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Heat Exposure May Lead to Inflammation and Weaken Immunity, Preliminary Research Finds

• Apr 16, 2024

It’s official: outdoor temperatures are hotter than ever. The National Oceanic and Atmospheric Administration (NOAA) declared 2023 the warmest year ever recorded. Rising temperatures have serious consequences for people who work outdoors. Heat is the leading cause of weather-related deaths, and people who work outdoors have up to a 35-fold higher chance of heat-related death than the average person.

While some of the health consequences of heat are readily apparent—such as heat stroke, heat exhaustion and heat rash—researchers are evaluating the less visible health consequences of heat. New preliminary research presented at the American Heart Association’s (AHA) EPI Lifestyle Scientific Sessions 2024 revealed immune system changes in people after short-term heat exposure. The immune system is a network of organs, tissues, cells and molecules that works to protect the body from infections and disease.

Researchers evaluated immune changes by taking a blood sample from more than 600 people in Kentucky during the summer months. The average 24-hour temperature on the days when researchers collected participant blood samples was 76 degrees F (24.5 degrees C). Researchers used the Universal Thermal Climate Index (UTCI) to assess environmental conditions. UTCI is a heat metric that considers environment humidity, temperature, ultraviolet radiation and windspeed, and a five-degree increase in UTCI is analogous to moving from an environment with no heat stress to an environment with moderate heat stress.

For every five-degree increase in UTCI, researchers found increased levels of inflammation markers in participants’ blood. Specifically, each five-degree increase in UTCI was associated with a 4.2 percent increase in monocytes, a 9.5 percent increase in eosinophils, a 9.9 percent increase in natural killer T-cells and a 7.0 percent increase in tumor necrosis factor-alpha. These inflammatory components are part of the innate immune system, which reacts quickly and nonspecifically. Inflammation is a typical part of a functioning immune system, but chronic inflammation can lead to an array of disorders, from atherosclerosis to diabetes and cancer. 

Researchers also reported that every five-degree increase in UTCI was associated with a 6.8 percent decrease in B-cells, the immune cells that produce antibodies. B-cells are part of the adaptive immune system, which remembers viruses and germs and reacts specifically.

The findings provided “evidence that short-term heat exposure could trigger an inflammatory response and impair the immune system,” Daniel Riggs, the study’s lead investigator and assistant professor at the University of Louisville, explained in an email. 

Riggs emphasized that this study provides information on immune system changes in response to short-term and minor exposure to heat. “Our study participants only had minor exposure to high temperatures on the day of their blood test. However, even minor exposure may contribute to changes in immune markers,” he said in an AHA statement.

The study had limitations. Participants’ blood samples were drawn only one time. Researchers also did not know how long participants had been exposed to outdoor heat before their blood draws.

Previous research has connected heat exposure to immune changes. It is known that heat exposure promotes inflammation, but Riggs explained that much of the prior research is focused on extreme heat exposure, rather than minor to moderate exposure.

Riggs explained that educating people exposed to heat—such as those who work outdoors—about its risks is important. He noted that summer’s first heat wave is frequently the most perilous because people haven’t yet acclimated to rising temperatures. He said, “Some practical strategies to minimize the harmful effects of heat exposure include increasing fluid intake, remaining in cool or air-conditioned environments, wearing loose-fitting clothes, and reducing activity levels.” 

Heat illness prevention is one of OSHA’s foremost concerns, an agency spokesperson wrote in an email. “The Biden-Harris Administration has been taking aggressive action from the start to protect workers from extreme heat, including increased enforcement where workers are exposed to heat hazards in high-risk industries like construction and agriculture,” the spokesperson wrote. 

OSHA is taking steps towards a final rule on regulations related to heat illness prevention and is fortifying efforts to spread awareness of heat illness and prevention to workers and employers. “OSHA also encourages employers to reach out to our compliance assistance specialists at their nearest area offices for information on how best to address heat-related hazards at their workplaces,” the spokesperson wrote.