key: cord-0817651-upr9si2k
authors: Shi, Dachuan; Song, Jiyun; Du, Ruiqing; Chan, Pak W.
title: Dual challenges of heat wave and protective facemask-induced thermal stress in Hong Kong
date: 2021-09-01
journal: Build Environ
DOI: 10.1016/j.buildenv.2021.108317
sha: ca209ba7e4bdcc50d7ec8a50d8179babe3941c09
doc_id: 817651
cord_uid: upr9si2k

During the COVID-19 pandemic, wearing protective facemasks (PFMs) can effectively reduce infection risk, but the use of PFMs can amplify heat-related health risks. We studied the amplified PFM-induced human thermal stress via both field measurements and model simulations over a typical subtropical mountainous city, Hong Kong. First, a hot and humid PFM microenvironment has been observed with high temperature (34–35 °C) and high humidity (80–95%), resulting in an aggravated facial thermal stress with a maximal PFM-covered facial heat flux of 500 W/m(2) under high-intensity activities. Second, to predict the overall PFM-inclusive human thermal stress, we developed a new facial thermal load model, S(PFM) and a new human-environment adaptive thermal stress (HEATS) model by coupling S(PFM) with an enhanced thermal comfort model to resolve modified human-environment interactions with the intervention of PFM under realistic climatic and topographical conditions. The model was then applied to predict spatiotemporal variations of PFM-inclusive physiological subjective temperature (PST) and corresponding heat stress levels during a typical heat wave event. It was found wearing PFM can significantly aggravate human thermal stress over Hong Kong with a spatially averaged PST increment of 5.0 °C and an additional spatial area of 158.4% exposed to the severest heat risks. Besides, PFM-inclusive PST was found to increase nonlinearly with terrain slopes at a rate of 1.3–3.9 °C/10°(slope), owing to elevated metabolic heat production. Furthermore, urban residents were found to have higher PFM-aggravated heat risks than rural residents, especially at night due to synergistic urban heat and moisture island effects.

where C, E, R, G denote convection, evaporation, radiation, and conduction, respectively, with 157 the subscripts "fsk" and "res" representing for facial skin and respiration, respectively. (C + E+ 158 R)fsk can be calculated according to general skin energy balance equations [14] . Gpfm can be 159 via Hpfm = (1apfm) cp (Tsk -Ta), and LE is the PFM surface latent heat flux. In current default 8 settings, LE = 0; in special settings if the PFM is wet or the PFM is made from phase change 163 materials, LE ≠ 0. Q is the convective heat exchange between PFM cavity and ambient air 164 due to leakage. (C + E)res is the suppressed heat dissipation from respiration [28] : 165 (C + E)res = 0.0014⋅M⋅(34 -Tin) + 0.0173⋅M⋅(5.87pin),

where Tin and pin denote air temperature and pressure inside the PFM, respectively. 167

Then, the modified governing equation of coupled SPFM-HEATS model is: 168

169

Detailed information about each variable in the SPFM model is shown in Table 1 . 170 

Second, due to the spatial variability of terrain gradients (Figure 1c 

where m is the average human body weight, kg (m = 60 kg in this study), l is the carriage loads, 193 kg, η is terrain coefficient (1.0 for hard surfaces or pavements). 194

In addition, to better represent the spatiotemporal variation of W, we determine 195 mechanical work (W, W/m 2 ) based on realistic walking speeds and terrain gradients [28] , as 196

