Extreme Heat and Humidity May Be Raising Stroke Risk Globally

For most of his life, Dattatrey Kamble believed the worst thing extreme heat could do was leave him exhausted. The 44-year-old farmer had spent decades working outdoors, sowing crops, irrigating fields, and tending to livestock under temperatures that frequently crossed 39°C (102.2°F). Like many farmers, he rarely took breaks. Then, one day in 2022, he slipped into a coma at the age of 40.

Kamble had spent seven hours working in the fields in his remote Jambhali village in western India before reporting for his second job as an ambulance driver. Shortly after reaching the hospital, he suffered a hemorrhagic stroke. He was admitted to a local hospital for a week, but his condition didn’t improve. Doctors transferred him to a better-equipped hospital, where he spent another two weeks. They were uncertain whether he would survive. Against the odds, he pulled through. Recovery took years.

His experience reflects a growing concern that heat may be contributing to stroke risk in ways that are only now becoming clear. A growing body of evidence suggests that both short- and long-term exposure to unusually hot temperatures can increase the risk of ischemic and hemorrhagic stroke. As temperatures continue to rise, researchers are working to understand who is most vulnerable, how heat affects the brain and blood vessels, and what can be done to reduce the risks.

Each year, an estimated 15 million people worldwide suffer a stroke. About five million die from it, and another five million are left permanently disabled. While people mostly associate high temperatures with heat stroke, that’s not always the case. Barrak Alahmad, a population health scientist at Harvard University, said that heat can quietly destabilize the cardiovascular system and trigger a heart attack, kidney injury, or ischemic or hemorrhagic stroke. “Heat stroke is just the tip of the iceberg. The true health burden of extreme heat is much broader and bigger than heat illness alone,” he added.

To understand the magnitude of the link between heat and stroke, his team analyzed nearly 5.9 million stroke deaths from 522 cities across 25 countries. Both extreme heat and cold increased the risk of dying from ischemic stroke, which is caused by blocked blood flow to the brain, and hemorrhagic stroke, caused by bleeding in the brain.¹ Although cold accounted for a larger share of excess deaths, heat was also linked to additional deaths. The burden of heat-related stroke was not distributed equally worldwide. In lower-income countries, hemorrhagic stroke mortality was higher.

A recent statement from the World Stroke Organization suggests that while cold temperatures continue to account for a larger share of stroke risk, the effects of heat are increasing over time.² Through several studies, scientists are beginning to quantify that risk.

When Heat Becomes Dangerous

In India, researchers analyzed data from almost 15,000 adults aged 45 and older. Rather than focusing on a single heatwave, the team examined long-term exposure to extreme heat.³ They calculated how often each district experienced unusually hot days over 12 years. These were defined as days when maximum temperatures exceeded the 90^th percentile of what was historically normal for that location.

Depending on the district, the number of extreme heat days ranged from about 13 to 43 per year, with a nationwide average of around 35 days. Those who experienced more of these hot days were more likely to report having had a stroke. After accounting for age, gender, smoking, alcohol use, socioeconomic status, and existing health conditions, the researchers found that a one-standard-deviation increase in extreme heat exposure, equivalent to about four additional unusually hot days per year, was associated with an 18 percent higher likelihood of a stroke. The association was strongest among women and urban residents, and they also observed it among people with diabetes and hypertension.

Pallavi Joshi, a public health researcher at the Energy and Resources Institute and a coauthor of the paper, said that studies suggest that diabetes and hypertension may make it harder for the body to cope with heat stress and maintain cardiovascular stability.^4,5 “Heat exposure may place additional strain on the circulatory system, potentially increasing stroke risk in these individuals,” she explained.

But scientists are increasingly finding that temperature alone may not tell the full story. An analysis of over 22,000 first-time stroke cases in Israel revealed that hotter days, more humid conditions, and the combination of the two were all associated with a greater chance of stroke.⁶ On days when temperatures reached 32°C (89.6°F) rather than 27°C (80.6°F), stroke odds were 32 percent higher.

Humidity independently increased stroke risk, and there was also a significant association between heat index—a measure that combines temperature and humidity—and stroke risk. A heat index of 100°F (37.8°C) was associated with a roughly 40 percent higher risk of stroke than 80°F (26.7°C).

A common assumption is that people living in hot climates are naturally protected from heat-related health risks. Maya Negev, an environmental health scientist at the University of Haifa and a coauthor of this paper, said her findings suggest that acclimatization has limits. Israel experiences hot summers and has widespread access to air conditioning. Yet stroke risk still rose significantly on hot and humid days. “Living in a hot country does not make people biologically immune to heat,” she explained.

