• DOI:10.7326/M19-0512
• Corpus ID: 199447588

The Effects of Electric Fan Use Under Differing Resting Heat Index Conditions: A Clinical Trial

@article{Morris2019TheEO,
  title={The Effects of Electric Fan Use Under Differing Resting Heat Index Conditions: A Clinical Trial},
  author={Nathan B. Morris and Timothy English and Lily Hospers and Anthony Guy Capon and Ollie Jay},
  journal={Annals of Internal Medicine},
  year={2019},
  volume={171},
  pages={675-677}
}

• N. Morris, Timothy English, +2 authors O. Jay
• Published 5 November 2019
• Medicine
• Annals of Internal Medicine

Background: The devastating effects of climate change on human health, particularly with respect to heat waves, are already evident (1). For example, heat wave events in Paris (2003) and Moscow (2010) were responsible for 4870 and 10860 deaths, respectively (2). As a result, delivery of evidence-based health advice by physicians and health care professionals to protect against heat-related illness has never been more important. Electric fans are simple, low-cost cooling devices. Yet, the U.S. Environmental Protection Agency states that fan use above a heat index (HI) of 37.2C actually increases the heat stress the body must respond to (3). The HI, a unified weather metric used widely in summer forecasts, expresses the combined effects of temperature and humidity (4), but its utility for recommending fan use may be low. A very hot day with low humidity may yield a lower HI than a slightly cooler but much more humid day. However, the more air temperature exceeds skin temperature (about 35C), the more fan use accelerates dry heat transfer toward the body via convection, and although fan use can greatly improve sweat evaporation in humid conditions, most sweat should evaporate freely without fan use when humidity is low (5). Objective: To examine the effect of electric fan use on thermal strain, cardiovascular strain, risk for dehydration, and thermal comfort during a 2-hour exposure to 2 simulated peak conditions of a heat wave: very hot and dry, and cooler and more humid with a higher HI. Methods and Findings: After University of Sydney ethics approval, volunteers were recruited on campus and gave written informed consent. Because the study was not determined to be a clinical trial during this review process, the trial was registered retrospectively (Australian New Zealand Clinical Trials Registry: ACTRN12619000663156). In this randomized crossover study, 12 men (Table) who were not receiving medication and had no conditions that altered their thermoregulation completed four 120-minute experimental trials, each on a separate day. Wearing shorts, participants sat in a climatic chamber that simulated either very hot, dry conditions or hot, humid conditions. Conditions were chosen to yield different HI valuescalculated using the National Weather Service method (4)that exceed the limits currently recommended for fan use (3) and that commonly occur during the peak of a heat wave (Table). Each condition was tested with and without a 44.5-cm fan (Chrome Pedestal TARCPF50, Target) placed 1.25 m in front of the participant (air velocity, 2.0 m/s). Table. Simulated Heat Wave Conditions With Real-World Examples and the Mean Participant Characteristics and Outcome Variables at Baseline and After 2-Hour Heat Exposures Thermal strain was assessed using rectal temperature, which was measured with a thermistor (Mon-a-therm, Covidien). Cardiovascular strain was assessed using 3-lead electrocardiography to measure heart rate, systolic blood pressure was measured using sphygmomanometry (Tango, SunTech), and rate pressure product was calculated as heart rate multiplied by systolic blood pressure. Rate of dehydration was assessed by whole-body sweat rate, which was measured using change in body mass from before to after the trial (Id1 MultiRange, Mettler-Toledo). Whole-body thermal comfort was assessed using a 120-mm visual analogue scale (ASHRAE [American Society of Heating, Refrigerating and Air-Conditioning Engineers] Standard 55). The 2-hour changes from baseline were compared between fan and control (without fan) trials, within each heat wave condition, for all variables. Normal distribution was assessed using ShapiroWilk tests, and normally and nonnormally distributed data were assessed using paired-samples t tests and Wilcoxon signed-rank tests, respectively. We used SPSS Statistics, version 25 (IBM), for all tests, and statistical significance was set at P 0.05. The Table shows all baseline and end-trial outcome variables, and the Figure illustrates differences in 2-hour change from baseline between fan and control trials. No adverse events were reported during the study. Fan use in very hot, dry conditions worsened rectal temperature, rate pressure product, heart rate, whole-body sweat rate, and whole-body thermal comfort. By contrast, fan use in hot, humid conditions improved each of these except whole-body sweat rate, which was worsened. Systolic blood pressure did not differ with fan use in either heat wave condition. Figure. Mean differences (with 95% CIs) and individual differences between fan and control trials in change in outcome variables from baseline after 2 h of VHD (circles) and HH (diamonds) heat exposure. Outcome variables are rectal temperature (A), rate pressure product (B), whole-body sweat rate (C), heart rate (D), SBP (E), and whole-body thermal comfort (F). For all comparisons, n= 12. Open symbols denote participants doing fan trials first, and gray symbols denote those doing fan trials second. Black symbols show the mean within-condition difference (i.e., fancontrol). HH= hot, humid; HI= heat index; SBP= systolic blood pressure; VHD= very hot, dry. A. Fancontrol difference exceeds clinical significance of 0.3C in VHD but not HH conditions. B. Fancontrol difference exceeds clinical significance of 1.3 beats/minmm Hg103 in VHD but not HH conditions. C. Fancontrol difference exceeds clinical significance of 125 g/h in both VHD and HH conditions. D. Fancontrol difference exceeds clinical significance of 5 beats/min in both VHD and HH conditions. E. Fancontrol difference does not exceed clinical significance of 5 mm Hg in either VHD or HH conditions. F. Fancontrol difference exceeds clinical significance of 15 mm in both VHD and HH conditions. * Derived from a Wilcoxon signed-rank test. All other P values were derived from paired-samples t tests. Discussion: In hot, humid conditions with an HI of 56C, fans reduced core temperature and cardiovascular strain and improved thermal comfort. Yet, fans were detrimental for all measures in very hot, dry conditions despite a lower HI of 46C. These findings highlight issues that may arise when HI values are used to recommend fan use during heat waves. Fan use during heat waves in very hot (45C) but arid (relative humidity <10%) regions, such as the South Central United States, South Australia, and the Middle East, is likely not advisable. However, in much of the remainder of the United States, Southeast Asia, South America, and Europe, temperatures rarely exceed 40C but are accompanied by moderately high humidity (relative humidity >30% to 40%) (2). In these regions, fans could be encouraged as a cheaper and accessible alternative to air conditioning that also limits net electricity demand and carbon pollution. The study was limited by its use of a small sample of young, healthy men who were at rest; were not taking medications, and had a normal sweating ability; as well as by its short exposure time. Future investigations are needed to fill remaining knowledge gaps.