Staying Safe When the Heat Is On: Understanding Heat-Related Illnesses

Dr. Francis O'Connor, a professor at the Uniformed Services University, provides essential insights into heat-related illnesses, emphasizing prevention and prompt treatment.

According to Dr. Francis O'Connor, extreme heat is the deadliest form of weather in the United States,

claiming more lives than hurricanes, floods, or tornadoes. (Photo by Sharilyn Wells/Paraglide, Fort Bragg, NC)

July 2, 2025 by Sharon Holland

As summer temperatures continue to soar and heat waves become more common, it’s more important than ever to understand how heat can threaten health. In a recent clinical review published June 26, 2025, in the Annals of Internal Medicine, Dr. Francis O’Connor, professor of  Military and Emergency Medicine and medical director for the Consortium for Health and Military Performance (CHAMP) at the Uniformed Services University (USU), offers a clear-eyed look at heat-related illnesses—their causes, signs, and crucial prevention strategies.

According to O’Connor’s review, extreme heat is the deadliest form of weather in the United States, claiming more lives than hurricanes, floods, or tornadoes. The risks are rising as climate change drives longer, more intense periods of high heat.

Our bodies rely on sweating and increased blood flow to the skin to cool down. But when heat and humidity are too high, these mechanisms can fail. Dehydration, physical exertion, and prolonged exposure all increase the risk.

O’Connor describes heat-related illness along a spectrum:

• Mild conditions like heat cramps, rash, or fainting signal early stress on the body
• Heat exhaustion is more serious, with symptoms like heavy sweating, weakness, dizziness, and nausea
• The most severe form is heat stroke, defined by a core temperature over 104 °F with confusion, disorientation, or even coma. Heat stroke is life-threatening and requires immediate action.

O’Connor also explains the difference between classic and exertional heat stroke. Classic heat stroke, he says, often affects older adults, young children, or people with chronic conditions who can’t escape extreme heat indoors. It develops over hours or days of exposure without adequate cooling or hydration. Exertional heat stroke, on the other hand, can strike healthy, active people—like athletes, military personnel, or laborers—who push themselves hard in high heat and humidity. It develops rapidly, often within an hour of intense activity.

Soldiers demonstrate how to use ice sheets to treat a heat casualty during an exercise hosted by the Maneuver

Center of Excellence, Fort Moore (formerly Fort Benning), GA. (Photo credit: Markeith Horace)

As O’Connor notes in his article, “Recognizing these distinct pathways is crucial for prompt diagnosis and effective treatment in both civilian and military settings.”

One of O’Connor’s key messages is that the danger isn’t just about how high the temperature gets but how long someone remains overheated. Prolonged high body temperature damages organs and can be fatal if not treated quickly.

The good news is that most heat-related illnesses can be prevented. O’Connor emphasizes staying hydrated, wearing light clothing, avoiding strenuous activity during peak heat, taking frequent breaks in shade or air conditioning, and checking on vulnerable neighbors.

If someone shows signs of serious heat illness, rapid cooling is critical. His review underlines the mantra “cool first, transport second”—meaning emergency responders should prioritize lowering body temperature immediately, even before heading to the hospital. Methods include cold-water immersion, ice packs, and wetting clothing combined with fans.

O’Connor also highlights the importance of community planning. Early warning systems, public education, and access to cooling centers can help protect those most at risk. As extreme heat events grow more frequent, these strategies will become even more essential.

“Heat, despite efforts at prevention, remains a significant threat to both civilian and military populations,” O’Connor says. “At USU CHAMP we are very proud to be contributing to translational research efforts to optimize prevention and performance strategies in the heat, and importantly, enhance the readiness of service members. While heat illness will continue to be a concern that confronts laborers, athletes and tactical athletes who need to perform in the heat, no one in 2025 should die from an exertional heat stroke.”