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08:30-10:30 Session 11: Occupational heat strain in a warming world
Managing occupational heat stress in a diverse working population

ABSTRACT. Despite extensive implementation of procedures to mitigate heat-illness in workers, heat-related morbidity/mortality remains elevated. This can in part be attributed to the fact that exposure limits for work in the heat recommended by occupational safety agencies worldwide assume a “one size fits all” approach to safeguard the health and safety of workers. They fail to consider key factors such as sex, age, health status and others which we reported can markedly alter a person’s tolerance to heat thereby leaving a large segment of the workforce under-protected. For instance, we showed that the body’s response to heat is impaired in healthy adults as young as 40 years. This impairment is worse in women and in older adults, especially in those with health conditions (e.g., diabetes, hypertension, other). In these individuals, heat tolerance is reduced as the body is unable to cool itself via the evaporation of sweat, placing them at increased risk of heat-induced illnesses/death. These limitations are exemplified by our findings on exposure limits for workers. For decades, industry has relied upon government recommended exposure limits such as the American Conference of Governmental Industrial Hygienists Threshold Limit Values to manage the health and safety of workers exposed to heat. However, we showed these guidelines fail to protect workers, especially older (≥40 years) workers, from potentially dangerous increases in core temperature which is an underlying cause for many workplace injuries and reduced productivity. This presentation examines the shortcomings of the current ‘one size fits all’ approach to managing occupational heat strain. Further, we evaluate future directions for preventing heat-related illness and maximising productivity in our diverse working population through the application of individualized work exposure limits, and the protection of workers through the application of technologies to track heat strain.

Support: Government of Ontario (ROP-16-R-036)

Inter-individual factors and screening criteria for occupational heat strain

ABSTRACT. Introduction: Climate-driven increases in heat exposure as well as societal changes such as population aging generate important public health risks, as specific population groups have increased susceptibility to heat stress. Most heat-induced deaths are of cardiovascular origin, yet there is no increase of cardiovascular admissions during heatwaves. This has led to the conclusion that deaths from cardiovascular disease during heatwaves occur rapidly before the patient is admitted to a hospital and that the first hours of heat exposure have a major impact on cardiovascular mortality. To address this vital public health issue, it is vital to develop criteria to identify and, ultimately, protect individuals who are more susceptible to heat stress, particularly those who are still part of the workforce and, thus, are physically active during periods of increased heat. This presentation will describe simple and practical sex-specific screening criteria for detecting susceptibility to heat stress during work and leisure activities in hot environments. The screening criteria have been developed from simple information derived from age, anthropometry, and cardiorespiratory fitness and have been recently validated during large studies in laboratory as well as occupational settings. This work has been supported by European Union's Horizon 2020 research and innovation programme under the project ‘Heat-Shield’ (grant agreement no. 668786). Conclusions: The developed criteria are based on robust risk factors that have been rigorously studied during the past decades and can be used as simple and effective means for detecting the vast majority of people who are less able to work or play in hot environments owing to their reduced capacity to dissipate heat. Therefore, the proposed criteria can play an important role in preventing and mitigating the public health risks caused by increased ambient temperatures.

How well do thermal Indices Quantify the Magnitude of Occupational Heat Strain?

ABSTRACT. Introduction: Hundreds of thermal indices have been introduced aiming to quantify the magnitude of heat stress/strain, reduce heat related illness, and enhance productivity. However, the exact number of thermal indices which are depended to at least one out of the four environmental factors (air temperature, humidity, radiant heat, and air velocity) remains unknown. Moreover, it is unclear which of these thermal indices actually reflect heat strain and physiological function in occupational settings. Therefore, the purpose of this study was to identify all the thermal indices developed during the last century, by conducting a systematic review and meta-analysis of the literature. Methods: Following PRISMA guidelines, we searched six databases for relevant studies. No language or any other study design limits were applied. Furthermore, we supplemented the electronic database searches with manual searches for published studies in international trial registers, ISO standards, and websites of international agencies (e.g., World Health Organization and World Meteorological Organization). Funding was provided by ‘Heat-Shield’, European Union's Horizon 2020 research and innovation programme under the Grant agreement no. 668786. Results: Of 556 records identified through our systematic search, 104 studies written in 9 different languages met the eligibility criteria for inclusion in the study. The analyzed studies included 172 thermal indices. The first index identified was developed in 1915 while the last one in 2016. Interestingly, a very strong relationship was found between the year of development and the number of thermal indices developed (r=0.97, p<0.01), indicating that more indices should be expected in the future. Finally, a computer software has been developed to simplify the calculation of the identified thermal indices. Conclusions: Based on the systematic review, this presentation will describe the relationship between thermal indices and the thermophysiological responses of workers who work in different industrial sectors around the world.

