A hand holds a thermometer in front of a black and white scan of a brain.
Credit: Madeline Monroe/C&EN/Shutterstock
Chhaya Gaikwad didn’t know that ignoring her father-in-law’s advice of not working in scorching heat could cost her over $1,000.
Gaikwad has been a farmer for over 30 years and has a wealth of knowledge about farming. For the past decade, she has been cultivating radish pods and has perfected the art of growing and harvesting them. She knows the precise timing for the best yield and how market demand shifts with the seasons.
Yet last year, she made a significant mistake for the first time.
Last summer, when the temperature reached 44 °C (111.2 °F) in her village in India, 50-year-old Gaikwad worked in the fields for 8 h a day. Her late father-in-law, Sakharam Gaikwad, had warned her against this, but she dismissed his concerns, convinced she was too young to be affected by the heat.
Gaikwad spent her days planting and watering crops, weeding the fields, harvesting produce, and carrying heavy bundles of livestock fodder on her head. She also worked applying fertilizers and spraying pesticides, collecting firewood, and tending to the cattle, without a minute’s break.
Then it came time to plant the radish pods. “As soon as winter arrives, radish pods are in high demand and fetch a premium price,” says Gaikwad, a resident of remote Jambhali, a village in India’s Maharashtra state.
To align with peak market demand, Gaikwad should have begun growing them in September 2024. But she had made a misjudgment in her preparation and work planning during the summer months that meant she planted the radish pods a month later. By the time the first harvest was ready, it was the last week of December, and the market was saturated. “There were days when I couldn’t sell even 5 kg as the price dropped rapidly,” she says.
While Gaikwad had anticipated a market rate of at least 100 Indian rupees ($1.15) per kilogram, after the first few weeks she could only sell for INR 30 ($0.35) and sometimes even less. “I couldn’t even cover the transportation costs to get to the market.” It was a tremendous loss caused by poor decision-making.
The answer to Gaikwad’s poor decisions may be found in a paper published last year by scholars in Australia (Econ. J., DOI: 10.1093/ej/ueae036). The researchers discovered that nights exceeding 25 °C in Indonesia led to irrational and impulsive behavior and poor economic decisions. Reduced sleep quality during these hot nights decreases cognitive function the following day, resulting in poor choices. Households with lower incomes and limited air-conditioning access are particularly susceptible.
India isn’t the only country grappling with rising temperatures. Parts of the US and nations in Central America, the Middle East, South Asia, and Africa reported unusually high temperatures in 2024, making it Earth’s hottest year. According to a report from the Red Cross Red Crescent Climate Centre, World Weather Attribution and Climate Central, these temperatures affected 6.8 billion people, or 84% of the world’s population. On average, individuals endured extreme heat for at least 31 days, a number that researchers project will grow further (Heliyon 2024, DOI: 10.1016/j.heliyon.2024.e26431).
A Lancet study found that over 489,075 heat-related deaths were reported between 2000 and 2019, with over half of them in Asia (2021, DOI: 10.1016/S2542-5196(21)00081-4). Now researchers are uncovering the complexities of how heat affects the human brain, revealing its far-reaching consequences (Int. J. Geriatr. Psychol., DOI: 10.1002/gps.6063).
Heat is affecting cognition
A growing body of research links high heat to cognitive problems. For example, researchers in Qatar, Greece, and Cyprus have investigated the effects of sun exposure on the physiology and cognition of outdoor workers (Int. J. Environ. Res. Public Health 2021, DOI: 10.3390/ijerph18147698). They found that while working in the sun increased heart rate, sweat rate, and skin blood flow, which you might expect, it also reduced attention and vigilance by 45% and 67% compared with similar heat stress in the shade. Despite identical heat levels, sun exposure significantly impaired cognitive function.
A woman in an orange sari stands facing away from us, looking at a field of radish pods.
Credit: Sanket Jain
Chhaya Gaikwad looks at the radish pods she cultivated. Incorrect decisions last year resulted in financial losses for the farmer
Study lead author Leonidas Ioannou, an environmental physiologist and assistant researcher at Slovenia’s Jožef Stefan Institute, says that cognitive performance is linked to elevated skin temperature. “When your skin temperature rises, mental capacity drops due to various physiological mechanisms, including blood being redirected toward the skin to dissipate heat and hyperventilation lowering the partial pressure of arterial carbon dioxide. Together, these mechanisms reduce cerebral blood flow and may limit oxygen delivery to the brain, impacting cognition,” he explains. Increased skin temperature also affects thermal comfort, which can further influence both cognitive and physical performance.
Similarly, a Taiwanese study found short- and long-term temperature exposure associated with moderate to severe cognitive impairment in elderly people (BMJ Open 2021, DOI: 10.1136/bmjopen-2021-049160). Its corresponding author, Han-Bin Huang, a professor at the School of Public Health at Taiwan’s National Defense Medical Center, says that extreme heat affects the hippocampus, a critical learning and memory region that is sensitive to body temperature changes.
