title: The biophysical and physiological basis for mitigated elevations in heart rate with electric fan use in extreme heat and humidity.
abstract: Electric fan use in extreme heat wave conditions has been thought to be disadvantageous because it might accelerate heat gain to the body via convection. However, it has been recently shown that fan use delays increases in heart rate even at high temperatures (42 °C) in young adults. We here assess the biophysical and physiological mechanisms underlying the apparently beneficial effects of fan use. Eight males (24 ± 3 y; 80.7 ± 11.7 kg; 2.0 ± 0.1 m2) rested at either 36 °C or 42 °C, with (F) or without (NF) electric fan use (4.2 m/s) for 120 min while humidity increased every 7.5 min by 0.3 kPa from a baseline value of 1.6 kPa. Heart rate (HR), local sweat rate (LSR), cutaneous vascular conductance (CVC), core and mean skin temperatures, and the combined convective/radiative heat loss (C+R), evaporative heat balance requirements (Ereq) and maximum evaporative potential (Emax) were assessed. C+R was greater with fan use at 36 °C (F 8 ± 6, NF 2 ± 2 W/m2; P = 0.04) and more negative (greater dry heat gain) with fan use at 42 °C (F -78 ± 4, NF -27 ± 2 W/m2; P < 0.01). Consequently, Ereqwas lower at 36 °C (F 38 ± 16, NF 45 ± 3 W/m2; P = 0.04) and greater at 42 °C (F 125 ± 1, NF 74 ± 3 W/m2; P < 0.01) with fan use. However, fan use resulted in a greater Emaxat baseline humidity at both 36 °C (F 343 ± 10, NF 153 ± 5 W/m2; P < 0.01) and 42 °C (F 376 ± 13, NF 161 ± 4 W/m2; P < 0.01) and throughout the incremental increases in humidity. Within the humidity range that a rise in HR was prevented by fan use but not without a fan, LSR was higher in NF at both 36 °C (P = 0.04) and 42 °C (P = 0.05), and skin temperature was higher in NF at 42 °C (P = 0.05), but no differences in CVC or core temperatures were observed (all P > 0.05). These results suggest that the delayed increase in heart rate with fan use during extreme heat and humidity is associated with improved evaporative efficiency.
authors.affiliation: School of Human Kinetics, University of Ottawa, 200 Lees Ave, Ottawa, K1N 6N5, Canada.|Thermal Ergonomics Laboratory, Faculty of Health Sciences, University of Sydney, Lidcombe, NSW, 2141, Australia.
authors.forename: Nicholas M
authors.initials: NM
authors.lastname: Ravanelli
authors.affiliation: Cardiovascular Prevention and Rehabilitation Centre (ÉPIC), Montreal Heart Institute Research Centre, Department of Molecular and Integrative Physiology, Faculty of Medicine, Université de Montréal, Montréal, QC, Canada.
authors.forename: Daniel
authors.initials: D
authors.lastname: Gagnon
authors.affiliation: Environmental Ergonomics Research Centre, Loughborough Design School, Loughborough University, Leics, LE11 3TU, UK.
authors.forename: Simon G
authors.initials: SG
authors.lastname: Hodder
authors.affiliation: Environmental Ergonomics Research Centre, Loughborough Design School, Loughborough University, Leics, LE11 3TU, UK.
authors.forename: George
authors.initials: G
authors.lastname: Havenith
authors.affiliation: Thermal Ergonomics Laboratory, Faculty of Health Sciences, University of Sydney, Lidcombe, NSW, 2141, Australia. ollie.jay@sydney.edu.au.|Charles Perkins Centre, University of Sydney, Camperdown, NSW, 2050, Australia. ollie.jay@sydney.edu.au.
authors.forename: Ollie
authors.initials: O
authors.lastname: Jay
mesh_terms: D000328:Adult; D001833:Body Temperature Regulation / Q000502:physiology*; D055615:Electrical Equipment and Supplies; D055874:Extreme Heat; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D006813:Humidity; D008297:Male; D012039:Regional Blood Flow; D012867:Skin / Q000098:blood supply; D013546:Sweating / Q000502:physiology; D014691:Ventilation; D055815:Young Adult
keywords: Cardiovascular strain; Heat waves; Sweating; Thermoregulation