Background Race starting time is often staggered over several hours in large mass-participation endurance sports events. Athlete exposure to environmental conditions changes as wet-bulb-globe-temperature (WBGT) changes during race day. Slower participants may be exposed to changing environmental conditions for a longer duration. In most studies environmental conditions are reported using an average WBGT for the race day.
Objective To calculate individual average WBGT (iWBGTavg) for each race participant and compare (iWBGTavg) to the average WBGT on race day (WBGTavg).
Design Retrospective, cross-sectional study.
Setting Cape Town Cycle Tour (109 km), South-Africa, 2012–2014.
Participants Race starters (n=97946).
Assessment of Risk factors WBGTavg for each year was calculated using the data over 11hours (race start at 6am; cut-off at 5pm) from the weather station at the geographical mid-point of the race. iWBGTavg for each race starter was calculated using individual start times, finishing times and data from automated weather stations on route. Factors that possibly affect the variation in measurement of WBGT are individual staggered start category (start time) (<07h00, 07h01–08h00, 08h01–09h00, >09h00) and total race exposure category (hours) (<3hr45min; 3hr45min-4hr30min; 4hr31min-5hr30min; >5hr30min).
Outcome measurements Difference between the two WBGT variables (WBGTdiff=iWBGTavg-WBGTavg) for start time and total race exposure (hours) categories (regression modelling).
Results There was a significant greater WBGTdiff (p<0.0001) in later start time categories (WBGT diff > in categories 07h01–08h00=0.38, 08h01–09h00=0.86, and >09h00=1.17 vs. reference <07h00). Similarly, there was a significant greater WBGTdiff (p<0.0001) in the longer race exposure categories (WBGT diff > in categories 3hr45min-4hr30min=0.24; 4hr31min-5hr30min=0.51; >5hr30min=0.56 vs. reference <3hr45min).
Conclusions Average race day WBGT underestimated environmental risk exposure in an endurance sports event characterized by staggered race start times and wide variations in individual total race times. We suggest that individualized environmental risk exposure be used in future studies relating risk of medical encounters to environmental exposure.
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