where g is the acceleration rate due to gravity (m/s 2 ), A is a standard value of body surface 198 area (A = 1.8 m 2 in this study [28] ). To drive the HEATS model, meteorological parameters, terrain data, physiological parameters, 206 clothing properties, and PFM thermal characteristics need to be collected as inputs (Table 2) Table 3 . In particular, Figure 3 shows the detailed 243 measurement set-up in the facial area. Figure 3a shows six measurement sites of skin-based 244 thermo-physiological variables on the facial area, including sites 1-4 inside the PFM and sites 245 5-6 outside the PFM. In addition, Figure 3b shows the set-up of two sensors including an 246 iButton sensor and a gas analyser for measuring (Tin, RHin) and (O2, CO2), respectively, in the 247 PFM microenvironment. The iButton sensor was fixed on the inner surface of PFM pointing 248 inward with a 0.5-cm distance from the mouth. The gas analyser was connected with a tube 249 positioned inside the PFM so that the air in the PFM microenvironment can be withdrawn and 250 analysed ( Figure 3a ). Experimental results can be used for model validation and will be 251 elaborated in section 3.1. 

In this section, we analysed the experimental results of facial thermal conditions due 287

to PFMs under three different activity levels (standing, walking, and climbing) as described in 288 detailed measured data about the PFM-induced facial thermal stress can be found in Table 5 . 298

The suppressed heat dissipation from facial skin and respiration due to PFMs can modify the 

To investigate the differences of PFM-aggravated thermal stresses in urban and rural Residents walking on a flat terrain (0°) with PFMs may feel equivalent thermal stress as 326 walking on an 30° inclined terrain without PFMs. On the other hand, it is noteworthy that urban 327 terrain slope significantly varies from 0° to 60° in the Hong Kong metropolitan area (Figure 4c) , 328 resulting in that urban residents tend to suffer from aggravated thermal stress with a maximum 329 PST increment of more than 20 °C due to the compound effects of PFM wearing and high-330 intensity activity levels such as climbing during their daily life (Figure 7a) . 

To investigate PFM aggravated heat risks over the whole Hong Kong metropolitan 356 area, we derived the spatial map of PST without and with PFM via the HEATS model for the 

Extreme heat events add extra heat risks for residents in PFM-wearing culture. Many 393 cases of the PFM-related heat stroke or even death was reported in 2020, which causes 394 widespread concern about the side effects of PFMs. During high-intensity activities like hiking, 395 human bodies need to consume more oxygen with accelerated or doubled breathing rates. 396

Since PFMs are not well ventilated and close to the face, they may quickly get damp due to 397 intensive breathing and sweating. Significant CO2 surges and O2 reductions were found inside 398 sweaty PFM during our field experiment (Figure 11 ), implying that sweaty PFMs can aggravate 399 the breathing discomfort sensation. Although wearing PFMs is uncomfortable in breathing 400 sensation and thermal sensation, it is unavoidable as a necessary personal protective 401 equipment during the pandemic. To deal with the dilemma, hikers sometimes use mask 402 brackets to improve their breathing and thermal comfort, since mask brackets can help keep 403 a constant-volume microenvironment for heat and air exchanges. With the use of mask 404 bracket, we found a decrease in Ta but an increase in RH inside the cavity (Figure 6a and 405 Figure 6b ), possibly due to enhanced sweat evaporative cooling. However, the accumulated 406 moisture inside the cavity may lead to another possibility of heatstroke due to a reduced 407 hydration level since the damp PFM microenvironment tends to make people feel fewer signs 408 of thirst [50] . 

The COVID-19 pandemic has amplified health risks in hot seasons for people 413 worldwide, especially for the following groups who are vulnerable to both infection risk and 414 heat stress risk, including people with underlying conditions (respiratory, cardiovascular and 415 cerebrovascular diseases as well as mental health issues), the elderly, children, pregnant 416 women, outdoor workers, athletes, health workers, the economically disadvantaged who might 417 lack of essential cooling and hygiene facilities, and those who have been first infected and 418 then discharged from hospitals after treatment with COVID-19 [1] [5] [41] . During the pandemic, 419 the use of PFMs in hot seasons can lead to more hospitalization rates of heatstroke due to an 420 aggravated level of thermal stress and a reduced level of hydration since the damp and humid 421 PFM microenvironment tends to make people feel less signs of thirst, according to a report 422 "Wearing masks in summer can lead to heatstroke; Japan doctors urge self-hydration" from 423 the Japan National Daily News [50] . Therefore, it is of great importance for the government 424 and public to be better prepared for the dual challenges of infection risk and heat risk. For 425 example, the Centers for Disease Control and Prevention (CDC) in the US published a 426 document "Employer information for heat stress prevention during the COVID-19 Pandemic" 427 on 26 Aug 2020 [52] , which provides heat illness prevention guidelines for employers to better 428 protect outdoor workers from the dual risks of the pandemic and heat stress. 429