Interactive effects of aging and other individual factors on occupational heat strain

ABSTRACT. Workers in many industries (e.g. mining, electric utilities, among others) often perform prolonged, strenuous work in the heat. Such conditions cause considerable heat strain, which can elevate heat illness risk. The challenge to temperature regulation posed by occupational heat stress is particularly great for the rising number of older workers, who demonstrate marked reductions in heat loss capacity (primarily via sweating) relative to their young counterparts. However, while our understanding of the independent effects of aging on thermoregulatory function is extensive, we know considerably less regarding the interactive effects of aging and other inter-individual factors (e.g., sex, chronic disease, others) and intra-individual factors that vary commonly both within and among work days (e.g., hydration and acclimation state, consecutive work shifts, work type). Although some of these factors may exert a restorative effect on thermoregulatory function in older adults, others may exacerbate occupational heat strain by increasing the magnitude of any age-related impairments in heat loss. Improving our understanding of these effects is therefore critical for stratifying risk of heat-related injury during occupational heat exposure. In recent years, work from our laboratory has begun to uncover the interactive effects of age and other inter-individual factors (e.g., aerobic fitness, chronic disease, sex) as well as intra-individual modulators (e.g., hypohydration, heat acclimation) on whole-body heat exchange. This presentation will describe these observations, providing attendees with a unique calorimetric perspective on the potentially divergent effects of these factors in young and older workers, while also highlighing important gaps in our understanding that warrant futher invesigtation.

Support: Government of Ontario (ROP-16-R-036)

Personalized Heat Warning - Alert and Advising Systems for improved Health

ABSTRACT. The present talk will provide two examples on personalized heat alert-advice systems, both aiming at improving information targeting the individual to minimize negative effects of thermal stress on human health and daily day functioning.

Excess deaths during heat waves signify the importance of weather-warning systems and accompanying preventive plans for minimizing hazardous effects of extreme thermal events. However, human health and performance are affected at much lower environmental heat strain levels than those directly associated with increased mortality. This presentation will discuss and present examples of the importance for individualized, appropriate heat alert. One example from the Heat-Shield project specifically focused on occupational health and the second system developed as a smartphone application (ClimApp) with several thermal models covering a broader range of thermal scenarios to support suitable thermoregulatory behavior during challenging climatic conditions. Both systems rely on thermo-physiological models and translation of general weather warnings into individual alert and personalized adaptation strategies. For a scenario representing a typical sunny summer day in southern Europe (30°C dry air and WBGTsun = 27°C; equal or higher heat observed more than 80 days in Italy in 2018), an outdoor worker performing demanding manual tasks may experience high heat stress and a total sweat loss of ~ 10 liter during a work shift, while an age and gender matched indoor worker performing light manual tasks may loose only ~1½ liter over an entire day. Considering, that inter-individual and gender differences will add further variation, it becomes clear that personalized guidance on hydration and behavior to secure health is of major importance. This should be incorporated in heat-alert systems aiming at supporting workers ability, maintaining healthy and productive lives, which in addition should be considered as integrated parts of public health.

11:00-12:30 Session 12: Heat exposure
The effect of ethnicity on whole-body heat exchange in first generation Black and White Canadians during exercise in the heat

ABSTRACT. Introduction: Studies show that Black individuals born in temperate climates (e.g., North America) display higher heat-related mortality than their White counterparts (CDC,2017:4), suggesting that Blacks may be at a thermoregulatory disadvantage during heat stress. However, evidence of ethnicity-related differences in heat loss is equivocal (JAP,2004,96:127; AJPA,1958,3:296). We therefore used direct calorimetry to evaluate the effects of ethnicity on whole-body heat exchange in first-generation Canadians during exercise in the heat, hypothesizing that Blacks would display reductions in total heat loss (evaporative ± dry heat exchange) that exacerbate body heat storage relative to Whites. Method: Whole-body total heat loss and body heat storage were assessed in 8 young, first-generation Black and White Canadians, with matched fitness and physical characteristics. Participants performed three, 30-min bouts of cycling at fixed rates of metabolic heat productions of 200 [Light], 250 [Moderate], 300 W/m2 [Vigorous] (equivalent to ~30, 45 and 60% of VO2peak), separated by 15-min recovery, in dry-heat (40°C, 15% relative humidity). Results: Total heat loss (mean ± SD) was similar between groups (all P>0.05), averaging 181±9 (Light), 224±11 (Moderate) and 251±21 W/m2 (Vigorous) in Blacks, and 176±15, 222±19 and 254±15 W/m2 in Whites. Accordingly, cumulative body heat storage across all exercise bouts did not differ significantly between Blacks (492±146 kJ) and Whites (535±125 kJ) (P=0.54). Conclusions: In first-generation Black and White Canadians, ethnicity does not significantly modulate whole-body total heat loss nor body heat storage, regardless of the exercise-induced heat load. These outcomes indicate that increases in vulnerability among first-generation Black Canadians are not owed to impairments in the body’s capacity to dissipate heat. Funding: Natural Sciences and Engineering Research Council of Canada (RGPIN-2014-06313).

