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 ...

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A greater ωmax occurred after 8 wk of aerobic training, but ωmax was further augmented with heat acclimation, indicating only a partially increased heat loss capacity with training. These ωmax values may assist future predictions of heat stress risk in untrained/trained unacclimated individuals and trained heat-acclimated individuals.

ObjectivePregnant women are advised to avoid heat stress (eg, excessive exercise and/or heat exposure) due to the risk of teratogenicity associated with maternal hyperthermia; defined as a core temperature (Tcore) ≥39.0°C. However, guidelines are ambiguous in terms of critical combinations of climate and activity to avoid and may therefore unnecessarily discourage physical activity during pregnancy. Thus, the primary aim was to assess Tcore elevations with different characteristics defining exercise and passive heat stress (intensity, mode, ambient conditions, duration) during pregnancy relative to the critical maternal Tcore of ≥39.0°C.DesignSystematic review with best evidence synthesis.Data sourcesEMBASE, MEDLINE, SCOPUS, CINAHL and Web of Science were searched from inception to 12 July 2017.Study eligibility criteriaStudies reporting the Tcore response of pregnant women, at any period of gestation, to exercise or passive heat stress, were included.Results12 studies satisfied our inclusion criteria (n=347). No woman exceeded a Tcore of 39.0°C. The highest Tcore was 38.9°C, reported during land-based exercise. The highest mean end-trial Tcore was 38.3°C (95% CI 37.7°C to 38.9°C) for land-based exercise, 37.5°C (95% CI 37.3°C to 37.7°C) for water immersion exercise, 36.9°C (95% CI 36.8°C to 37.0°C) for hot water bathing and 37.6°C (95% CI 37.5°C to 37.7°C) for sauna exposure.ConclusionThe highest individual core temperature reported was 38.9°C. Immediately after exercise (either land based or water immersion), the highest mean core temperature was 38.3°C; 0.7°C below the proposed teratogenic threshold. Pregnant women can safely engage in: (1) exercise for up to 35 min at 80%–90% of their maximum heart rate in 25°C and 45% relative humidity (RH); (2) water immersion (≤33.4°C) exercise for up to 45 min; and (3) sitting in hot baths (40°C) or hot/dry saunas (70°C; 15% RH) for up to 20 min, irrespective of pregnancy stage, without reaching a core temperature exceeding the teratogenic threshold.

This study assessed whether, notwithstanding lower resting absolute core temperatures, alterations in time-dependent changes in thermoregulatory responses following partial and complete heat acclimation (HA) are only evident during uncompensable heat stress. Eight untrained individuals underwent 8 wk of aerobic training (i.e., partial HA) followed by 6 days of HA in 38°C/65% relative humidity (RH) (i.e., complete HA). On separate days, esophageal temperature (Tes), arm (LSRarm), and back (LSRback) sweat rate, and whole body sweat rate (WBSR) were measured during a 45-min compensable (37°C/30% RH) and 60-min uncompensable (37°C/60% RH) heat stress trial pre-training (PRE-TRN), post-training (POST-TRN), and post–heat acclimation (POST-HA). For compensable heat stress trials, resting Tes was lower POST-TRN (36.74 ± 0.27°C, P = 0.05) and POST-HA (36.60 ± 0.27°C, P = 0.001) compared with PRE-TRN (36.99 ± 0.19°C); however, ΔTes was similar in all trials (PRE-TRN:0.40 ± 0.23°C; POST-TRN:0.42 ± 0.20°C; POST-HA:0.43 ± 0.12°C, P = 0.97). While LSRback was unaltered by HA ( P = 0.94), end-exercise LSRarm was higher POST-TRN (0.70 ± 0.14 mg·cm−2·min−1, P < 0.001) and POST-HA (0.75 ± 0.16 mg·cm−2·min−1, P < 0.001) compared with PRE-TRN (0.61 ± 0.15 mg·cm−2·min−1). Despite matched evaporative heat balance requirements, steady-state WBSR (31st–45th min) was greater POST-TRN (12.7 ± 1.0 g/min, P = 0.02) and POST-HA (12.9 ± 0.8 g/min, P = 0.004), compared with PRE-TRN (11.7 ± 0.9 g/min). For uncompensable heat stress trials, resting Tes was lower POST-TRN (36.77 ± 0.22°C, P = 0.05) and POST-HA (36.62 ± 0.15°C, P = 0.03) compared with PRE-TRN (36.86 ± 0.24°C). But ΔTes was smaller POST-TRN (0.77 ± 0.19°C, P = 0.05) and POST-HA (0.75 ± 0.15°C, P = 0.04) compared with PRE-TRN (1.10 ± 0.32°C). LSRback and LSRarm increased with HA ( P < 0.007), supporting the greater WBSR with HA (POST-TRN:14.4 ± 2.4 g/min, P < 0.001; POST-HA:16.8 ± 2.8 g/min, P < 0.001) compared with PRE-TRN (12.7 ± 3.2 g/min). In conclusion, the thermal benefits of HA are primarily evident when conditions challenge the physiological capacity to dissipate heat. NEW & NOTEWORTHY We demonstrate that neither partial nor complete heat acclimation alters the change in core temperature during compensable heat stress compared with an unacclimated state, despite a marginally greater whole body sweat rate. However, the greater local and whole body sweat rate with partial and complete heat acclimation reduced the rise in core temperature during 60 min of uncompensable heat stress compared with an unacclimated state, suggesting the improvements in heat dissipation associated with heat acclimation are best observed when the upper physiological limits for evaporative heat loss are challenged.

