Nation & World
How hot is too hot?

A worker labors in the heat of Uttar Pradesh, India, hauling bricks from a kiln.

Photos by David Trilling

David Trilling

Salata Institute Communications

May 2, 2025 8 min read

Teaming up with grassroots organizers in India, Harvard researchers are collecting data to help workers adapt to dangerous spikes in heat

When it gets hot in Ahmedabad, bats pass out and fall from the trees.

Climate change is forcing temperatures into the upper limits of what many mammals, including humans, can bear. Heatwaves are more frequent and last longer. People in the world’s hottest places — like Ahmedabad, a 15th-century city of about 8 million in western India — now routinely struggle for months at a time. Air conditioning there is rare.

While extreme daytime temperatures grab headlines, “being indoors, at home, during the rest hours can be just as dangerous and deadly,” said Satchit Balsari, associate professor in emergency medicine at Harvard Medical School.

Adapting to this new reality is a matter of life or death. But to test adaptation strategies — say, how much does a white roof cool the room below? — urban planners need basic data. For example, how hot is it really in the places where people live and work? Ahmedabad’s official temperature is measured at an airport weather station standing out in the open, not in the dense urban microclimates where homes and livelihoods are concentrated. And how does the body respond to prolonged high temperatures? Or, as Balsari put it: “How do you define how hot is too hot?”

Harvard researchers are working with community leaders in India to build one of the largest datasets ever recorded on extreme heat and human well-being.

Cities and states across India are developing heat action plans to mitigate the impacts on their populations, though they often overlook informal workers.

The data will let civil society groups lobby on behalf of policy solutions, because there are no precise statistics in India on how heat affects work.

To answer these questions, Harvard researchers are collaborating with community leaders in Ahmedabad to build one of the largest datasets ever recorded on extreme heat and human well-being — data that could help millions of others facing rising temperatures around the world. The research team is placing thumb-size heat and humidity sensors in the workplaces and homes of hundreds of local women — in urban dwellings, on streets, and on farms — and monitoring their health with Fitbits and regular checkups over a year.

“This study was born out of an attempt to quantify the lived experience, the temperatures people are experiencing day after day in their homes, and what that means for their health, their heart rate, kidney function, their sleep,” said Caroline Buckee, professor of epidemiology at Harvard T.H. Chan School of Public Health.

Policies meant to offset extreme heat often focus on the risk of heat stroke, “but there’s a range of other health impacts that injure and kill people more slowly,” she added.

A woman harvests wheat at twilight, after the worst of the afternoon heat, in Uttar Pradesh.

A man erects shade to protect worshippers ahead of Friday prayers at a mosque in Delhi.

Balsari and Buckee lead a research cluster — funded by the Salata Institute for Climate and Sustainability and supported by the Lakshmi Mittal and Family South Asia Institute — studying the impact of warming microenvironments on the lives and livelihoods of the Indian working poor.

“During our pilot study, in certain homes we found temperatures up to 10 degrees hotter than the airport weather station data,” said Balsari. “We realized that a lot of global modeling efforts were dependent on data at a temporal and spatial resolution that may not reflect the lived reality of the millions of people most at risk in South Asia.”

Inside one home their team recorded a heat index of 137 degrees Fahrenheit — a measure that combines temperature and humidity to reflect how hot it feels. “That’s extreme, but the fact that the numbers cross 120 F routinely is terrifying. These thresholds, in many contexts, are unlivable,” Balsari said.

Building resilience with data

Study participants are tenant farmers and piece-rate workers. Data collectors are local social workers who visit participants every two weeks in their homes and workplaces to download data onto a customized smartphone app.

Cities and states across India are developing heat action plans to mitigate the impacts on their populations, though they often overlook informal workers.

The data will let civil society groups lobby on behalf of durable policy solutions, like heat action plans that take their members into account, because there are no precise statistics in India on how heat affects work.

Already these groups offer a growing menu of novel responses to heat: affordable finance to paint roofs a heat-reflecting white, buy umbrellas to shade market stalls, and install vents that let heat escape concrete-block homes. Another innovation is parametric heat insurance: Members buy a policy that will pay a day’s wage when the temperature exceeds a preset threshold, to allow them to stay home during the most brutal heatwaves and not have to choose between feeding their families and protecting their health.

Air conditioning is rare in the 15th-century city of Ahmedabad.

These insurance policies are designed to cover the kind of losses suffered last year by Ramila, who sells okra and tomatoes, bottle gourds, and fresh fenugreek from a cart she has pushed around Ahmedabad for 28 years.

Summers have gotten hotter and longer, she said, especially in the last few years. “But I still have to run my business.”

Last May, on one of the many days the mercury topped 110 F, Ramila, 47, passed out and hit her head on the ground. A bystander called an ambulance, which took her to a hospital. “That day my vegetables spoiled, so that was a loss. Plus, I incurred hospital costs — a double loss,” she recalled.

This year 250,000 women bought a policy ahead of the heat season, which peaks in May.

No escape

After a deadly 2010 heatwave, Ahmedabad adopted a heat action plan, the first in South Asia. It encourages public awareness of the dangers, trains doctors to spot heat stroke, and expands supplies of drinking water.

Still, argued one of its authors, Dileep Mavalankar of the Indian Institute of Public Health, “there is gross underreporting of heatwave-related deaths” across India, which might be recorded as heart attacks or other emergencies. “The combination of very high daytime temperature and warm nights kills the most. It is so hot that bats are falling unconscious and dropping out of the trees.”

Researchers are placing heat and humidity sensors in the workplaces and homes of hundreds of local women and monitoring their health with Fitbits and regular checkups.

Data collectors visit participants every two weeks to download data onto a smartphone app.

Initial data from the project is validating concerns about the home environments, which remain hot and humid well into monsoon season.

Initial data from the project — known as “Community Heat Adaptation and Treatment Strategies” — is validating concerns about the home environments, which remain hot and humid well into monsoon season.

“The way homes are designed, they absorb and store a lot of heat,” said Robert Meade, a postdoctoral research fellow with the Salata cluster. “Imagine a street vendor. They go to work in the hottest period of the day. Then they return to overheated homes where they must recover, take care of family, clean — all in an environment that is actually hotter than outdoors and doesn’t get that same nighttime drop in temperatures measured out at the airport weather station.”

An adaptation model

Down a lane wide enough only for pedestrians and mopeds, Karunisha, 55, works as a seamstress in her one-room home in Ahmedabad.

On the cornflower-blue wall, a white sensor records the humidity and temperature, 24 hours per day, year-round. A thermometer reads 93 F on a March afternoon.

“My neighbors gossip. They say the sensor is a camera, that I am being spied upon. But I know the data is being collected to help us,” says Karunisha, describing how it “gets hotter every year, so hot I get too weak to work.”

Without local trust in civil society, the Harvard researchers would not be able to reach so many vulnerable women in India’s poorest neighborhoods, said Buckee, the principal investigator.

“It is extremely difficult to implement this kind of study. You have hundreds of participants, sensors that need to be checked, private data. You have to do lab tests regularly. This level of coordination is simply not possible in most places. It’s possible here because our partners are doing the work and they care about finding the answers,” said Buckee.

The research cluster is now scaling up — expanding to other parts of India, distributing Fitbits and sensors in time for peak heat this year. As they collect empirical observations to inform a ground-up picture of the health risks from climate change, they are pioneering a new type of adaptation model by and for workers.

“We see this as a research platform that our partners themselves can use to test their own adaptations, to decide which work best for them,” Buckee said.