Sweat evaporation in humans: A molecular and thermodynamic perspective

Evaporative heat loss through sweating is essential for maintaining thermal balance in humans, particularly during exercise or in hot environments. Although the physiological mechanisms regulating sweat production and skin blood flow are well documented, the molecular processes underpinning sweat evaporation are less often considered. This review explores the physics of sweat evaporation from first principles, examining how energy is transferred, how water molecules escape the liquid phase and how this process is shaped by local and systemic factors. At the molecular level, evaporation occurs when surface water molecules attain sufficient kinetic energy to overcome hydrogen bonding. The energy required for this phase change, the latent heat of vaporisation, is supplied via conduction from the skin and, ultimately, from core body heat. The molecular energy within the sweat layer follows a Boltzmann distribution, meaning that only a subset of molecules have sufficient energy to evaporate at any time. As these high-energy molecules escape, the remaining sweat cools, helping to lower body temperature. This process continues as long as heat is resupplied via skin blood flow. Environmental conditions, such as humidity, airflow and clothing, affect the likelihood that evaporated molecules will remain in the vapour phase, while electrolytes in sweat can slightly reduce vapour pressure by locally altering the bonding structure of water. These factors determine how effectively sweat can evaporate by influencing surface area and liquid retention. By linking classical thermodynamics to human physiology, this review presents a unified framework for understanding how molecular interactions, statistical physics and environmental conditions converge to influence heat loss.