Intermittent wetting clothing as a cooling strategy for body heat strain alleviation of vulnerable populations during a severe heatwave incident

Highlights

• Intermittently wetting clothing (WEC) to alleviate heat strain in an extreme heatwave was examined.

• WEC could significantly reduce mean skin and core temperatures compared with CON.

• WEC could remarkably lower skin temperatures at local body sites covered by wet clothing.

• No significant difference was found on perceptual responses between WEC and CON.

• WEC could serve as an ecologically valid cooling strategy to reduce heat strain of vulnerable populations while seating during the studied heatwave.

Abstract

Many documented studies have demonstrated the human mortality rate increases during severe heatwaves. There remains a need for further explore ecologically valid cooling strategies to alleviate body heat strain during extreme heatwaves. The main aim of this work was to explore whether intermittent wetting clothing can be served as an ecologically valid cooling strategy to mitigate heat stress on inactive vulnerable populations not having access to air-conditioning during a severe heatwave. Ten young male subjects underwent two 90-min separate trials: a dry clothing trial (i.e., CON) and a wetted clothing cooling trial (i.e., WEC). A set of light summer wear was chosen and intermittently wetted by tap water at intervals of every 30 min. Physiological and perceptual responses of subjects were examined and compared. All trials were performed in a chamber with an air temperature of 43 ± 0.5 °C, RH= 57 ± 5% and an air velocity of 0.15 ± 0.05 m/s (WBGT=37.35 °C). Results demonstrated that WEC, compared with CON, could significantly reduce both the mean skin temperature and the core temperature throughout the 5–90th min and 25–90th min of the trial, respectively (p < 0.05). Besides, WEC could also remarkable reduce local skin temperatures at those body sites covered by wet clothing (p < 0.05). In comparison, no significant difference was found between WEC and CON on perceptual responses. Further, it was also found from PHS simulations that conditions with a partial water vapour pressure ≤ 3.1–3.5 kPa would not induce pronounced core temperature rises at 43 °C. Finally, it may be concluded that intermittent wetting clothing could be served as an ecologically valid cooling strategy to reduce thermophysiological strain of vulnerable populations while seating during humid heatwaves and thereby improve their health and safety.

Introduction

Heatwaves (i.e., prolonged periods of extremely hot weather) are becoming increasingly frequent and intense in recent years due to global warming and climate change (Li et al., 2015). It is now well established that human mortality and morbidity rates increase significantly during extreme heatwaves (Robine et al., 2008, Knowlton et al., 2009, Shaposhnikov et al., 2014, Guo et al., 2017). In the year of 2003, the deadliest heatwaves in Europe led to over 70,000 deaths (Robine et al., 2008). The California's 2006 heatwave killed at least 140 people and led to 1182 hospitalizations (Knowlton et al., 2009). The 2010 severe heatwave killed 55,736 people in Russia (Shaposhnikov et al., 2014). More recently in 2015, heatwaves in India and Pakistan claimed more than 4500 lives (Murari et al., 2015). Obviously, heatwaves have become a global concern, and they severely threaten human health and safety (Li et al., 2015, Mora et al., 2017).

In extremely hot environments (T_air≥ 40 °C), people like the poor and the homeless in backward areas do not have a chance to access air-conditioning. Hence, they have a high risk of suffering heat stress during prolonged heatwave incidents. In fact, statistical data showed that those populations account for a large proportion of heat-induced death tolls (Åström et al., 2011, Gronlund, 2014, Gubernot et al., 2014). Besides, extreme heatwaves put strains on the electrical power grid and cause power outages in some regions which renders electrically powered cooling devices (e.g., air-conditioning, electric fans and water pumps) useless. Thus, there is a great need to seek effective cooling strategies to combat heat stress on vulnerable people during extreme heatwave incidents (Gubernot et al., 2014). Presently, most government agencies just published qualitative guidelines for helping the general populations manage health and safety in heatwaves (CDC, 2013). Some commonly suggested instructions may include drinking lots of water when the weather is hot, terminating work in the hottest hours of the day and wearing thin and light clothing (Gubernot et al., 2014). Unfortunately, the effectiveness and feasibility of the above qualitative measures remain uninvestigated. Therefore, there is a great need to scientifically explore effective but feasible cooling strategies to mitigate heat stress on vulnerable populations during heatwaves.

Based on the human heat balance equation (i.e., M-W=(K+C+R+E_sk) + (C_res+E_res)+S [W/m²], where M, W, K, C, R, E_sk, C_res, E_res and S are the metabolic heat production, mechanic work, conductive heat transfer, convective heat transfer, radiative heat transfer, evaporative heat transfer from the skin, convective respiratory heat transfer, evaporative respiratory heat transfer and the heat storage, respectively), the alleviation of heat strain (e.g., reducing the body heat storage S) in hot environments may be implemented from three aspects, i.e., provide a relatively comfortable working environment to the individuals (e.g., using air conditioning), reduce the body metabolic heat production (e.g., adjusting exercise/work intensities) and create a comfortable clothing microenvironment around the human body (e.g., using personal wearable cooling systems). Air conditioning (AC) is regarded as an effective cooling strategy to mitigate heat stress, but it tends to bring about huge cooling energy consumptions. Besides, the use of AC systems induces ecological and environmental protection issues (Zhang et al., 2015). More important, it is not easily accessible for many population groups such as people living in economically backward areas. Lowering down the body energy expenditure seems less useful under extremely hot environments because the body heat gain in such conditions always outweighs the body heat dissipation. Documented studies (Yang and Yan, 2014, Lu et al., 2015, Song and Wang, 2016) on personal wearable cooling strategies (e.g., personal cooling clothing [PCC]) have demonstrated that PCC systems are effective in alleviating heat strain while working in various hot environments. Hence, personal wearable cooling clothing has great potential for heat stress mitigation on the general population during heatwaves. To our knowledge, whether personal wearable cooling clothing could effectively reduce body heat strain of vulnerable groups during extreme heatwaves remains unknown.