He even pinpoints a specific protein that is implicated in those changes. “A temperature-sensitive channel, known as transient receptor potential vanilloid 4 [TRPV4] . . . is activated at physiological temperatures within the hippocampus,” he says.
TRPV4 is like a temperature sensor. Found in the hippocampus and other body tissues, it activates at normal body temperature and plays a role in sensing heat and pain and maintaining body functions like hydration. “Prolonged exposure to high temperatures may disrupt hippocampal function by altering TRPV4 activity, potentially impairing cognitive processes such as memory and learning,” Huang says.
Moreover, extended heat exposure has also been linked to increased inflammation and oxidative stress markers in the body. Oxidative stress happens when there are too many harmful free radicals and insufficient antioxidants to balance them. “These processes are known to contribute to cognitive decline,” Huang says. Researchers emphasize the need for more long-term studies to understand how heat affects brain function precisely.
The effect of overexposure to heat can also be immediate. A recent Chinese study that tracked 53,000 people over 8 years found that extreme heat was associated with reduced cognitive function on an hourly, daily, and annual basis (Ecotoxicol. Environ. Saf. 2024, DOI: 10.1016/j.ecoenv.2024.116238). During heat stress, brain support cells like microglia and astrocytes become active and release inflammatory substances that can damage neurons and impair the formation of new neurons, affecting memory.
Heat makes seizures prominent
While there’s an uptick in research on the impact of heat on cognition, researchers have also found that climate change is worsening other brain conditions. Rising temperatures and humidity can aggravate neurological disorders such as stroke, multiple sclerosis, Alzheimer’s, Parkinson’s, and epilepsy.
Sunil Varekar, 47, has been living with epilepsy, a brain condition that has caused recurring seizures since he was 15 years old. He avoids going outside alone. “I can experience a seizure anytime, so I don’t risk it,” he says.
A man sits on a low wall in front of a field of tall green plants.
Credit: Sanket Jain
Sunil Varekar has been living with epilepsy since he was 15 years old. He says extreme heat has made seizures frequent, severely impacting his mental and physical health.
But Varekar doesn’t always have the luxury of being around others. In the third week of January this year, while working in a sugarcane field, he experienced a seizure. “There was no one around. I regained consciousness after 10 min and continued to work.” Such incidents have resulted in multiple injuries for him. “There’s always a risk, but I have no choice.” He pointed out that the heat has intensified over the past 4 years in his village, Jambhali.
Last year, when temperatures soared past 40 °C in April and May, Varekar sometimes experienced seizures up to five times a day. The seizures usually last for 2–3 min, but during extreme heat, they can last for 5 min or more. “It becomes quite scary during that time,” says his nephew’s wife, Suchita Varekar. “When I have more than four seizures in a day, I feel extremely weak and struggle with laborious tasks in the field,” Sunil says.
According to the World Health Organization, 50 million people globally have epilepsy, with 80% of them living in low- and middle-income countries. Varekar’s experience reflects a broader pattern observed in research from other countries showing that heat-related complications of epilepsy are affecting more people.
A study published in Urban Climate examined 225,699 epilepsy-related hospitalizations in Brazil from 2000 to 2015. It revealed that for every 1 °C increase above 26 °C, the risk of hospitalization rose by 4.3%, with the effect peaking on the second day of heat exposure (2023, DOI: 10.1016/j.uclim.2023.101497).
Its lead author, Yiwen Zhang, a graduate researcher at Australia’s Monash University , says that “rising temperatures and heat can lower the brain’s resistance to seizures, while extreme weather events like heat waves can increase stress, which in turn affects the brain’s response to stress hormones, raising the risk of seizures.” Climate change can also boost inflammation in the body, making seizures more likely. Moreover, heat can also lead to stress, fatigue, lack of sleep, and fever, which are common seizure triggers.
Zhang’s study found that women were more vulnerable than men to heat- induced epileptic seizures because of a mix of biological, hormonal, and physical factors. “The female brain tends to have more connections between its two hemispheres, which can make it more excitable and lower the seizure threshold,” Zhang explains.
Additionally, she notes that men generally regulate heat more efficiently, with smaller temperature changes and faster stabilization. “Females have higher body fat and sweat less, which makes it harder to cool down.” Moreover, hormonal shifts during the menstrual cycle, pregnancy, or menopause make female individuals more vulnerable to seizures when exposed to heat. “Differences in immune system activity and brain cell responses between males and females further contribute to this heightened risk,” she adds.
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Climate change can induce stress and sleep deprivation, increasing the risk of seizures in people with epilepsy. For Varekar, rising heat has been more challenging than the seizures. “I’ve learned to live with seizures,” he said. “But I can’t adjust to this heat.”
Heat not only affects his health but also leads to farm losses. “Many crops dry up during extreme heat, causing severe losses,” he explained. The stress of farm losses directly affects his seizures. To manage this, he turns to meditation, which he says has been a great help. Every day, without fail, he spends 30 min at a nearby meditation center.
