Heat exhaustion vs. heat stroke: the signs runners must not ignore
Most hot-weather running advice is about going faster. This one is about not ending up in an ambulance. Every runner who trains through summer should be able to tell heat exhaustion from heat stroke, because the response — and the stakes — are completely different.
The line between them is the brain. Heat exhaustion is miserable but your head stays clear: heavy sweating, dizziness, nausea, a pounding heart, weakness. Exertional heat stroke is defined by central-nervous-system dysfunction. The hallmark signs are mental: irritability, confusion, apathy, belligerence, irrational behavior, staggering or loss of coordination — and, untreated, progression to delirium, seizures, or coma. If a running partner stops making sense, that is the emergency, not the sweat.
The numbers track the danger. Early heat-stroke features appear once core temperature climbs past about 104–105°F (40.0–40.6°C). Above roughly 106–107°F (41.1–41.7°C), serious neurological deterioration sets in, and collapse can occur at 108°F (42.2°C) or higher. You can't read your own core temperature mid-run, which is exactly why the behavioral signs matter so much — they're your early-warning system.
The response is where lives are saved, and it's counterintuitive: cool first, transport second. Aggressive on-site cooling should begin before the person is sent to a hospital, because how fast you cool them determines whether they survive. The fastest method is cold- or ice-water immersion, which drops core temperature roughly twice as fast as standing in the air under wet towels. Immersion can bring someone from a dangerous 108–110°F (42.2–43.3°C) down to a safe 102°F (38.9°C) in about 15–30 minutes — and you stop cooling once they hit that 102°F mark to avoid overshooting. The payoff for getting this right is stark: with rapid field cooling, survival approaches 100%.
You'll likely never need this. Know it anyway. On the hottest days, recognizing the signs and starting to cool immediately is the most important running skill there is.
RunNerd's job on hot days is to keep you on the safe side of this line. When dew point and temperature stack into dangerous territory, it pulls back prescribed paces, builds in cooling, and tells you plainly to run by effort. But the recognition piece is yours to own: confusion, irritability, or staggering during a hot run is never "push through" territory — it's a stop-and-cool emergency. This article exists so you know the signs and the cool-first response before you ever need them.