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A new study reports heat exposure may harm the immune system and increase inflammation, potentially impacting heart health. Martin Harvey/Getty Images
  • 2023 was the hottest year on record for planet Earth, and by the middle of the 21st century, the U.S. will experience 27–50 days of temperatures exceeding 90 degrees each year.
  • Prolonged heat exposure may lead to heat-related illnesses with complications such as increased risk for heart disease.
  • A new study reports that high heat exposure may harm the body’s immune system and increase inflammation, potentially harming a person’s cardiovascular health.

Scientists reported that 2023 was the warmest year on record for planet Earth, and the world’s median temperature is increasing much more rapidly than it was at the start of the 20th century.

If this warming trend continues, experts believe that by the middle of the 21st century, the United States will experience between 27 to 50 days of over 90 degrees each year.

Almost 33% of working adults in the U.S. have a job where they are regularly exposed to the outdoors, including heat.

In 2020, the U.S. Bureau of Labor Statistics reported about 2,330 cases of illness or injury caused by heat exposure. And each year, about 40 working adults die from extreme heat exposure.

Prolonged exposure to high heat can lead to several heat-related illnesses, including:

“Additionally, exposure to heat can also lead to further complications. For example, heat can negatively impact pre-existing cardiovascular disease.”

A new study reports that high heat exposure may harm the body’s immune system and increase inflammation, potentially harming a person’s cardiovascular health.

The findings were recently presented by University of Louisville researchers at the American Heart Association’s Epidemiology and Prevention│Lifestyle and Cardiometabolic Scientific Sessions 2024. The study results have yet to be published in a peer-reviewed journal.

Lead study author Dr. Daniel W. Riggs, assistant professor of medicine in the Christina Lee Brown Envirome Institute at the University of Louisville explained that heat exposure is an important yet underappreciated risk factor contributing to heart disease.

“In the context of warming global temperatures the frequency of extreme temperature events is increasing and will continue to negatively impact population health,” Dr. Riggs told Medical News Today.

“A better understanding of the effects of heat on health, including heart health, is necessary to develop evidence-based approaches to help mitigate and prevent future climate-related impacts on health and well-being.”

“Although the link between cardiovascular disease and exposure to high heat is well established, the mechanisms and exposure pathways that promote the development of cardiovascular disease are complex and require more research. Therefore, our interest was to try to develop a better understanding of how exposure to high temperatures may contribute to immune-inflammatory activation.”

— Dr. Daniel W. Riggs, lead study author

For this study, Dr. Riggs and his team recruited 624 adults with an average age of 49.5 years. More than half of the participants were women, and 77% identified as white.

Study participants visited study sites in the Louisville, KY area during the summer months, with each day having a median temperature of 76 degrees Fahrenheit.

Researchers took blood samples from study participants, which were analyzed for cytokine levels linked to inflammation as well as the levels of several types of white blood cells, including monocytes, eosinophils, natural killer cells, and B cells.

Using the blood test data, scientists then looked for associations between what was found in the blood to environmental heat levels, including the Universal Thermal Climate Index (UTCI) that day.

The UTCI factors in temperature, humidity, and ultraviolet radiation levels.

At the study’s conclusion, researchers found that for every five-degree increase in UTCI, there was also an increase in levels of key markers of inflammation in participants’ blood samples.

“Previous research has established a relationship between higher temperatures and an increase in inflammatory markers,” Dr. Riggs explained. “However, previous studies have primarily focused on ambient temperature and a limited number of inflammatory markers.”

“The objective of our study was to assess heat exposure using more physiologically relevant markers of heat, such as the UTCI and a larger panel of inflammatory and immune markers, with the goal of getting a more complete and accurate picture of the relationship between heat and inflammation.”

— Dr. Daniel W. Riggs, lead study author

“Participants in our study were only exposed to moderate levels of heat, and we were surprised to find that these moderate levels were related to a variety of markers reflective of changes to inflammation, and innate and adaptive immune responses,” Dr. Riggs noted.

This study also found that study participants experienced a decrease in B cells, which researchers say also indicates a lowering of the capabilities of the body’s immune system.

“Temperature and humidity are known to be important environmental drivers of infectious, airborne disease transmission,” Dr. Riggs said.

“This could suggest that not only are people at higher risk of exposure to infectious disease during high temperatures, but they could also be more vulnerable to disease or inflammation.”

“Dysregulation of the immune system and inflammatory pathways are known to be a leading mechanism in many types of cardiovascular disease,” he continued.

“Our findings suggest that heat exposure could be contributing to these pathways that ultimately lead to greater risk of cardiovascular disease.”

After reviewing this study, Dr. Justin Lee, an interventional cardiologist at Hackensack University Medical Center in New Jersey told MNT he felt the authors propose an interesting hypothesis that may warrant further research with better randomization and statistical analysis.

“Interesting hypothesis; however, there are much stronger cardiovascular disease risk factors — i.e., cigarette smoking, obesity, hypertension, diabetes, high cholesterol, family history — that have proven evidence with robust cause and effect relationship,” Dr. Lee explained.

“It does not appear that the authors in the study conduct thorough sample analysis and propensity matching to ensure true randomization of the subjects. Hence, the results are not immune to bias and confounders.”

MNT also spoke with Dr. Cheng-Han Chen, a board-certified interventional cardiologist and medical director of the Structural Heart Program at MemorialCare Saddleback Medical Center in Laguna Hills, CA, about this study.

“We have known for a long time that heat stress has negative effects on someone’s health, including cardiovascular health,” Dr. Chen said.

“We’ve also known that inflammation in the body also has effects on cardiovascular health. This study is useful because it directly ties changes in inflammatory markers in the bloodstream of patients in response to these short-term heat stress situations. So it makes sense that that would be so, but it’s good to have the data showing this.”

— Dr. Cheng-Han Chen, cardiologist

To protect yourself from high heat exposure, Dr. Chen recommended the following:

  • try to stay indoors and in an air-conditioned environment as much as possible
  • stay away from direct sunlight
  • drink plenty of water
  • wear loose-fitting clothing