Zhang advises individuals with epilepsy to monitor their health, stay hydrated, avoid outdoor activities during extreme heat, and seek medical help. “Workplaces should implement heat-safety protocols, conduct regular health checks for employees with epilepsy, and provide seizure first-aid training.”
For Varekar, the link between heat and his seizures is clear. “The moment the temperature crosses 40 °C, I know seizures will become common.” This leaves him helpless and angry, but he asks, “What can I do?”
Heat affects disproportionately
The impacts of climate change are experienced differently within the same region because of socioeconomic, cultural, and systemic factors that intensify its effects on vulnerable populations.
A 2023 study from the US found that exposure to extreme heat was associated with faster cognitive decline in Black individuals and those living in low-income neighborhoods than in White or Hispanic people or residents of wealthier communities (J. Epidemiol. Community Health, DOI: 10.1136/jech-2023-220675).
Eunyoung Choi, a postdoctoral associate at the University of Southern California’s Leonard Davis School of Gerontology who was part of the team that conducted the study, says that many older Black adults have faced lifelong chronic stress and systemic disadvantages.
Every year, the maximum temperature is rising, and we have no choice but to find a way to survive it
Chhaya Gaikwad, farmer
This accumulated disadvantage contributes to persistent chronic stress exposure, weakening cognitive reserve over time. “Consequently, insults such as extreme heat exposure in later adulthood may compromise Black individuals’ cognitive reserve at a faster rate,” she says. The observed disparity is not just about the immediate effects of heat. It’s about a lifetime of exposure to conditions that make certain groups more vulnerable to the heat, she says.
Moreover, access to air-conditioning is marked by significant inequalities. People in regions in the world’s lowest quartile of income are 40% more exposed to heat waves than the wealthiest people (Earth’s Future 2022, DOI: 10.1029/2021EF002488). A study published last year found that up to 4 billion people may lack air-conditioning in 2050 (Nat. Comm., DOI: 10.1038/s41467-024-52028-8). A report by the Food and Agriculture Organization of the United Nations highlights that rural households in the lowest quartile of a relative wealth index lose 2.4% of their on-farm incomes and 1.1% of the value of their crops in every day of extreme heat compared with those of households in other quartiles.
Choi cautions that prolonged and repeated exposure to extreme heat could negatively affect cognitive function in the longer term. Heat-related illnesses, such as heat stroke and exhaustion, can raise the risk of cardiovascular diseases, which may restrict blood flow to the brain and lead to cognitive decline, says Choi.
She describes how extremely high temperatures can break down the blood-brain barrier, which protects the brain from harmful substances. This may allow unwanted proteins and ions to accumulate, leading to inflammation and permanent cell death in the brain.
Brain health in a warmed world
Gaikwad’s family has urged her not to work in the fields during the hottest days of the summer.
Still, Gaikwad knows she must be out in the fields, no matter how extreme the heat. In such instances, Ioannou suggests that a simple yet effective strategy is to wear light-colored clothing and stay hydrated. Taking frequent breaks during a shift and managing workload intensity can also help. “If you know that tomorrow it’s going to be very hot, then you can specifically choose to do the lighter tasks then and leave the heavier ones for another day,” he says.
Another approach is using real-life examples to educate workers. When Ioannou interviewed workers, he asked them if they believed heat affected their physical or cognitive capacity. “The majority said no, which was unexpected.” To address this, he started educating workers on the impact of heat exposure on their jobs.
For example, in winter, when the temperature is around 20 °C, workers are highly efficient and productive on the farm. “But if you return in the summer and show them that their productivity has dropped by half, they begin to understand the impact of heat,” Ioannou says. “When presented with data, they realize how heat affects their productivity.”
Huang suggests policy-level initiatives, such as developing a comprehensive heat-action plan and strengthening surveillance and early-warning systems to notify communities of impending heat waves. Communities should also receive clear guidelines and implement targeted measures to safeguard vulnerable populations, says Huang.
He also advocates for proper urban planning that incorporates “blue and green spaces, such as parks, trees, and water features,” to mitigate the urban heat island effect and create cooler microclimates. Additionally, building design should focus on heat-reflective materials and maximizing natural ventilation to keep indoor spaces cooler, recommends Huang.
That’s easier in more well-resourced homes. “Smart home systems and ambient technology can further enhance heat resilience by monitoring indoor temperature and alerting caregivers of potential risks by offering real-time feedback and proactive support to older adults,” Huang says.
In India, anxiety is growing among outdoor workers. Gaikwad says she feels helpless about rising heat and has started talking to fellow farmers about how they are coping. With no concrete answers yet, she often feels scared. “Every year, the maximum temperature is rising, and we have no choice but to find a way to survive it,” Gaikwad says.
Sanket Jain is a journalist and documentary photographer from Maharashtra, India.
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