A Wet Sheet over the Window: How Much a Room Can Be Cooled by Evaporation, and When It Makes a Fever Worse

This simulated study calculates what a wetted sheet hung at an open window does to a small room and to a fevered patient inside it. The room measures 3 m by 4 m by 2.6 m, with 5 air changes per hour passing through a sheet of 2 m². At an outdoor temperature of 35 °C and 30 percent relative humidity, the wet-bulb temperature is 22.1 °C and the room settles at 27.7 °C, a depression of 7.3 °C, while the room humidity rises by 36.5 percentage points and the sheet consumes 0.9 litres of water per hour. The patient's net heat loss improves by 35.0 W in that case despite weaker evaporation from the patient's own skin, because the cooler air restores dry heat loss. At 35 °C and 70 percent humidity the room cools by only 0.8 °C and the patient's net heat loss worsens by 14.7 W. Scanning humidity at 35 °C places the crossing point at 56 percent outdoor relative humidity: above it, the sheet raises the patient's heat stress instead of lowering it. The study is simulated, not measured.