How do you tell heat exhaustion from heat stroke?

As the air around you gets hotter, your body stops shedding heat into it and starts taking heat on, leaving evaporating sweat as the only cooling left. That evaporation costs water and salt, and heat exhaustion is what happens when sweating spends more of both than the body can spare. Symptoms vary, and may include weakness, fatigue, thirst, headache, nausea and dizziness, and the person's core temperature may still read normal. Notice what hasn't changed. Heat exhaustion leaves the central nervous system uninjured, so they answer you and they know where they are. Out of the heat, off their feet and drinking fluids with salt in them, mild cases usually settle. Left untreated, this can climb into heat stroke.

In heat stroke the body loses control of its temperature: it climbs fast, and the body can't cool itself down. The classic definition is a core temperature above 104°F together with a nervous system that has stopped working properly. What you see is confusion, slurred speech, a seizure or loss of consciousness. Do not count on a warning phase, though. Heat stroke can arrive suddenly, with no identifiable phase of heat exhaustion before it.

Two familiar rules will steer you wrong here, and the first is dry skin. But sweating does not mean you're safe from heat stroke. Sweating is usually absent in the classic heat stroke that overtakes a sedentary person, but in an active person they might keep sweating even when their body is shutting down. The second is the thermometer. Household thermometers that read at the forehead, ear, mouth or armpit aren't reliable here, and a normal number is no proof it isn't heat stroke. So cool on suspicion alone, no matter what the thermometer says.

For heat exhaustion reduce your activity, rest in the shade for 30 to 45 minutes, and drink water with electrolytes. Soak a cloth or a towel in water and fan the person to help cool down. If they aren't getting better or start to get confused or show other signs of worsening it's time to call for emergency help. How much damage heat stroke leaves behind depends on how high the temperature climbed and how long it stayed there. Cool them aggressively where they are, and get help as quickly as you can.