In this study, blood work from volunteers was examined for immune biomarkers, and the findings mapped against environmental data.
“With rising global temperatures, the association between heat exposure and a temporarily weakened response from the immune system is a concern because temperature and humidity are known to be important environmental drivers of infectious, airborne disease transmission,” lead author Daniel W. Riggs, PhD, with the Christina Lee Brown Envirome Institute, University of Louisville in Louisville, Kentucky, said in a news release.
“In this study, even exposure to relatively modest increases in temperature were associated with acute changes in immune system functioning indexed by low-grade inflammation known to be linked to cardiovascular disorders, as well as potential secondary effects on the ability to optimally protect against infection,” said Rosalind J. Wright, MD, MPH, who wasn’t involved in the study.
“Further elucidation of the effects of both acute and more prolonged heat exposures (heat waves) on immune signaling will be important given potential broad health implications beyond the heart,” said Dr. Wright, dean of public health and professor and chair, Department of Public Health, Mount Sinai Health System.
The study was presented at the American Heart Association (AHA) Epidemiology and Prevention | Lifestyle and Cardiometabolic Scientific Sessions 2024.
High Temps Hard on Multiple Organs
Extreme-heat events have been shown to increase mortality, and excessive deaths due to heat waves are overwhelmingly cardiovascular in origin. Many prior studies only considered ambient temperature, which fails to capture the actual heat stress experienced by individuals, Dr. Riggs and colleagues wrote.
They designed their study to gauge how short-term heat exposures are related to markers of inflammation and the immune response.
They recruited 624 adults (mean age 49 years, 59% women) from a neighborhood in Louisville during the summer months, when median temperatures over 24 hours were 24.5 °C (76 °F).
They obtained blood samples to measure circulating cytokines and immune cells during clinic visits. Heat metrics, collected on the same day as blood draws, included 24-hour averages of temperature, net effective temperature, and the Universal Thermal Climate Index (UTCI), a metric that incorporates temperature, humidity, wind speed, and ultraviolet radiation, to determine the physiological comfort of the human body under specific weather conditions.
The results were adjusted for multiple factors, including sex, age, race, education, body mass index, smoking status, anti-inflammatory medication use, and daily air pollution (PM 2.5).
In adjusted analyses, for every five-degree increase in UTCI, there was an increase in levels of several inflammatory markers, including monocytes (4.2%), eosinophils (9.5%), natural killer T cells (9.9%), and tumor necrosis factor-alpha (7.0%) and a decrease in infection-fighting B cells (−6.8%).
Study Raises Important Questions
“We’re finding that heat is associated with health effects across a wide range of organ systems and outcomes, but this study helps start to get at the ‘how,’” said Perry E. Sheffield, MD, MPH, with the Departments of Pediatrics and Environmental Medicine and Public Health, Icahn School of Medicine at Mount Sinai in New York City, who wasn’t involved in the study.