Skin Wettedness Independently Modulates Thermal Behavior during Passive Heat Stress
PRESENTER: Nicole Vargas

ABSTRACT. Introduction: Increased skin wettedness contributes to thermal behavior. We tested the hypothesis that greater skin wettedness augments thermal behavior during passive heat stress (HS). Methods: Eleven healthy subjects (25+/-2 y; 5 females) donned a water perfused suit (WPS) and completed three trials in a 28.5+/-0.4ºC environment that involved a 20 min baseline period, a 60 min period in which ambient humidity was elevated to 26+/-3 (HUM30), 49+/-4 (HUM50) or 67+/-5% RH (HUM70), and then 60 min of progressive HS with a stable ambient humidity. 34ºC water perfused the WPS for the first 80 min. Subjects thermally behaved when their neck was thermally uncomfortable by pressing a button. Each button press initiated 30 s of -20ºC antifreeze perfusing through a custom-made device on the dorsal neck. Mean skin temperature (Tsk, 10-site), intestinal temperature (Tcore, telemetry pill), mean skin wettedness (Wsk, 10-site), and neck device temperature (Tdevice) were measured continuously. Thermal behavior was determined from Tdevice, while the motivation to behaviorally thermoregulate was determined by cumulative button presses. Data are presented as mean +/- SD. Results: Tsk (by +1.9+/-0.1ºC) and Tcore (by +0.4+/-0.0ºC) increased during HS (P≤0.04), but were not different between conditions (P≥0.15). Wsk was elevated in HUM70 vs. HUM30 at 30-60 min (60 min: by +0.06+/-0.09 a.u.), and in HUM70 vs. HUM50 at 40-50 min (50 min: by +0.10+/-0.08 a.u.) of HS (P≤0.03). Tdevice was lower for HUM70 vs. HUM30 at 40-50 min (50 min: -3.4+/-5.7ºC), and in HUM70 vs. HUM50 at 30-40 min (40 min: -5.5+/-5.6ºC) of HS (P≤0.04). Cumulative button presses were greater for HUM70 vs. HUM30 (60 min: +3+/-4 presses) and HUM50 (60 min: +2+/-7 presses) from 40-60 min of HS (P≤0.02). Conclusions: Skin wettedness independently modulates thermal behavior and increases the motivation to thermally behave during passive HS.

Quantifying Physical Work Capacity in the Heat: One Hour versus Full-Day Heat Exposure
PRESENTER: James Smallcombe

ABSTRACT. Introduction: Physiological responses to heat stress are commonly assessed during short experimental exposures. Under the climate change context, this approach has been used in laboratories to quantify the effects of high workplace temperatures on physical work capacity (PWC). However, in real world occupational settings, workers typically experience heat stress over much longer periods (i.e. up to a full shift). This ongoing study examines both the changes in PWC and physiological responses during a 7-hour simulated working day in the heat.

Methods: Four males (age = 24.7 (2.6 SD)) completed six 50-min work-bouts, interspersed by 10-min rest intervals. Between the third and fourth work-bout, a standardised meal was provided during a 1-hour lunch break. Work was performed on a treadmill at a fixed cardiovascular strain (heart rate of 130 b·min-1). Rectal (Tre) temperature was monitored continuously, and PWC during each work-bout was quantified as the energy expended (kJ). Counterbalanced work days were performed in a cool (reference) environment (WBGT = 12°C) and in heat (WGBT = 29; 33; 36°C).

Results: PWC declined as WBGT increased. Relative to the reference condition, percentage PWC as quantified from the first work-bout of the day versus the sum of all 6 work-bouts was: 29°C WBGT: 90.4 vs. 89.6%; 33°C WBGT: 68.9 vs. 66.6%; 36°C WBGT: 37.9 vs. 27.9%. Hence, a synergistic degradation of PWC with repeated work bouts was observed at 36°C WBGT only. Additionally, Tre increased with WBGT but did not differ within conditions across repeated work-bouts.

