Do Older Firefighters Show Long-Term Adaptations to Work in the Heat?

Older experienced firefighters may show signs of heat adaptation, and thus reduced physiological strain, due to repeated occupational heat stress exposure. The aim was to examine physiological and perceptual strain, and hydration, responses to intermittent exercise in the heat in 12 older Non-Firefighter (Non-FF) and experienced Firefighter (FF) males, pair matched for age (Group mean +/- SE: Non-FF = 51.7 +/- 1.5, FF = 49.8 +/- 1.1 years), VO2peak (Non-FF = 39.4 +/- 2.2, FF = 40.7 +/- 1.8 mL center dot kg(-1)center dot min(-1)), body surface area (Non-FF = 1.94 +/- 0.04, FF = 2.03 +/- 0.03 m(2)), and percent body fat (Non-FF = 24.4 +/- 2.3, FF = 19.3 +/- 1.8%). Rectal (T-re) and mean skin (MTsk) temperatures, heart rate (HR), local sweat rate (LSR), hydration indices, and ratings of thermal sensation and perceived exertion were measured during 4x15-min (rest 15-min) moderate-to-heavy cycling bouts (400W heat production) in Dry and Humid heat (35 degrees C, similar to 20 and similar to 60% relative humidity, respectively). No differences were observed between the Non-FF and FF for T-re, T-re change, MTsk, HR,% max HR, LSR, physiological strain index (PhSI), or % plasma volume change. Plasma protein concentration was reduced at baseline for the Non-FF (7.6 +/- 0.1 g center dot 100mL(-1)) than FF (8.0 +/- 0.1 g center dot 100mL(-1)). The Perceptual Strain Index overestimated PhSI for Non-FF and FF in both thermal conditions. At the end of exercise, the Non-FF showed a greater T-re difference between thermal conditions (0.27 +/- 0.05 degrees C) compared to the FF (0.10 +/- 0.09 degrees C). Although the Non-Firefighters and Firefighters demonstrate similar cardiovascular and hydration responses during moderate-to-heavy intensity exercise within each of the thermal conditions, the attenuated thermal effects between the two heat stress conditions in the Firefighters suggests a protective adaptation.