Army Medical Researchers Hunt for Indicators of Heat Stress Risk Published Aug. 28, 2025 By Paul Lagasse, Army Medical Research and Development Command Heat-related injuries are a persistent threat to warfighters, especially during ruck marches and timed runs. Physiologists have long sought to understand why some people are able to withstand the effects of heat for longer periods, while others of the same age and physical condition are prone to experiencing potentially dangerous symptoms. A team of researchers at the U.S. Army Research Institute of Environmental Medicine is seeking to solve that mystery by identifying specific physiological characteristics of people who can withstand extreme heat stress during training. By helping to identify risk factors and prevention strategies for heat-related illnesses, the research could lead to improved recovery and return-to-duty protocols that better protect the health of warfighters, thereby improving their readiness, endurance and lethality. Exertional heatstroke — a severe form of heat illness typically marked by high body temperature and altered mental status such as disorientation and loss of consciousness — affects nearly 500 service members per year. The Medical Surveillance Monthly Report recently found that the overall incidence rate of EHS among military personnel increased in 2024 after three years of steady decline. Gabrielle Giersch, a research physiologist in USARIEM's Thermal and Mountain Medicine Division, is leading the study and said that the research into identifying risk factors and biomarkers associated with resilience to heat stress will help fill key gaps in the Defense Department's understanding of heat-related illness. "Heat illnesses pose a serious threat not just to individuals, but also to units and deployability, and they impose a significant financial cost as well," Giersch said. "We don't have a lot of data on the factors that contribute to the onset of heat illness or optimal recovery time. This study is designed to help us develop better criteria for determining the return-to-duty requirements for heat illness [sufferers] by identifying what puts people at risk and how those risk factors affect them." Earlier this summer, Giersch and her team traveled to Fort Rucker, Alabama, where they recruited 24 volunteer soldiers assigned to the 1st Battalion, 145th Aviation Regiment, to serve as the study's control group. Prior to participating in a 5-mile run and an 8-mile ruck march as part of their regular physical training, the volunteers provided blood and urine samples and were fitted with instrumentation to monitor their heart rate, skin temperature and core temperature. They also completed a brief questionnaire on their health history. In follow-up visits, the research team collected blood and urine samples from the volunteers at six-hour intervals for an additional 24 hours — the same frequency that care providers in emergency departments and hospitals take samples from actual heat illness patients. The samples will be analyzed to identify the prevalence of biological molecules that can be correlated with a volunteer's ability to better withstand the onset of EHS. "The 24 individuals who participated in the initial data collection represented a pretty wide spectrum of race, sex, fitness status and body mass index," Giersch said. "We haven't analyzed the blood samples yet, but the core temperature data showed that several people reached high temperatures without experiencing any symptoms of EHS and cooled off very quickly, which gives us a very good indication that we have a really good control group to identify what individuals who don't become heat illness casualties look like relative to those who do." Giersch said the team hopes to enlist special operations forces as volunteers to participate in similar data collection events throughout the summer. The team is specifically seeking data from the elite warfighter population because they tend to experience a higher rate of heat-related illnesses thanks to the intensity of their training. The team aims to publish their findings next year. The outcomes of this study will also be used to inform future updates to the Army's medical bulletin, "Heat Stress Control and Heat Casualty Management," which provides guidance for developing and implementing programs for preventing, diagnosing and treating heat-related injuries — crucial for ensuring a swift return to duty and improving unit readiness rates. In addition to informing policy decisions and pushing the boundaries of our understanding of heat-related injury, Giersch said the study has also been of intense personal interest to the soldiers who volunteered to participate in it. "There is so much intrinsic value for the volunteers in this study," she said. "People were constantly coming up to me and asking what their core temperature was, when we'd have their blood data and similar questions. They want to see the papers when they get published and learn how our study impacts policy. They really want to make life better for their future battle buddies, whether they know them or not. That makes it all worthwhile."