Conclusion: The preliminary evidence suggests that changes in PWC during a full working day may be estimated from a single work-bout. Only under the most severe, uncompensable heat stress conditions may this approach underestimate the degradation of PWC. Ongoing data collection aims to extend these findings.

Acknowledgements: Funding was provided by ‘Heat-Shield’, European Union's Horizon 2020 research and innovation programme under the Grant agreement no. 668786.

Drinking to thirst sufficiently off-sets dehydration during a 3-h simulated heatwave exposure in young healthy individuals

ABSTRACT. Rationale: To compare the impact of different fluid replacement practices on the development of dehydration and the associated changes in thermal and cardiovascular strain during an ecologically valid heatwave simulation.

Methods: Twelve participants (25±4 years) completed four separate 180-min trials, exercising at a fixed average intensity of 3 METs in 40.1±0.6°C, 40.4±2.1%RH. In each trial a different hydration plan was employed; i) ad libitum consumption of 20°C water (ALTAP); ii) ad libitum consumption of 4°C water (ALCHILL); iii) no fluid replacement (NOFR); iv) full replacement of sweat loss (FULLFR). Fluid consumption (FC), resultant dehydration (%DEH), rectal temperature (Tre), rate pressure product (RPP), mean skin temperature (Tsk), and local sweat rate (LSR) were measured/determined.

Results: FC was greater in ALTAP (1.30±0.41 L) than ALCHILL (1.03±0.32 L; P=0.003). %DEH in NOFR was 1.93±0.28%. In comparison, %DEH was lower in ALCHILL (0.43±0.64%; P<0.0001), ALTAP (0.11±0.76%; P<0.0001), and FULLFR (0.01±0.12%; P<0.0001). The change in Tre from rest was greater in NOFR trial (1.05±0.27˚C) compared to ALTAP (0.72±0.30˚C; P<0.0001), FULLFR (0.74±0.35˚C; P<0.0001) and ALCHILL (0.76±0.25˚C; P<0.0001). After 180 min, RPP was higher in NOFR (12389±1578 mmHg·min-1) than in FULLFR (11067±1292 mmHg·min-1; P<0.0001), ALCHILL (11089±1795 mmHg·min-1; P<0.0001) and ALTAP (11214±2078 mmHg·min-1; P<0.001). No differences in Tsk or LSR were observed between trials.

Conclusions: No fluid replacement throughout a 180-min exposure to realistic heatwave conditions at a work rate matching everyday activity levels exacerbated both thermal and cardiovascular strain. Ad libitum consumption of 4°C or 20°C water was sufficient to prevent levels of dehydration that exacerbate physiological heat strain, with 4°C water seeming to blunt thirst more, and hence fluid intake relative to 20°C water.

Extending work tolerance time in the heat in protective ensembles with commercially available cooling methods
PRESENTER: Matthew Maley

ABSTRACT. Introduction Physical roles necessitating the use of chemical, biological, radiological, or nuclear (CBRN) protective ensembles in the heat accentuate thermal and cardiovascular strain during work. As a result, work tolerance times are shortened with an increased threat of heat exhaustion if work is maintained. We, therefore, investigated commercially available cooling methods and their ability to reduce physiological strain and/or extend work tolerance time in those working in the heat dressed in a CBRN protective ensemble.

Methods Eight males wore a CBRN ensemble (MT94, Lion Apparel, USA; 15.3 kg) and walked for a maximum of 120 minutes at 35 °C, 50 % relative humidity. In a randomised order, participants completed the trial with no cooling and one of four cooling protocols: 1) ice-based cooling vest (IV), 2) a non-ice-based cooling vest (PCM), 3) ice slushy consumed before work, combined with IV (SLIV) and 4) a portable battery-operated water-perfused suit (WPS). Mean with 95 % confidence intervals are presented.

Results Tolerance time was extended in PCM (46 [36–56] mins, P = 0.018), SLIV (56 [46–67] mins, P < 0.001) and WPS (62 [53–70] mins, P < 0.001), compared with control (39 [30–48] mins). Tolerance time was longer in SLIV and WPS compared with both IV (48 [39–58 mins]) and PCM (P ≤ 0.011). After 20 min of work, HR was lower in SLIV (121[105–136] beats·min–1) and WPS (117[101–133] beats·min–1) compared with control (137[120–155] beats·min–1), IV (130[116–143] beats·min–1) and PCM (133[116–151] beats·min–1) (all P < 0.001).

Conclusion All cooling methods utilised in the present study can reduce physiological strain, while SLIV and WPS are most likely to extend tolerance time for those working in the heat dressed in a CBRN ensemble.