key: cord-269704-ax306loy
authors: Hospers, Lily; Smallcombe, James W.; Morris, Nathan B.; Capon, Anthony; Jay, Ollie
title: Electric fans: A potential stay-at-home cooling strategy during the COVID-19 pandemic this summer?
date: 2020-07-25
journal: Sci Total Environ
DOI: 10.1016/j.scitotenv.2020.141180
sha: 
doc_id: 269704
cord_uid: ax306loy

Abstract Current public health guidance designed to protect individuals against extreme heat and the ongoing COVID-19 pandemic is seemingly discordant, yet during the northern hemisphere summer, we are faced with the imminent threat of their simultaneous existence. Here we examine the environmental limits of electric fan-use in the context of the United States summer as a potential stay-at-home cooling strategy that aligns with existing efforts to mitigate the spread of SARS-COV-2.

Mitigating Extreme Heat at Home During the COVID- 19 Pandemic 4 A growing body of scientific evidence strongly supports the efficacy of several low-resource home-based cooling solutions. For example, skin-wetting has been shown to reduce physiological heat strain, dehydration, and thermal discomfort at temperatures up to 47°C, irrespective of humidity (9) . Electric fans are another low-cost, low-energy demand (i.e. ~20-50-times less A/C) (10, 11) cooling strategy.

However, their cooling effect during a heatwave is dependent on the prevailing combination of temperature and humidity (12) , which can vary greatly across the United States. Fans improve skin surface evaporation in humid conditions, but in low humidity conditions sweat evaporates readily, even without a fan, and therefore fans provide no additional benefit (12) . When air temperature exceeds skin temperature (~35˚C/95˚F) fans also accelerate dry heat transfer towards the body, via convection (13) . A recent clinical trial (12) showed that increases in physiological heat strain and thermal discomfort were lower with fan use during an acute exposure to the peak conditions of the most deadly hot/humid heat wave in recent US history (Chicago, 1995; 40˚C/104˚F, 50%RH). While in very hot/arid heat wave conditions (e.g. Los Angeles, 2018; 47˚C/117˚F, 10%RH) fan-use was clearly detrimental, accelerating body heating and exacerbating cardiovascular strain and discomfort relative to a no-fan control condition.

Other studies have shown that fans can provide physiological cooling up to air temperatures of 42°C/108°F with ~50%RH (13) . Heat loss may also be compromised by a reduced physiological capacity to secrete sweat, common in older adults (14, 15) and individuals taking certain medications (e.g. anticholinergics) (16) , effectively reducing the range of conditions under which a fan is beneficial (15) .

However, any potential decrements in sweating can be compensated by externally applying water directly J o u r n a l P r e -p r o o f Mitigating Extreme Heat at Home During the COVID-19 Pandemic 6 ( Fig.2A) . According to our model, electric fan use would have been detrimental in a further 15 metropolises, mainly in the South (e.g. Austin, TX), on <0.5% of summer days; equivalent to 1-10 days in 20 years (Fig.2B ). In the remaining 10 metropolises, mostly in the hot-arid interior of the Southwest, fan use would have exerted a heating effect on up to 43.5% of summer days (Phoenix, AZ; Fig.2B ).

