Heart Rate and Body Temperature Responses to Extreme Heat and Humidity With and Without Electric Fans

Abstract

Patz et al 1 described the projected effects of more prolonged and severe heat waves on human health. A simple, low-cost cooling device is an electric fan. A Cochrane review 2 concluded "no evidence currently exists supporting or refuting the use of electric fans during heat waves" for mortality and morbidity. However, public health guidance typically warns against fan use in hot weather. Recommended upper limits range from 32.3°C (90°F) at 35% relative humidity (RH) to the high 90s (96-99°F; 35.6-37.2°C, no RH stated 2 ).The skin-to-air temperature gradient reverses with rising environmental temperature, causing dry heat transfer toward the body via convection rather than away from it. Fan use would increase this dry heat transfer, potentially accelerating body heating 3,4 ; however, the efficiency of sweat evaporation from the skin would be simultaneously increased. Thus, fans could still improve net heat loss.Sweat evaporation declines with increasing humidity, so in more humid environments fans may not prevent heatinduced elevations in cardiovascular (heart rate, HR) and thermal (core temperature) strain. This study examined the influence of fan use on the critical humidities at which hot environments can no longer be physiologically tolerated without rapid increases in HR and core temperature.Methods | After University of Ottawa ethics approval, written informed consent was obtained from student volunteers. Each participant completed four 135-minute trials presented in randomized order and separated by more than 48 hours. Euhydration was confirmed prior to each trial (urine-specific gravity <1.025). Wearing shorts and t-shirts, participants sat in a chamber maintained at temperatures equal to (36°C; 97°F) or exceeding (42°C; 108°F) the limits currently recommended for fan use.Each temperature was tested with and without an 18-in fan (Whirlpool) facing the participant from 1 m (air speed: 4.0 m/s). After a 20-minute baseline period, RH was increased in 15 equal steps (7.5 minutes each) from 25% to 95% at 36°C and from 20% to 70% at 42°C.5 Heart rate (Polar) and core (esophageal) temperature (Covidien) were measured throughout. Whole-body sweat rate was determined using the 135-minute pre-to-post trial change in body mass (Sartorious).The RH values at which an upward inflection in first HR and then core temperature occurred were determined (Figure 1) separately for each individual trial using segmented linear regression (GraphPad). These critical RH values and wholebody sweat rates were compared between fan and no fan trials at each temperature using paired-sample t tests (P < .05, 2-sided).Results | Eight healthy males (mean [SD] age of 23 [3] years and weight of 80.7 [11.7] kg) participated between June 5 and November 6, 2013. The critical RH for an upward inflection in HR at 36°C was higher with fans (83%; 95% CI, 78%-87%) than without fans (62%; 95% CI, 56%-68%) (P < .001) and at 42°C (47% [95% CI, 42%-51%] vs 38% [95% CI, 33%-42%], respectively) (P = .01; Figure 2). Change in Heart Rate, beats/min Heart ...

The biophysical and physiological basis for mitigated elevations in heart rate with electric fan use in extreme heat and humidity

“…The data presented in the current manuscript were collected as part for a larger study 105 examining humidity inflection points for heart rate and core temperature with and without fan 106 use (Ravanelli et al 2015). Eight healthy, normotensive, non-smoking young males, with no pre-107 existing cardiovascular, metabolic, or neurological issues participated in the study (age: 24±3 y; 108 mass: 80.7±11.7 kg; height: 1.77±0.05 m; BSA: 1.98±0.14 m 2 ).…”

“…However, at both ambient temperatures the greater potential for 333 evaporation with fan use increased evaporative efficiency. While evaporative efficiency was not 334 directly quantified, the additional ~70 W/m 2 of dry heat gain with fan use at 42°C must have 335 been offset by at least an equally greater evaporative heat loss as the increase in core temperature 336 was delayed relative to the no fan condition (Ravanelli et al 2015). From a physiological 337 perspective, the different inflection points for increases in heart rate between the fan and no fan 338 conditions seemed to coincide with elevations in sudomotor output at both 36°C and 42°C.…”

“…Over the past 20 years, heat waves -characterized by extended bouts of extreme heat and 59 humidity -have led to high levels of excess morbidity and mortality in the United States 60 (Whitman et al 1997 electric fan use at air temperatures up to 42°C delays heat-induced elevations in heart rate in 79 young healthy males (Ravanelli et al 2015). However, the underlying physiological and 80…”

“…P sk,sat -P a is the 228 difference in water vapor pressure between the skin and air in kPa. While P a is a measured in 229 absolute terms (in kPa), P sk,sat can be derived from Antoine's equation As reported previously (Ravanelli et al 2015), the critical humidity at which elevations in 243 heart rate were observed was higher with fan use at both 36°C (F: 4.9±0.4 kPa, NF: 3.7±0.5 kPa; 244 P<0.001) and 42°C (F: 3.8±0.6 kPa, NF: 3.1±0.6; P=0.01). The temperature and humidity ranges 245 for stages during which an elevation in heart rate was observed are presented in Table 1.…”

Electric fan use during heat waves: Turn off for the elderly?

“…However, as Ravanelli & Jay 1 discuss, greater heat gain during fan use is far outweighed by increased sweat evaporation, the latter of which can be offset by drinking less than one cup of water per hour. Consequently, fan use delayed the increase in heart rate and internal body temperature of young adults exposed to a humidity ramp protocol performed at 36 C (97 F) and 42 C (108 F) 2 . These findings are particularly important given a general lack of knowledge regarding the effectiveness of fan use during heat waves.…”