Letter to the editor regarding Hospers et al. (2020): Electric fans: A potential stay-at-home cooling strategy during the COVID-19 pandemic this summer?

In Hospers and colleagues' recent article (Hospers et al., 2020), electric fans have been proposed as a potential stay-at-home cooling strategy during the COVID-19 pandemic under heat wave conditions. Besides, the authors defined the threshold temperatures for electric fan-use so that the public could use this as a guideline. In this letter, I would like to challenge the rigorousness of the methodology used in their work to determine threshold temperature and relative humidity (RH) zone for electric fan-use during heatwave temperatures.

Hospers et al. (2020) used a standard conceptual human heat balance model to analyze the heat transfer between the body and ambient environments with and without an electric fan. If the convective heat load with fan-use exceeded the maximum evaporative cooling potential (which was calculated from the maximum sweat production of an individual person), electric fan-use under the specific is not advisable. The above methodology seems flawed because in some recommended climatic conditions which fan-use is advisable, fanning could accelerate human sweating and thereby hasten dehydration. Hence, it's debatable to include those conditions for fan-use.

In this letter, a rational model, i.e., the predicted heat strain model (ISO 7933, 2004) was selected to examine the effect of fanning on temporal variations of thermophysiological responses at various temperatures from 36 to 50 °C. The model was first validated by documented human trials in literatures (Ravanelli et al., 2015; Morris et al., 2019). Also, a guideline on the fan use to bring remarkable benefits of body cooling was proposed by performing simulations under various relative humidity (RH) levels (range: 10–90%, elevation intervals: 5%) at temperatures of 36–50 °C. To determine the threshold temperature-RH zone for the fan-use, the two most important thermophysiological parameters, i.e., the predicted core temperature and the total sweating production, in Fan (i.e., with the electric fan) and No Fan were compared. Fan use in a specific condition is advisable if any of the following criteria were met.

• i)The use of electric fan should decrease the core temperature by at least 0.3 °C within 2 h on a standard lightly-clothed healthy man shown in Hospers et al.'s article (i.e., body mass 70 kg, height 1.73 m, body surface area 1.83 m², clothing insulation 0.1291 m²·K·W⁻¹, evaporative resistance 23.7 m²·kPa⁻¹·W⁻¹, metabolic rate 65 W·m⁻²) and the core temperature with fan-use should never exceed 38.0 ± 0.2 °C (safety threshold core temperature (WHO, 1969));
• ii)The electric fan should bring remarkable sweating suppression, i.e., the total sweat production should be decreased by 87.5 g per hour (thirst sensation is triggered with a body water loss of about 1% (Saltmarsh, 2008), i.e., 700 g).

Morris et al. (2019) examined the effect of fan use on thermal and cardiovascular strain, risk of dehydration and thermal comfort of 12 healthy men during the peak condition in two heat wave weather conditions (i.e., 40 and 47 °C). The rectal temperatures at the end of 2-h exposure at 40 °C & 50%RH were 37.4 and 37.5 °C in Fan and No Fan, respectively. Post-exposure rectal temperatures at 47 °C & 10%RH were 37.7 and 37.4 °C, respectively. Simulation results obtained in two example heatwave conditions (i.e., 40 and 47 °C) are listed in Table 1 . It is evident that simulation results shown in Table 1 are quite close to observed clinical trial data reported in Morris et al.'s (2019) work and the maximal difference on core temperatures was 0.2 °C. Further, the model was validated by the human trial data reported in Ravanelli et al. (2015), see Fig. 1 . The predicted core temperature changes in different RH levels showed perfect match with those observed on human participants with the maximum temperature difference of <0.10 °C.

Table 1.

Post-simulation rectal temperatures and sweat production at 40 & 47 °C under various RH levels.

Relative humidity, RH (%)	Temperature: 40 °C				Temperature: 47 °C
	Core temperature (°C)		Total sweat production (g)		Core temperature (°C)		Total sweat production (g)
	Fan	No Fan	Fan	No Fan	Fan	No Fan	Fan	No Fan
10	37.4	37.2	690	560	37.6	37.2	1100	920
15	37.4	37.1	690	570	37.6	37.1	1110	970
20	37.4	37.1	690	580	37.6	37.0	1110	1070
25	37.4	37.1	690	590	37.5	37.4	1130	1230
30	37.4	37.1	690	610	37.5	38.1	1160	1250
35	37.3	37.1	680	640	37.4	38.8	1220	1250
40	37.3	37.0	690	680	37.8	39.6	1250	1250
45	37.3	37.0	690	780	38.6	40.4	1240	1240
50	37.3	37.4	700	930	39.8	41.2	1240	1240
55	37.3	37.9	720	1190	–	–	–	–
60	37.2	38.5	780	1240	–	–	–	–
65	37.1	39.0	1010	1240	–	–	–	–
70	38.2	39.6	1200	1240	–	–	–	–
75	39.5	40.1	1230	1240	–	–	–	–
80	–	–	–	–	–	–	–	–
85	–	–	–	–	–	–	–	–
90	–	–	–	–	–	–	–	–