Discussion. The present analysis indicates that electric fan-use with light water-spraying potentially offers a feasible stay-at-home cooling strategy during heat extremes for large parts of the US historically experiencing hot-humid summer conditions. In comparison to existing experimental data that demonstrates fan-use providing physiological cooling up to air temperatures of 42°C/108°F with ~50%RH (11) our modelled thresholds appear conservative. The aforementioned study (12) and others (13) were undertaken using young, healthy participants, but it is known that other factors such as age alter the environmental limits for fan-use (16) likely due to age-related decrements in sweating (15) , that limit the potential increase in evaporative heat loss a fan can provide. To establish environmental limits more generalizable to the American public that can be easily adopted in at-home settings we chose to incorporate several conservative components in the current model. Examples include the assumption of a low maximal sweat rate (440 mLh -1 ), more representative of an older adult (15) and considerably lower than sweat rates reported in previous experimental research examining fan effectiveness (i.e. average sweat rate in 47˚C/117˚F, 10%RH fan condition = 691 mLh -1 , (12)) and a low volume of water used for skin-wetting (115 mL⸱h -1 / 0.5 cup⸱h -1 ), relative to previously reported self-dousing values (i.e. = 698 J o u r n a l P r e -p r o o f Mitigating Extreme Heat at Home During the COVID-19 Pandemic 7 targeted as a stay-at-home cooling solution, rather than for use in public spaces. There is a lack of evidence suggesting fan-use may aid virus transmission, but given the suggested occurrences where air ventilation systems may have acted as a vector for virus transmission (18, 19) and our constantly developing knowledge of the nature of its spread (20, 21) , it is possible that fan-use may accelerate the distribution of virus particles present in the home. There is indeed inherent transmission risk associated with co-habitation (23, 24) . Importantly though, fan use during heat extremes in the home prevents people seeking cooling in public places among individuals whose virus status is less likely to be known than cohabitants, thus limiting personal risk of transmission and further spread in the community. Finally, our model identifies the point at which using a fan is better than not using a fan, and therefore does not quantify the amount of cooling, and whether it is sufficient to maintain body temperature within safe limits. Nevertheless, even with the conservative approach taken in developing the current model, for 200 million of the ~221.5 million residents (according to 2018 census estimates) in the 105 metropolis areas assessed, fan-use with light water-spraying would have been beneficial (i.e. exerted a cooling effect) relative to not using a fan on more than 199 of every 200 summer days in the past 20 years (Fig.2C ). It is therefore clear that public health officials should not advise people to turns fans off during heat waves as is current practice in a range of jurisdictions (25,26).

While public health officials strive to protect all citizens during the current pandemic, parallel efforts are also required to proactively prepare for the likely overlap of COVID-19 with extreme heat. Heatwave preparation plans are increasingly centred on building community resilience and protecting the most vulnerable members of society (27, 28, 29) . Whilst this approach must continue, we require adaptive, yet evidence-based, efforts to protect against the ill-effects of extreme heat that align with current public Methodological Overview. The present model was created based around an elderly adult (+65 y), with a body mass of 70 kg, a height of 1.73 m, and a calculated body surface area (BSA) (30) of 1.83 m 2 , seated at rest, in light clothing, while wetting their skin either with or without the use of an electric fan in a variety of heatwave conditions. The model was created using standard partitional calorimetry equations (13, 31) and has been updated from an earlier model4 based upon the findings of several clinical laboratory experiments (9, 12, 14, 16, 32) . The partitional calorimetry method relies upon first principles thermodynamic heat transfer equations, which determine the body's net heat flow by comparing the total amount of heat produced within the body to the total amount of heat gained or lost to the environment, through all available avenues of heat transfer (i.e. conduction, convection, radiation and evaporation) (27) . Accordingly, fan use was determined to be detrimental when the net amount of heat lost to the environment was greater with the fan off compared to having the fan on. Below are the detailed equations and assumptions used to produce the model.

Partitional calorimetry equationsdetermining the required amount of evaporation. The primary argument informing this model is that, once ambient air temperature exceeds skin temperature, fan use will increase heat gain from the environment through dry heat transfer via convection (i.e. like a convection oven), necessitating a greater required amount of evaporation to maintain heat balance.