In this study, we predicted a fine-resolution PFM-inclusive thermal stress and heat 430 warning map covering Hong Kong, which can offer significant guidance for the public and the Interactive Employment Service of Labor Department (IESLD). The system can provide heat 440 illness prevention guidelines for employers to better protect outdoor workers from the dual 441 risks of the pandemic and heat stress. The procedures may include adjusting work/rest 442 schedules and daily work completion targets during hot weather, providing more cooling and 443 rehydration stations, and making emergency first-aid plans [52] . On the other hand, the public 444

can also benefit from this study, such as planning or altering their outdoor activity schedules 445 according to the PFM-inclusive heat risk forecasts. For example, traffic policemen and 446 building/road construction workers shall be aware of the aggravated thermal stress in their 447 working areas and make good preparations beforehand, pedestrians can choose paths 448 crossing more comfortable neighbourhoods (shaded areas, green/blue spaces, relatively flat 449 paths) with less heat risks, and hikers can make less dangerous plans such as choosing hiking 450 trails with smaller terrain slopes, more tree canopy shade, or more pavilions, using mask 451 frame to improve thermal and breathing comfort, and bringing spare clean masks to replace 452 sweated ones. 453

In this study, we investigated the aggravated human thermal stress due to PFMs over swealtering). Particularly, urban residents may suffer more intensive PFM-aggravated thermal 469 stress at night than rural residents due to synergistic urban heat and moisture island effects. 470

Besides, PFM-inclusive PST was found to increase nonlinearly with terrain slopes at a rate of 471 1.3~3.9 o C/10°, owing to increased metabolic heat production when climbing on inclined 472 terrains, which is usually overlooked but very important for residents in mountainous cities. 473

Residents walking on a flat terrain (0°) with PFMs may feel equivalent thermal stress as 474 walking on an 30° inclined terrain without PFMs. In addition, we found that PFMs not only 475 affect people's thermal comfort, but also breathing comfort. Particularly, a high concentration 476 of CO2 and a low concentration of O2 were found inside sweaty PFMs, which may cause where m is the average human body weight, kg, l is the carriage load, kg, η is terrain 513 coefficient (1.0 for hard surfaces or pavements), v is the walking speed, slope is the terrain 514 gradient. The total metabolic cost M combines 4 sub-components, M1 is the metabolic cost of 515 J o u r n a l P r e -p r o o f 29 standing without load, which is proportional to the body's weight (m), M2 is the metabolic cost 516 of standing with load, which is affected by the total weight of body and load (m + l) and is fitted 517 as a function of the load-to-weight ratio squared ((l/m) 2 ), M3 is the metabolic cost of walking 518 on a flat terrain, which is related to a specific terrain form (η), and is fitted as a function of the 519 walking speed squared (v 2 ), M4 is the metabolic cost of climbing on an inclined terrain, which 520 is related to a specific terrain form (η) and total weight (m + l) and is fitted as a linear function 521 of the walking speed (v) and terrain gradient (slope). 522 523 J o u r n a l P r e -p r o o f 

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 We studied PFM-induced human thermo-physiological responses via experiments.  We proposed a new PFM-inclusive human thermal stress model.  Wearing PFMs can lead to additional 158.4% of residents exposed to 'sweltering' heat risk.  Urban residents tend to suffer more PFM-aggravated heat risks than rural residents at nights.  PFM-aggravated thermal stress increases nonlinearly with terrain slopes.

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:J o u r n a l P r e -p r o o f