Note: –, conditions are rarely seen on the Earth. Hence, simulations were not performed under those conditions Baseline rectal temperature: 36.8 °C; model inputting variables: clothing insulation: 0.33 clo, permeability index: 0.38, metabolic rate: 70 W/m² fanning velocity: 2.0 m·s⁻¹, simulation duration: 2 h.

Fig. 1.

Core temperature changes reported in human trials conducted by Ravanelli et al.'s study (i.e., ΔTcore_exp) and those predicted by the PHS model (i.e., ΔTcore_sim).

Thus, the PHS model could be able to generate accurate thermophysiological data for electric fan use studies.

Table 1 shows that fan use is advisable at 40 °C when RHs are within 50–70%. Interestingly, the total sweat production of Fan is also lower than those of No Fan. Fanning greatly prompts sweat evaporation, which brings greater evaporative body cooling compared to No Fan. Thereby, less sweating is required to maintain the body heat balance. Contrary to existing guideline (Gupta et al., 2012), fan cooling could still be effective for the public without access to HVAC in heat wave periods if RH values fall within 50–70% at 40 °C. Similarly, fans are advisable if RHs fall within 30–40% at 47 °C. The recommended RH zone for fanning to alleviate physiological strain on healthy individuals under other heat wave conditions (36–50 °C) is presented in Fig. 2 (Left). It should be noted that a fanning speed of 2.5 m/s was used in the simulations because most published literatures on electric fans had a maximum speed of about this value (Yang et al., 2015; Liu et al., 2018). Obviously, recommended fanning application RH values decrease with the increasing environmental temperature. The maximum advisable fan-use temperature is 48 °C (with RHs = 30–35%). Fanning is advisable when the environment water vapour pressure is within 3.1–5.1 kPa. For the oelderly people, the maximum advisable fan-use air temperature is 42 °C (with RHs = 45–50%).

Fig. 2.

Left: Recommended fanning zone for the healthy young people (the blue shaded region) and the elderly people (i.e., OLD, the interior region bounded by the closed red curve) under various heat wave conditions (fanning speed: 2.5 m·s⁻¹); Right: Temporal variations of rectal temperatures in scenarios of No Fan and Fan at 40 °C and RH = 50% (or RH = 70%). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Gupta et al. (2012) suggested that fan use might lead to increased dehydration risk. Ravanelli et al. (2015) and Ravanelli and Jay (2016) confirmed fanning could hasten dehydration. However, above statements and findings seem only partly correct. For instance, at 40 °C and 10–40%RH or at 47 °C & 10–20%RH, fanning did induce greater sweat production than No Fan. However, when RH goes above those mentioned ranges, fanning didn't actually lead to greater sweating production compared to No Fan. Besides, Fig. 2 (Right) demonstrates that fan use elevated rectal temperature at the initial exposure stage, the duration of which depends on the RH level. This is because sweating is still in its development phase in the initial heat exposure period. Hence, convective body heat gain outweighs the evaporative cooling induced by fanning. With the increasing saturation on the skin surface, heat dissipation through sweat evaporation becomes greater than convective heat gain due to fanning. Thus, a body heat balance might still be maintained if fanning is used in certain heat wave conditions (e.g., 40 °C & 50%RH).

It is clear from Fig. 1 presented in Hospers et al.'s study (Hospers et al., 2020) that the threshold temperature-RH zone for fanning should be revised and narrowed down because fanning under some recommended environmental conditions could not bring remarkable cooling effect to the occupants. Conversely, fanning could hasten dehydration in some recommended conditions. Besides, the core temperature could be increased due to fanning. In particular, Hospers et al. used a surprisingly high fanning speed, 4.5 m/s. The threshold temperature-RH zone for effective fanning should be further narrowed down because the convective heat load due to fan-use is much greater at a speed of 4.5 m/s compared to that at 2.5 m/s.

In summary, electric fans might be served as effective and economical personal cooling devices to reduce heat strain during heat waves if environmental RH levels are appropriate. Fanning during heat waves with either too low or too high RHs could worsen human thermophysiological status on lightly-clothed healthy people. Therefore, caution should still be taken when choosing fanning as an effective and economical strategy to mitigate heat strain under various heat wave conditions.

Declaration of competing interest

None.

Editor: Jay Gan