Effect of intermittent work in the heat on hydration indices in older Firefighters versus Non-Firefighters

Working in the heat, or when sweat output is high, can lead to dehydration which increases thermal and cardiovascular strain, impairs vigilance and psychomotor skills, and thus increases the risk for accidents and injury. It is unclear however, whether repeated exposure to occupational heat stress (i.e., firefighting) reduces or increases strain in older workers.PURPOSE: To examine the effects of intermittent exercise, at a fixed rate of heat production, in a Warm/Dry environment on the hydration changes in older Firefighters (FF) and Non-Firefighters (Non-FF).METHODS: Twelve FF and 12 Non-FF, matched for age (Mean\ub1SE; 52\ub11 and 50\ub11 yrs, respectively) and VO2max (39.3\ub12.2 and 40.4\ub11.8 mL\ub7kg-1\ub7min-1, respectively), performed four successive 15-min cycling bouts at a constant rate of metabolic heat production (400 W), wearing shorts & sandals, in dry heat (35\ub0C, 20% relative humidity). Exercise bouts were separated by 15 min rest with a 60-min final rest. Rectal temperature (Tre) and heart rate were measured continuously. Blood and urine were analyzed prior to exercise (PRE) and the end of the final recovery (POST) for hemoglobin, hematocrit, and osmolality (OSMO), and urine specific gravity (USG), respectively.RESULTS: The Tre change from PRE to the end of exercise (FF: +0.46\ub10.06, Non-FF: +0.49\ub10.09\ub0C) and POST (FF: +0.27\ub10.05, Non-FF: +0.21\ub10.09\ub0C) was similar between FF and Non-FF. A group 7 time interaction was observed for heart rate. The Non-FF, compared to the FF, had a greater % change in plasma (-8.3\ub10.7 vs. -3.9\ub10.6%) and blood (-4.4\ub10.4 vs. -1.9\ub10.3%) volumes. Similar USG and OSMO PRE-POST changes were observed in the Non-FF (+0.005\ub10.002 and +4.2\ub10.4 mOsm/kg, respectively) and FF (+0.005\ub10.001 and +4.2\ub10.8 mOsm/kg, respectively).CONCLUSION: The greater shift in blood and plasma volumes in the Non-FF suggests that the FF may possibly be better adapted to maintaining hydration status, and thus cardiovascular integrity, during work in dry heat for a given change in thermal strain. Support: Workplace Safety and Insurance Board of Ontario and Canada Foundation for Innovation-Leaders Opportunity Fund (G.P. Kenny), MITACS Accelerate (H.E. Wright).Peer reviewed: YesNRC publication: N