by Sanket Jain, Yale Climate Connections
July 21, 2026
Sadashiv Yadav, 46, collapsed from a heart attack in 2024 after working long hours in western India’s sorghum and soybean fields, where heat rises in the summer to over 40°C (104°F). Yadav realized he was in trouble when he felt a sharp pain in his chest, and sweat poured down his body.
“Even in peak summers, I hadn’t experienced that kind of sweating,” he recalled.
At the hospital, doctors found blockages in the arteries supplying blood to his heart and surmised that years of working in extreme heat likely strained his cardiovascular system.
Many forms of heart disease are linked to chronic inflammation, and scientists are probing whether global warming has worsened this condition for people around the world, even among younger adults in northern Europe where extreme heat is much less common than in India.
Unlike acute inflammation, the immune system’s tool to fight infection or heal an injury, chronic inflammation is a prolonged immune response that can damage healthy tissues over time. Chronic inflammation has been linked to conditions ranging from arthritis to heart disease, diabetes, stroke, and other major illnesses.
“People often think heat is only dangerous when it causes heat stroke or during a heat wave, but that is probably too narrow,” said environmental epidemiologist Daniel Riggs, who is an assistant professor in the division of environmental medicine and the Christina Lee Brown Envirome Institute at the University of Louisville.
Emerging evidence suggests that even routine exposure to elevated temperatures may alter biological markers linked to inflammation, immune responses, and the functioning of blood vessels.
Cardiovascular diseases are the world’s leading killer, claiming an estimated 19.8 million lives in 2022. Around 85% of these deaths resulted from heart attacks and strokes, with more than three-quarters occurring in low-and middle-income countries.
How heat fuels inflammation
Recent studies have revealed evidence of these subtle biological changes. In Louisville, Kentucky, researchers analyzed blood samples from 624 adults and linked hotter conditions, measured using a heat metric that accounted for temperature, humidity, and wind, with signs of increased inflammation and measurable changes in the immune system.
Hotter conditions were associated with nearly 10% higher levels of some immune cells involved in the body’s inflammatory response, while levels of TNF-alpha, a molecule that helps drive inflammation, were about 7% higher.
The study did not track disease outcomes, yet the researchers say the findings raise concerns that repeated exposure to high temperatures could alter immune responses and make people more vulnerable to infections. Over time, these changes may also increase the risk of cardiovascular disease.
Riggs, who co-authored the study, found that even short-term exposure to heat appeared to affect both blood vessels and the immune system. Changes associated with inflammation included the cells lining blood vessels becoming more reactive, while immune cells were more likely to attach to vessel walls. The study also suggested that extreme heat may influence how immune cells communicate and respond to stress.
In a large Chinese study that examined some 3,000 proteins in nearly 4,000 adults, exposure to temperatures around 29.5°C (85.1°F) was associated with increases in some proteins and decreases in others, affecting the levels of 770 proteins, many of which were involved in inflammation, immune responses, and blood clotting. Many of these proteins were also linked to blood pressure and future risk of heart disease.
Riggs explained that repeated exposure to high temperatures and temperature fluctuations may gradually affect the lining of blood vessels, disrupt normal immune function, and increase the tendency for blood clot formation. These are all biological pathways linked to cardiovascular disease.
A climate change connection
Scientists are beginning to uncover biological clues hinting that cardiovascular problems could become more common in a warming world. In Germany, researchers analyzed blood samples from 359 women with an average age of 74. They examined temperature exposures over periods ranging from a few days to two weeks before the samples were collected.
A 5°C rise in ambient temperature was associated with roughly 10% higher levels of inflammatory molecules involved in immune regulation and the body’s response to stress. Some of them have previously been linked to cardiovascular disease, diabetes, and other chronic conditions.
The risks may increase when heat interacts with other environmental stressors.
“People experiencing co-exposures, especially air pollutants like PM2.5, may also be at higher risk because heat and pollution can act on overlapping inflammatory and vascular pathways,” Riggs said. PM2.5 refers to tiny air pollution particles that can be inhaled deep into the lungs.
Zhebin Yu, an environmental epidemiologist and assistant professor at Sweden’s Karolinska Institute, added that regulators should also take steps to reduce air pollution, which also worsens inflammation.
Wildfires, which are also growing much more common as the climate warms, may offer a glimpse of what happens when multiple environmental stressors converge. In Portugal, scientists reported higher levels of inflammation and signs of elevated cardiovascular risk among wildland firefighters after wildfire exposure. Although it could not disentangle the effects of heat from wildfire smoke, physical exertion, and other hazards associated with firefighting, it adds to evidence that climate-related exposures can activate inflammatory pathways.
Not just an older person’s problem
Although older adults are vulnerable to heat, emerging evidence suggests they are not the only ones affected. In Sweden, researchers followed 807 participants with a mean age of about 26 years and collected blood samples on three occasions between 2020 and 2022.
The team examined 365 inflammation-related proteins and compared them with short-term outdoor temperature exposures estimated at each participant’s home address. Some 58 proteins, about 16% of those studied, varied with temperature exposure. Many of them were involved in immune responses and inflammation, and several were also linked to measures of lung function, blood pressure, and blood sugar control. Exposure to both unusually hot and cold temperatures was associated with signs of accelerated biological aging.
“Many believe temperature stress only affects the elderly, but this study proves that young, healthy adults experience significant inflammatory changes and biological aging,” said Yu, who led the study.
At 40, Vilas Kamble is far from elderly, yet the farmer from Maharashtra’s Kerle village in India began experiencing recurring chest pain several years after being diagnosed with hypertension, or high blood pressure. Doctors found blockages in his arteries, and he underwent coronary artery bypass surgery last year. Now he avoids working during the hottest parts of the day whenever possible.
Kamble cultivates rice, sugarcane, corn, and other crops, often spending more than seven hours outdoors doing physically demanding work.
“Earlier, it never used to get so hot in our region,” he said.
Reducing risk in a warming world
Riggs said access to cooling, hydration, shade, and limiting heavy exertion during hot periods can help reduce heat strain. However, individual actions alone are not enough.
“More broadly, stronger heat warning systems, occupational protections, and cooler built environments are all important,” he said.
He noted that older adults, outdoor workers, people with cardiovascular, kidney, or metabolic disease, and those taking medications that affect hydration or thermoregulation are more vulnerable.
Yadav must farm to earn a living, so avoiding heat is easier said than done. Since his heart attack, he now tries to finish farmwork before noon and avoids working during the hottest part of the day. He has also stopped taking on additional work in construction. His wife, Lalita Yadav, 42, says she takes more frequent breaks and tries to limit prolonged heat exposure.
“Now you can’t pass summers peacefully,” Yadav said. “Earlier, summers were meant to rest. But now, they have become extremely dangerous.”
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