Heat, Sweat and Salt: Two Runners After the 800 m
At a school athletics meet in August the shade temperature is 33 °C and the relative humidity is 80 %. Devan, aged 15 and 62.0 kg, runs the 800 m at 1:15 p.m. He drinks about 200 mL of water in the hour before the race. The school nurse weighs him before and after: 62.0 kg before, 60.8 kg after. He complains of a throbbing headache, dizziness and cramps in both calves, and his urine is dark yellow. His skin is cool and clammy, he is sweating heavily and he answers questions sensibly. His coach says, 'Drink two litres of plain water quickly and you'll be fine.' Another parent offers salt tablets. Ninety minutes earlier, a runner in the same race collapsed; she was confused, her skin was hot and dry, and she had stopped sweating.
1. Estimate how much fluid Devan lost during the race and explain why 80 % humidity makes the loss worse. Then say what and how much he should drink, and how quickly.
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Fluid loss: 62.0 − 60.8 = 1.2 kg. Sweat is mostly water with a density close to 1 kg per litre, so he lost roughly 1.2 L - about (1.2 ÷ 62) × 100 = 1.9 % of body mass. A 2 % loss is already enough to cause headache, dizziness and dark, concentrated urine.
Why humidity matters: sweating only cools the body when the sweat evaporates from the skin. In 80 % humidity the air is nearly saturated, so much of the sweat drips off instead. Devan must sweat more - and lose more water and salt - to get the same cooling. Combined with 33 °C and a 1:15 p.m. race, his loss per minute is high.
What to drink: about 1.25–1.5 L over the next 1–2 hours (his 1.2 L deficit plus ongoing losses), in small amounts every 10–15 minutes rather than all at once, as a drink containing roughly 0.5–0.7 g of sodium per litre and 4–8 % carbohydrate. A normal meal or salty snack plus plain water is also acceptable once the immediate deficit is partly replaced. A child of 62 kg should not be pushed to drink 2 L in a few minutes.
2. Explain why drinking two litres of plain water quickly could be risky, and why a drink containing some sodium and carbohydrate is better after heavy sweating.
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Sweat is not pure water: it contains sodium, chloride and some potassium, typically around 0.5–1.5 g of sodium per litre. Devan has lost both water and salt.
If he replaces a large sweat loss with a large volume of plain water in a short time, the water dilutes the sodium already in his blood. The result is dilutional hyponatraemia. The kidneys can only excrete about 0.8–1 L of excess water per hour, so a rapid 2 L load can outrun them. Early symptoms - headache, nausea, bloating, dizziness - look almost exactly like dehydration, which makes the mistake easy to repeat. In rare cases it causes confusion, seizures and swelling of the brain.
A drink with sodium and a little carbohydrate replaces both losses. Glucose also speeds water absorption in the small intestine, because sodium and glucose are absorbed together, so the fluid gets into the body faster. The carbohydrate provides some fuel.
Salt tablets on their own are a poor idea: concentrated salt in an under-filled stomach draws water into the gut, causing nausea and vomiting, which makes the dehydration worse. Salt belongs in a drink, or in food.
3. Distinguish Devan's condition from that of the collapsed runner, and state the correct action for each.
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Devan - heat exhaustion with dehydration. The deciding features are that he is sweating heavily, his skin is cool and clammy, and he is talking sensibly. Heat exhaustion is a warning, not yet an emergency. Action: move him to shade or an air-conditioned room, loosen clothing, fan him and apply cool wet cloths, let him sit and then lie with legs slightly raised, give oral rehydration fluid in small sips, and check him every 10–15 minutes. If he becomes confused, vomits repeatedly or stops sweating, treat it as heat stroke and call an ambulance.
The collapsed runner - heat stroke until proven otherwise. She has the two red flags: altered mental state (confusion) and hot, dry skin with sweating stopped. When sweating stops, the body has lost its main cooling mechanism and core temperature is rising fast; this damages organs and can be fatal. Action: this is a medical emergency. Call an ambulance immediately, start cooling at once - cold water poured over her, ice packs to the neck, armpits and groin, fanning - and do not give her drinks because she is confused and may choke. Do not wait for her to 'come round'.
The rule to remember: dehydration and heat exhaustion are managed by cooling and drinking; confusion plus hot dry skin is a 999/911 call.