Acute Heat Exposure-Related Illness: A Unified Emergency Medicine Framework for Hot Baths, Hot Springs, and Saunas-A Narrative Review.

Hot springs, hot-water bathing, and saunas are widely practiced forms of acute heat exposure and are often perceived as health-promoting. However, emergency clinicians frequently encounter patients in whom these exposures precipitate syncope, hypotension, drowning/aspiration, heat-related illness, and renal or electrolyte disturbances. This narrative review integrates these modalities within a unified "acute heat exposure" framework and summarizes pathophysiology and clinical implications from an emergency medicine perspective. We searched PubMed/MEDLINE from inception to January 2026 using controlled vocabulary and free-text terms related to heat stress, thermoregulation, hot-water immersion, sauna exposure, and acute clinical outcomes; evidence was synthesized qualitatively. Across modalities, acute heat exposure induces shared physiological responses-peripheral vasodilation, relative hypovolemia, circulatory stress, and internal heat storage-that can trigger diverse emergency presentations. We classify acute heat exposure-related illness into four domains: (1) cardiovascular events, including syncope, hypotension, and arrhythmic/ischemic complications in vulnerable individuals; (2) the heat-illness spectrum from exhaustion to heat stroke with organ dysfunction; (3) renal and electrolyte disturbances related to dehydration and hypoperfusion; and (4) neurological and traumatic complications, including falls, drowning, and aspiration. This framework may support risk stratification, evaluation, management, and prevention after hot spring, hot bath, or sauna use.