The geography of extreme heat hazards for American football players

• We examine the best football practice times to minimize extreme heat exposure.
• We develop a unique wet bulb globe temperature climatology for the contiguous U.S.
• Early morning and late evening practices are best for minimizing heat exposure.
• Delaying afternoon practices may reduce extreme heat hazards by over 90%.

American Football players are among the most susceptible athletes to heat-related illnesses. Environmental conditions are an important factor when considering risk rates for these illnesses. Thus, we examine the spatio-temporal variations in the wet bulb globe temperature (WBGT), a commonly used metric for heat exposure, and quantify the hazard for extreme heat using safety thresholds specifically derived for athletes from the American College of Sports Medicine (ACSM). The objective is to provide better information on heat-related hazards to help mitigate the risk of exertional heat illnesses (EHI) among football players. We created a unique 15-year climatology (1991–2005) of August WBGTs for 217 locations across the contiguous United States using weather station observations and a WBGT model. Thirteen 3-h overlapping training session times ranging from 6–9 a.m. to 6–9 p.m. were examined to identify how the WBGT varies with the time of day the practice session was held and how frequently the WBGT during those sessions posed a hazard for extreme heat by exceeding two ACSM safety thresholds (30.1 °C and 32.3 °C). Maximum hazards for extreme heat are located in an arc across the Southern tier of the country, stretching from eastern Texas through to South Carolina as well as across southern Arizona and southeastern California. Climatologically, practice sessions early in the morning and later in the evening were best for minimizing heat exposure while those held from late morning through afternoon, particularly the noon-3 p.m. and 1–4 p.m. periods, had the highest WBGT values and were the practice periods that most frequently exceeded safety thresholds. Delaying the start of afternoon practices a few hours, however, may substantially reduce the likelihood of oppressive conditions and reduce the risk for heat illnesses.