Exertional heat illness in Marine Corps recruit training

Background: Exertional heat illness (EHI) is a recurring problem in military training resulting in loss of manpower and training effectiveness. A scientific approach to this problem requires quantitative analysis of factors related to its occurrence. Methods: Review of clinic and hospital records identified all cases of EHI occurring among 217,000 Marine Corps recruits (90% male, 80% age 18-20 yr) entering 12-week basic training at Parris Island, SC during 1982-1991. Results: There were 1454 cases, 89% males and 11% females; 11% of male and no female cases were hospitalized. The majority (88%) of cases occurred during May-September, with a peak rate of 2% of recruits in summer. Most of the cases occurred during the cooler early morning hours when recruits performed strenuous exercise. During 0700-0900 hours the rate of heat casualties increased substantially as the Wet Bulb Globe Temperature Index (WBGT) increased, beginning at levels as low as 65 degrees F. EHI rates increased 26- and 39-fold at WBGT 75-<80 degrees F (over baseline rates at <65 degrees F) for immediate and prior day exposures, respectively. Exposure to WBGT of 80+degrees F was infrequent (25%) among the early morning cases at the time of illness, while it was common (87%) on the day prior to illness, suggesting a lasting effect of heat exposure. Conclusions: Risk for EHI in military training increases markedly as WBGT levels rise above 65 degrees F; cases occur primarily with strenuous exercise (e.g., running); and heat stress exposure on prior days may be important.