Exposure to High Heat May Impair Immune System Response, Study Finds

Short-term exposure to high heat may negatively affect immune system function and increase inflammation, according to new research presented at the American Heart Association’s Epidemiology and Prevention—Lifestyle and Cardiometabolic Scientific Sessions. Researchers say the preliminary evidence indicates that high heat may heighten susceptibility to infections and accelerate the progression of cardiovascular disease.

Inflammation serves as a natural component of the body's protective mechanisms against injury or infection. Nonetheless, the study's researchers explained that prolonged or unwarranted inflammatory reactions, persisting for weeks to months or arising in healthy tissues, can be detrimental and contribute significantly to arterial plaque accumulation, possibly leading to atherosclerosis.

While heat waves are presumed to exacerbate inflammation, investigations into the correlation between air temperature and inflammation biomarkers have yielded inconclusive findings. For this study, researchers aimed to understand better the relationship between heat and inflammation and how it manifests throughout the body.

"Most research only considers temperature as the exposure of interest, which may not be adequate to capture a person's response to heat," said lead study author Daniel W. Riggs, PhD, an assistant professor of medicine in the Christina Lee Brown Envirome Institute at the University of Louisville in Louisville, Kentucky. "In our study, we used alternative measurements of heat in relation to multiple markers of inflammation and immune response in the body to investigate the short-term effects of heat exposure and produce a more complete picture of its health impact."

Participants traveled to study sites in Louisville during the summer to undergo blood tests, where researchers assessed various markers of immune system function. Researchers then analyzed the relationships between the immune system markers and heat levels, such as temperature, net effective temperature (which integrates relative humidity, air temperature, and windspeed), and the Universal Thermal Climate Index (UTCI) for the respective day. UTCI, the authors explained, incorporates factors like temperature, humidity, wind speed, and ultraviolet radiation levels to gauge participants' physical comfort.

The study’s results showed that key markers of inflammation in the blood increased significantly for every five-degree increase in UTCI—equivalent to transitioning from a day with no thermal stress to one with moderate thermal stress. According to researchers, these markers denote activation of the body's innate immune system, prompting a rapid and non-specific inflammatory reaction to combat pathogens and injuries.

Conversely, a decrease in B-cells, indicative of the body's adaptive immune system, was observed. The study showed that heat's impact on the immune system was less pronounced when assessed by average 24-hour temperature or net effective temperature, which incorporates humidity and wind but not sunshine.

"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," Riggs said. “Thus, during the hottest days of summer, people may be at higher risk of heat exposure; they may also be more vulnerable to disease or inflammation."

Dr. Riggs highlighted that individuals over 60 and those with pre-existing cardiovascular conditions are more susceptible to heat-related cardiovascular incidents and fatalities. To mitigate risk during heat waves, it's advised to limit outdoor exposure during peak temperatures and intense sunlight, seek shade when outdoors, wear lightweight and breathable attire, and ensure adequate hydration by consuming ample water.

"It's important for physicians to communicate with patients about the risk of adverse health effects from heat exposure. For example, cardiologists could conduct customized consultations and assessments to increase patient awareness about their susceptibility to the effects of high temperatures,” Riggs added. “Also, changes to treatment regimens may be important to consider to address other risks. For example, some medications could make people more susceptible to heat-related illness, or some may not be as effective when the body is exposed to high temperatures.”