• Robert F Wallace, David Kriebel, Laura Punnett, David H Wegman, C Bruce Wenger, John W Gardner, and Richard R Gonzalez.
• Biophysics and Biomedical Modeling Division, U.S. Army Research Institute of Environmental Medicine, 42 Kansas Street, Natick, MA 01760-5007, USA. robert.wallace@na.amedd.army.mil
• Med Sci Sports Exerc. 2005 Jan 1; 37 (1): 84-90.

PurposeTo determine whether cumulative daily average wet-bulb globe temperature (WBGT) index, over one or two preceding days, is a better measure for predicting cases of exertional heat illness (EHI) than current daily average WBGT, which is the standard heat index used by the Marine Corps; and to identify the most accurate index of heat stress to prevent and predict future cases of EHI.MethodsA case-crossover study was conducted in male and female Marine Corps recruits in basic training at Marine Corps Recruit Depot, Parris Island, SC. Weather measurements were obtained for 2069 cases of EHI during 1979-1997 and for randomly selected control periods before and after each EHI episode.ResultsThe risk of EHI increased with WBGT (OR = 1.11 degrees F(-1); 95% CI, 1.10-1.13). EHI risk was associated not only with the WBGT at the time of the event (OR = 1.10 degrees F(-1); 95% CI, 1.08-1.11) but with the previous day's average WBGT as well (OR = 1.03 degrees F(-1); 95% CI, 1.02-1.05). Alternative combinations of WBGT components were identified that better predicted EHI risk.ConclusionOur results provide evidence for a cumulative effect of previous day's heat exposure on EHI risk in these Marine Corps recruits. A simple index for use in predicting EHI risk is proposed that includes the dry-bulb temperature and the relative humidity.

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