Timing and predictors of mild and severe heat illness among new military enlistees

Purpose Heat illnesses are important and potentially fatal conditions among physically active individuals. We determined predictors of heat illness among enlistees in a large military population experiencing common physical activity patterns. Methods We estimated the adjusted odds of mild (MHI) and severe (SHI) heat illness associated with demographic, health-related, and geographic factors among active-duty, US Army soldiers enlisting between January 2011 and December 2014 (N = 238,168) using discrete-time multivariable logistic regression analyses. Results We observed 2612 incident cases of MHI and 732 incident cases of SHI during 427,922 person-years of follow-up, with a mean and median of 21.6 and 21 months per subject. During the first 6 duty months, 71.3% of the MHIs and 60.2% of the SHIs occurred, peaking at month 2. The odds of MHI quadrupled among those with prior SHI (odds ratio [OR], 4.02; 95% confidence interval [CI], 2.67-6.03). Body mass index (BMI) extremes increased the odds substantially (OR at BMI ≥30 kg·m^-2: For MHI, 1.41 (CI, 1.19-1.67); for SHI, 1.94 (CI, 1.47-2.56); OR at BMI <18.5 kg·m^-2: For MHI, 1.50 (CI, 1.01-2.21); for SHI, 2.26 (CI, 1.16-4.39)). Tobacco use was associated with a 55% increase (CI, 1.37-1.77) in MHI odds. The odds of MHI increased if taking nonsteroidal anti-inflammatory drugs, opioids, or methylphenidate stimulants. Lower age and lower entry aptitude scores were associated with progressively increased MHI odds. Conclusions Most heat illnesses occurred at the outset of service, indicating the need for focused prevention methods at the initiation of military duty. Prior heat illness, BMI extremes, medications, and tobacco use represent potentially actionable risk factors to address by education, policy, and/or clinician intervention.