheet metal commonly used for roofing in many informal settlements, particularly across the Global South. Studies from Ghana, South Africa, India, and elsewhere show that building materials and their properties can make a big difference.
Back at home, I lead an NIH-funded study called “Beat the Heat,” which provides air conditioners – matched with funds to offset energy costs – to high-risk adults in Boston. These are not luxuries or handouts in a changing climate. Access to cooling is becoming a form of essential health care.
We are using indoor sensors to track thermal comfort and we are recording residents’ health conditions and signs of heat stress to see exactly how these air conditioners impact health.
The study, a partnership with The Family Van and CREW, has already opened our eyes to lived experiences that many public health researchers and physicians rarely see. Most Bostonians consider air conditioning a necessary and standard part of managing New England summers and annual heatwaves. For Bostonians struggling to cover rent, food, and other basic expenses, buying and running an air conditioner can be out of reach. Heating is protected by law for renters; cooling is not yet treated as a basic need.
Imagine your air conditioner were coin-operated. For many families, that is effectively how it works: Every hour of cooling competes with rent, groceries, phone bills, and the other costs of daily life.
Turning knowledge into protection
We cannot change what we don’t measure. In the year 2026, we have tools to identify, diagnose, and address these risks. Health surveillance, epidemiological methods, and genuine two-way engagement with communities can guide interventions that make a difference.
If our data show that low-cost upgrades and clear heat warnings prevent illness, Boston officials – and eventually state or federal agencies – could require cooler, cleaner buildings, and greener neighborhoods; steer climate funds toward the neighborhoods that need them most; and set stricter standards for private and public housing. Bottom line – let’s not let our homes be a blind spot for intervention. Our study also examines how such programs can be implemented effectively and expanded. That work may not make headlines, but it is essential to turning research into public health practice.
There is no single solution to the public health consequences of climate change. This is an all-of-the-above moment. When I was a graduate student in the 1990s, we theorized about what a human-altered climate might bring in the 21st century. It is now unfolding much as predicted.
All perspectives expressed in the Harvard Climate Brief are those of the authors and not of Harvard University or the Salata Institute for Climate and Sustainability. Any errors are the authors’ own. The Harvard Climate Brief is edited by an interdisciplinary team of Harvard faculty.
