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

Abstract

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), VO_2peak (Non-FF = 39.4 ± 2.2, FF = 40.7 ± 1.8 mL·kg⁻¹·min⁻¹), body surface area (Non-FF = 1.94 ± 0.04, FF = 2.03 ± 0.03 m²), and percent body fat (Non-FF = 24.4 ± 2.3, FF = 19.3 ± 1.8%). Rectal (T_re) and mean skin (MT_sk) temperatures, heart rate (HR), local sweat rate (LSR), hydration indices, and ratings of thermal sensation and perceived exertion were measured during 4 ×15-min (rest 15-min) moderate-to-heavy cycling bouts (400 W heat production) in Dry and Humid heat (35°C, ∼20 and ∼60% relative humidity, respectively). No differences were observed between the Non-FF and FF for T_re, T_re change, MT_sk, 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·100 mL⁻¹) than FF (8.0 ± 0.1 g·100 mL⁻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°C) compared to the FF (0.10 ± 0.09°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.

Keywords:

• core temperature
• physiological strain
• perceptual strain
• dry and humid heat
• repeated heat stress

ACKNOWLEDGMENTS

This study was funded by a research grant provided by the Workplace Safety and Insurance Board (Ontario) and Canada Foundation for Innovation-Leaders Opportunity Fund (held by Dr. Glen P. Kenny). The authors wish to acknowledge Drs. M.B. DuCharme, A.J. Seely, and J.E. Yardley as part of the grant writing team. Dr. Glen Kenny was supported by a University of Ottawa Research Chair Award. Dr. Heather Wright was supported by a MITACS-Accelerate fellowship. The authors wish to thank the participants for their time and effort, and Dr. Francois Haman of the University of Ottawa for the use of his laboratory freezer.