We sought to identify the appropriate exercise intensity for unbiased comparisons of changes in rectal temperature (ΔTre) and local sweat rates (LSR) between groups unmatched for body size during uncompensable heat stress. Sixteen males vastly different in body morphology were separated into two equal groups [small (SM): 65.8 ± 6.2 kg, 1.8 ± 0.1 m2; large (LG): 100.0 ± 13.1 kg, 2.3 ± 0.1 m2], but matched for sudomotor thermosensitivity (SM: 1.3 ± 0.6; LG: 1.1 ± 0.4 mg·cm−2·min−1·°C−1). The maximum potential for evaporation (Emax) for each participant was assessed using an incremental humidity protocol. On separate occasions, participants then completed 60 min of cycling in a 35°C and 70% RH environment at (1) 50% of VO 2max, (2) a heat production (Hprod) of 520 W, (3) Hprod relative to mass (6 W·kg−1), and (4) Hprod relative to mass above Emax (3 W·kg−1>Emax). Emax was similar between LG (347 ± 39 W, 154 ± 15 W·m−2) and SM (313 ± 63 W, 176 ± 34 W·m−2, P > 0.12). ΔTre was greater in SM compared to LG at 520 W (SM: 1.5 ± 0.5; LG 0.8 ± 0.3°C, P < 0.001) and at 50% of VO 2max (SM: 1.4 ± 0.5; LG 0.9 ± 0.3°C, P < 0.001). However, ΔTre was similar between groups when Hprod was either 6 W·kg−1 (SM: 0.9 ± 0.3; LG 0.9 ± 0.2°C, P = 0.98) and 3 W·kg−1>Emax (SM: 1.4 ± 0.5; LG 1.3 ± 0.4°C, P = 0.99). LSR was similar between LG and SM irrespective of condition, suggesting maximum LSR was attained (SM: 1.10 ± 0.23; LG: 1.07 ± 0.35 mg·cm−2·min−1, P = 0.50). In conclusion, systematic differences in ΔTre and LSR between groups unmatched for body size during uncompensable heat stress can be avoided by a fixed Hprod in W·kg−1 or W·kg−1>Emax.

When exercise was prescribed to elicit a fixed evaporative heat balance requirement (E req ), no differences in steady-state sweat rates were observed with different absolute oesophageal and/or skin temperatures, secondary to differences in time of the day (i.e. morning (AM) vs. afternoon (PM)) and ambient temperature (i.e. 23°C vs. 33°C).r Exercise at a fixed metabolic heat production (H prod ), but a different E req (due to differences in air temperature), yielded higher steady-state sweat rates with a higher E req , irrespective of absolute oesophageal temperature.r Circadian rhythm did not alter the change in core temperature prior to the onset for sudomotor activation, nor the thermosensitivity, resulting in similar cumulative whole-body sweat rates irrespective of time of day at a fixed E req .r Collectively, these data indicate that during exercise in a compensable environment, steady-state sudomotor responses are influenced by E req rather than absolute core and skin temperatures, or H prod .

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