At what water temperature will an unprotected swimmer lose heat faster than their body can create it?

• A. 80°F (27°C)
• B. 70°F (21°C)
• C. 60°F (16°C)
• D. 90°F (32°C)

Answer: (B)

Heat balance in the body is a tug-of-war between the heat the body can generate and the heat it loses to the environment. Water conducts heat away from the body far more efficiently than air, so immersion increases heat loss dramatically. For an unprotected swimmer, there’s a tipping point where heat loss to the water outpaces the body's ability to produce heat through metabolism and activity. That tipping point is about 70°F (21°C). At this temperature, the rate of heat loss begins to exceed the body’s heat production, so over time the swimmer cools faster than they can warm themselves. Warmer water (80°F or 90°F) reduces heat loss and tends to preserve body heat, while cooler water (60°F) speeds cooling even more. Individual factors like body fat, fitness, and how much the person is moving matter, and wearing a wetsuit dramatically changes this balance, but for an unprotected swimmer, around 70°F is the general threshold.