Perspiration vapor lightens near-skin air, but hinders human evaporative cooling in arid heat.

Sweat evaporation is the body's primary cooling mechanism, yet the physical factors governing it are not fully understood. We identify a "dueling buoyancy" effect in the context of the human body, in which perspiration vapor reduces the near-skin air density, counteracting the downward flow driven by cooling of warm air upon contact with the skin. In hot, arid, stagnant environments, this opposing buoyancy suppresses free convection and can reduce sweat evaporation by more than half. As a result, commonly used thermoregulation models can substantially underpredict body temperature (e.g., by 1°C after 2 hours of exposure to typical Arizona summer conditions). We develop compact, physics-informed models for free-convective heat transfer coefficients across wide temperature and humidity ranges, enabling improved thermoregulation modeling and thermal audits. These results enhance understanding of human heat balance and support more accurate heat-stress assessment to inform behavioral, infrastructural, and policy decisions for extreme-heat adaptations.