The most commonly used personal cooling device (PCD) at homes and in offices is electric fans. Electric fans (e.g., ceiling fan, desk fan and floor fan) help individuals dissipate body heat through forced convection under a range of indoor environmental conditions (Centers for disease control and prevention (CDC), 2013, Scheatzle et al., 1989, Ravanelli et al., 2015, Yang et al., 2015, Schiavon et al., 2017). The U.S. Environmental Protection Agency (EPA) suggests that the indoor temperature upper limit to use electric fans is 35.6–37.2 °C with no relative humidity values given (CDC, 2013). Controversially, several researchers recommend wider and higher temperature upper limits (i.e., >40 °C) to use electric fans in low relative humidity conditions (Scheatzle et al., 1989, Ravanelli et al., 2015). Electric fans are usually less portable and hence, it is impractical for active populations to use electric fan cooling.

A potential personal cooling strategy to provide body cooling to both inactive and active populations is to wear personal cooling clothing (PCC). Presently, existing PCC systems include phase change material (PCM) cooling clothing, air cooling clothing (ACC), liquid cooling clothing (LCC), evaporative cooling clothing (ECC) and hybrid cooling clothing (HCC) that combines at least two aforementioned cooling techniques (Heled et al., 2004, Lu et al., 2015; Mokhtari and Sheikhzadeh, 2014; Wang and Song, 2017). Various types of PCC were found to bring significant body cooling benefits in hot environments (Lu et al., 2015; Mokhtari and Sheikhzadeh, 2014; Wang and Song, 2017). Nevertheless, it should be emphasized that a number of ergonomic problems brought by PCC (e.g., weight and movement restriction of PCC, convenience of use, expense and logistics) largely limit the actual wearable usability of PCC for vulnerable populations (Parsons, 2009, Sampson et al., 2013, Chan et al., 2015). For example, most ACC, LCC and HCC systems are normally complicated and expensive, and also some PCC systems are non-portable due to the use of connected immovable compressors for providing cold air/liquid (Mokhtari and Sheikhzadeh, 2014). PCM cooling clothing suffers from drawbacks of cold storage requirement for PCM solidification, and also, miscellaneous replacements of PCMs during usage to realize prolonged cooling. In view of the above issues, there is an urgent need to seek ecologically valid personal cooling strategies with less ergonomic problems for the general public to mitigate heat stress during heatwave incidents.

Further, for such vulnerable populations as the poor, the homeless and those who do not have access to cooling devices and locations, an ecologically valid cooling strategy is important to manage individual health and safety while being exposed to prolonged heatwaves. It has been well recognized that water evaporation is a natural and ecologically valid strategy for body cooling. In cold conditions, wet clothing (i.e., another type of PCC containing water, referred as passive cooling clothing) can induce the body ‘chilling effect’ and thereby, increases the risk for an individual to develop hypothermia. Laboratory trials discovered wet clothing could absorb a large amount of body heat or promote great body heat losses, and thus results in a great body heat debt in cold environments (Craig, 1972, Havenith et al., 2013; Bakkevig et al., 1994; Richards et al., 2008; Wang et al., 2016). Greatly enhanced evaporative (due to the promoted moisture evaporation) and conductive heat transfer (due to the reduced clothing thermal insulation) through wearing wet clothing may be the explanation of the above phenomenon (Elson and Eckels, 2018). In hot conditions, the only documented human trial study (Heled et al., 2004) has demonstrated that spraying tap water on subjects in impermeable nuclear, biological, and chemical (NBC) clothing was effective in reducing body heat strain under an extremely hot environment (i.e., 40 °C, 40% RH). Nevertheless, the effectiveness of water spraying of normally clothing (e.g., light summer wear) on the heat strain alleviation of vulnerable populations remains unclear. Given these observations, it is also expected that using wet clothing in heatwaves may also promote evaporative heat transfer, and thus brings significant cooling to individual persons in heatwave events. Besides, this strategy is practical and easy to enforce due to its simplicity, i.e., the wetting of clothing may easily be achieved by using accessible water sources such as water reservoirs, water pools or water hydrants on the street.

Therefore, a study was conducted to investigate the effectiveness of intermittent wetting clothing on body heat strain alleviation in a simulated extreme heatwave event. Human trials were conducted in a controlled climate chamber with an extremely high air temperature and a high relative humidity (i.e., 43 °C, 57%). Thermophysiological responses and subjective perceptions of ten healthy male subjects in wet and dry summer wear were examined and compared. It was hypothesized that intermittent wetting clothing is able to improve both human thermophysiological responses and subjective perceptions in extreme heatwave incidents. Besides, due to the neglection of the contribution of environmental relative humidity (RH) to the magnitude of heat waves (Guo et al., 2017, Russo et al., 2017, Mora et al., 2017), the RH effect on the human thermophysiological load was also investigated using the PHS (Predicted Heat Strain) model (ISO, 7933, 2004).