She noted that people are not constantly shielded by air conditioning. They spend time outdoors, commute, work, shop, and carry out daily activities in environments that are not always adequately cooled. Others may struggle to afford sufficient cooling at home.

Another common misconception is that heat-related stroke is a concern only for the old or those already living with serious health conditions. While certain groups remain at higher risk, “our study suggests that heat stress can act as an acute trigger for stroke across broad population groups,” she added.

What Heat Does to the Brain and Body

The exact mechanisms through which heat increases stroke risk are still being uncovered. But evidence suggests that the body can come under stress through several pathways at once.

Galit Weinstein, a neuroepidemiologist at the University of Haifa who coauthored the study that was based in Israel, explained that when people are exposed to high temperatures, the body tries to cool itself by sweating and increasing blood flow to the skin. “If this continues, especially when people do not drink enough or cannot cool down properly, it can lead to dehydration, changes in the balance of salts in the body, and thicker blood.” This may increase the tendency to form clots, triggering ischemic stroke.

Studies are also uncovering what may be happening at the cellular level. In one recent study, researchers combined data from stroke patients in China, animal experiments, and laboratory analyses, and they found that heatwave exposure increased platelet activation and accelerated clot formation.⁷ The analysis identified heat shock factor 1 (HSF1), a protein involved in the cellular stress response, as a key mediator of this process. When activated by heat, HSF1 appeared to make platelets more reactive, increasing their tendency to clump together and form clots.

Heat also strains the heart and blood vessels. As the body works to dissipate heat, the heart has to work harder, and the vascular system comes under stress. According to Weinstein, repeated episodes of heat exposure may create recurring periods of physiological strain, particularly among older adults and people with underlying vascular risk factors.

Heat also poses a risk beyond the hottest hours of the day. In Germany, researchers analyzed over 11,000 stroke cases recorded in Augsburg between 2006 and 2020.⁸ Unusually warm nights—when nighttime temperatures stayed above normal for the region—were associated with 14 percent higher odds of stroke, even after accounting for daytime temperatures.

Comparing 2006–2012 with 2013–2020, the estimated annual number of excess stroke cases associated with hot nights rose sharply, from two to 33.

What Can Be Done?

While the evidence linking heat and stroke is growing, experts said that many of the interventions needed to reduce the risk are well understood.

“Anything that can lower your exposure to heat should be considered,” said Alahmad. At the workplace level, measures like rest breaks, shade, and water interventions have been shown to be effective.⁹ Negev added that hot and humid days should be taken seriously, especially by older adults and people with cardiovascular risk factors. “This includes avoiding unnecessary outdoor activity during the hottest hours, drinking enough water, staying in cool places, and seeking medical attention quickly if neurological symptoms appear,” she said.

Those lessons resonate with Kamble. The stroke left him bedridden and unable to speak or walk. For a year, he depended on others for everyday tasks, from brushing his teeth to eating meals, recalled his wife, Jayashree Kamble. Yet he refused to give up. Today, he exercises without fail, goes to the gym regularly, and takes precautions to avoid prolonged heat exposure. He has progressed to walking on his own and lifting 3kg dumbbells.

However, reducing risk cannot rely solely on individual actions. Protecting vulnerable populations will also require stronger public health measures and better heat preparedness. Joshi said that heat action plans should identify older adults and individuals with cardiovascular risk factors as priority populations. “This could include targeted risk communication, integration of heat-health advisories into routine clinical care, community outreach through frontline health workers, and strengthening surveillance of heat-related illnesses during summer months,” she said. Negev added that health systems also need heat preparedness plans that account for humidity and heat index, not just temperature.

To help lower population-level heat exposure, Joshi suggested expanding green spaces, improving housing quality and ventilation, expanding access to cooling infrastructure, creating shaded public spaces, and reducing urban heat island effects.

Efforts are also underway to predict stroke risk before it occurs. In China, a stroke-specific heat-risk model was developed using data from over 28,000 stroke deaths across 304 counties.¹⁰ Instead of relying on a single temperature threshold, the model estimates stroke risk using demographic and local climate information. It then classifies risk into four levels, from low to extremely high. The analysis suggested that the model outperformed China’s existing heat-warning system, with the potential to prevent about 49 percent of heat-related stroke deaths compared to 17 percent under current warnings.

But for Kamble, it is not just about predicting risk. It is also about rebuilding a life that stroke nearly took away. Each morning, he meditates and repeats the same message to himself.

“I will recover completely in the next six months,” he said.

Whether or not that timeline proves accurate, his determination has already carried him further than many once thought possible.