WESTERN INDIA — Researchers in Louisville, Kentucky, analyzed blood samples from 624 adults and linked hotter conditions with signs of increased inflammation and measurable changes in the immune system. The Louisville study found that short-term exposure to heat appeared to affect both blood vessels and the immune system.

Hotter conditions were associated with nearly 10 percent higher levels of some immune cells involved in the body’s inflammatory response in the Louisville study. Levels of TNF-alpha, a molecule that helps drive inflammation, were about 7 percent higher in hotter conditions in the Louisville study.

The Louisville study used a heat metric that accounted for temperature, humidity, and wind. Changes associated with inflammation in the Louisville study included the cells lining blood vessels becoming more reactive. Changes associated with inflammation in the Louisville study included immune cells being more likely to attach to vessel walls.

The Louisville study suggested that extreme heat may influence how immune cells communicate and respond to stress. The Louisville study did not track disease outcomes.

"People often think heat is only dangerous when it causes heat stroke or during a heat wave, but that is probably too narrow," said Daniel Riggs, assistant professor in the division of environmental medicine and the Christina Lee Brown Envirome Institute at the University of Louisville. Riggs co-authored the Louisville study.

Emerging evidence suggests that routine exposure to elevated temperatures may alter biological markers linked to inflammation, immune responses, and the functioning of blood vessels. "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," Riggs said. "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," he said. PM2.5 refers to tiny air pollution particles that can be inhaled deep into the lungs.

A large Chinese study examined some 3,000 proteins in nearly 4,000 adults. In the Chinese study, 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 the proteins affected by heat exposure in the Chinese study were involved in inflammation, immune responses, and blood clotting. Many of the proteins affected by heat exposure in the Chinese study were linked to blood pressure and future risk of heart disease.

In Germany, researchers analyzed blood samples from 359 women with an average age of 74. The German study examined temperature exposures over periods ranging from a few days to two weeks before the blood samples were collected. In the German study, a 5°C rise in ambient temperature was associated with roughly 10 percent higher levels of inflammatory molecules involved in immune regulation and the body’s response to stress. Some of the inflammatory molecules studied in Germany have previously been linked to cardiovascular disease, diabetes, and other chronic conditions.

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 Swedish study team examined 365 inflammation-related proteins and compared them with short-term outdoor temperature exposures estimated at each participant’s home address. In the Swedish study, 58 proteins, about 16 percent of those studied, varied with temperature exposure.

Many of the proteins that varied with temperature in the Swedish study were involved in immune responses and inflammation. Several proteins that varied with temperature in the Swedish study were 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 in the Swedish study.

In Portugal, scientists reported higher levels of inflammation and signs of elevated cardiovascular risk among wildland firefighters after wildfire exposure. The Portugal study could not disentangle the effects of heat from wildfire smoke, physical exertion, and other hazards associated with firefighting.

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. Cardiovascular diseases claimed an estimated 19.8 million lives in 2022.

Around 85 percent of cardiovascular disease deaths in 2022 resulted from heart attacks and strokes. More than three-quarters of cardiovascular disease deaths occur in low-and middle-income countries.