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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, Ereq was 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 Emax at 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.

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Acknowledgments

The authors thank the volunteers for their participation in the present study.

Author contributions

N.M.R., O.J., S.H., and G.H. were involved in the concept and design of the research question and methodology; N.M.R. performed all data collection; N.M.R. analyzed the data; N.M.R., O.J., S.H., G.H., and D.G. interpreted the results; N.M.R. prepared the figures; N.M.R. and O.J. drafted the manuscript; N.M.R. and O.J. edited the manuscript; N.M.R., O.J., D.G., S.H., and G.H. approved the final version of manuscript.

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Correspondence to Ollie Jay.

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All participants provided written informed consent prior to their participation in the study.

Grants

This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (no. 386143-2010; O. Jay). N.M. Ravanelli is supported by a University of Ottawa Excellence Scholarship and a NSERC Postgraduate Scholarship (PGS-D).

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The authors declare that they have no conflict of interest.

Ethical approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

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Ravanelli, N.M., Gagnon, D., Hodder, S.G. et al. The biophysical and physiological basis for mitigated elevations in heart rate with electric fan use in extreme heat and humidity. Int J Biometeorol 61, 313–323 (2017). https://doi.org/10.1007/s00484-016-1213-0

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Keywords

  • Heat waves
  • Cardiovascular strain
  • Thermoregulation
  • Sweating