However, fan use will simultaneously increase the maximal amount of heat that can be lost to the environment through evaporation, as well as improve sweating efficiency (10) . Accordingly, to determine whether fan use is overall beneficial or detrimental during a heatwave, the amount of total heat gain and loss from fan use needs to be assessed. This was done using the conceptual heat balance equation ( 

As evaporation is the primary method by which humans lose heat during moderate and more severe heat stress conditions (34) , and that this is the primary heat loss avenue controlled by the autonomic nervous system (34) , this equation was subsequently reorganized to determine the required amount of evaporation (E req ) to maintain heat balance (i.e. establish a steady-state core temperature):

Eq2:

here, when all other variables are held constant (as they would be at rest), an increase in dry heat transfer (primarily via convection) would necessitate an increase in E req . Within the present model (seated rest) W can be eliminated as no external work is performed. For M, we assumed a resting metabolic rate of 65

W·m -2 , which is equivalent to a person standing (35). A typical value for M when seated could be as low as 58 W·m -2 , however, the highest potential value was selected to represent the worst-case scenario in terms of metabolic heat that must be dissipated to maintain a stable body temperature (35). In order to account for heat loss due to respiration, the following equation was used (36):

Eq3:

where P a is the partial pressure of water vapor of ambient air in kP a and t a is ambient air temperature in ºC. Next, dry heat transfer (the combined effect of convection and radiation) was accounted for (37):

where t sk is mean skin temperature in ºC (assumed to be 35.5°C based on the literature (38, 39) ; t o is operative temperature in ºC, which in this case was equal to ambient air temperature; R cl is dry heat transfer resistance of clothing in m 2 ·K·W -1 ; f cl is the unitless clothing area factor (see Eq5) and h is the where is skin wettedness (i.e. proportion (0 through to 1) of BSA covered with sweat (48); P a the water vapor pressure in the ambient air in kPa; P sk,s is the partial water vapor pressure at the skin in kPa (equal to saturated water vapor pressure at skin temperature (35.5ºC), i.e. 5.78 kPa); R e,cl is the evaporative resistance of clothing in m 2 ·kPa -1 ·W -1 ; f cl is clothing area factor [see Eq. (2) employed. Similarly to R cl , this value was determined using ISO 9920 (2007) (40) and was equivalent to a typical summer ensemble, inclusive of air layers. For the "fan off" condition, an R e,cl value of 0.0237 m2·kPa -1 ·W -1 was used for the whole body.

In addition to E max , which is determined by the physical properties (i.e. temperature, water vapor content and air speed) of the skin and the surrounding the environment as well as reductions alterations in actual attainable skin wettedness, the actual attainable E max will be dictated by the maximum amount of sweat which can be produced. For the purpose of our model this was assumed to be 440 mLh -1 based upon the J o u r n a l P r e -p r o o f where SR is the sweat rate (in our model assumed to be (440 mLh -1 ), SW latent is the latent heat of the vaporization of sweat (2426 Jg -1 ) (46), 3600 is the factor needed to convert mLh -1 to gs -1 and SW eff is the sweating efficiency, i.e. the proportion of sweat produced that is evaporated from the skin surface (thereby contributing to evaporative heat loss) as opposed to sweat that drips off the body and does not contribute to heat loss. Sweating efficiency is calculated by (52):

where  req is the skin wettedness required for heat balance determined by (47):

Eq13: = (no units)

Weather data analysis. Finally, to determine whether (in conjunction with exogenous skin wetting) an electric fan should be used in given environmental conditions, a final equation was generated:

where E reqoff and E reqon are the required amount of evaporation with a fan off or on, respectively, and E maxloff and E maxlon was the lower value of the calculated E max and SW max terms with fan off or on, respectively. This equation was subsequently entered into an environmental conditions matrix that ranged from 30°C to 50°C in 2°C increments and from 5% to 100% relative humidity in 10% increments (with the exception of 5% to 10% relative humidity). In order to ensure only the hottest weather was included in the analysis, only weather from daylight hours during the months of June, July and August were analyzed. From this data, the following metrics were ascertained: the peak temperature and corresponding relative humidity, the number of days where recorded temperatures exceed the calculated upper temperature limit at which point fan use is no longer beneficial and the total number of days included in the analysis (typically 1839 days, but this differed slightly due to missing data from select weather stations). These data are displayed in supplementary table A1 as well as within the manuscript Figure 1 . 

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