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Exertional heat illness can affect athletes during high-intensity or long-duration exercise and result in withdrawal from activity or collapse during or soon after activity. These maladies include exercise associated muscle cramping, heat exhaustion, or exertional heatstroke. While certain individuals are more prone to collapse from exhaustion in the heat (i.e., not acclimatized, using certain medications, dehydrated, or recently ill), exertional heatstroke (EHS) can affect seemingly healthy athletes even when the environment is relatively cool. EHS is defined as a rectal temperature greater than 40°C accompanied by symptoms or signs of organ system failure, most frequently central nervous system dysfunction. Early recognition and rapid cooling can reduce both the morbidity and mortality associated with EHS. The clinical changes associated with EHS can be subtle and easy to miss if coaches, medical personnel, and athletes do not maintain a high level of awareness and monitor at-risk athletes closely. Fatigue and exhaustion during exercise occur more rapidly as heat stress increases and are the most common causes of withdrawal from activity in hot conditions. When athletes collapse from exhaustion in hot conditions, the term heat exhaustion is often applied. In some cases, rectal temperature is the only discernable difference between severe heat exhaustion and EHS in on-site evaluations. Heat exhaustion will generally resolve with symptomatic care and oral fluid support. Exercise associated muscle cramping can occur with exhaustive work in any temperature range, but appears to be more prevalent in hot and humid conditions. Muscle cramping usually responds to rest and replacement of fluid and salt (sodium). Prevention strategies are essential to reducing the incidence of EHS, heat exhaustion, and exercise associated muscle cramping.This document replaces, in part, the 1996 Position Stand titled "Heat and Cold Illnesses during Distance Running" (9) and considers selected heat related medical conditions (EHS, heat exhaustion, and exercise associated muscle cramping) that may affect active people in warm or hot environments. These recommendations are intended to reduce the morbidity and mortality of exertional heat-related illness during physical activity, but individual physiologic responses to exercise and daily health status are variable, so compliance with these recommendations will not guarantee protection.Heat illness occurs world wide with prolonged intense activity in almost every venue (e.g., cycling, running races, American football, soccer). EHS (1,27,62,64,65,109,132,154,160,164) and heat exhaustion (54,71,149,150) occur most frequently in hot-humid conditions, but can occur in cool conditions, during intense or prolonged exercise (133). Heat exhaustion and exercise related muscle cramps do not typically involve excessive hyperthermia, but rather are a result of fatigue, body water and/or electrolyte depletion, and/or central regulatory changes that fail in the face of exhaustion.This document will address recognition, treatment, and incidence reduction for heat exhaustion, EHS, and exercise associated muscle cramping, but does not include anesthesia-induced malignant hyperthermia, sunburn, anhidrotic heat exhaustion, or sweat gland disorders that are classified in other disease categories, because these disorders may or may not involve exercise or be solely related to heat exposure. Hyponatremia also occurs more frequently during prolonged activity in hot conditions, but is usually associated with excessive fluid intake and is addressed in the ACSM Exercise and Fluid Replacement Position Stand.Evidence statements in this document are based on the strength of scientific evidence with regard to clinical outcomes.