And on a brutally hot summer afternoon, sometimes the most important thing was knowing the difference.… By two o’clock in the afternoon, the asphalt outside the stadium looked liquid. Mika could feel heat radiating through the soles of her shoes as she crossed the service entrance. The summer festival had brought thousands of people into the city, and the temporary food stalls were working without a break. Even under the tent, the air barely moved. Her supervisor, Sato, handed her a bottle of water. “Drink.” “I’ve been drinking all morning.” “Then drink some more.” Mika laughed. “I thought too much water was dangerous.” Sato shook his head. “That depends on the circumstances. Heat illness isn’t simply about not drinking enough water. You lose water and electrolytes through sweat, and what you need depends on how much you’re sweating, how hard you’re working, and how long you’re exposed to the heat.” That summer, Japanese...