Metabolism- and sex-dependent critical WBGT limits at rest and during exercise in the heat

Heat stress vulnerability and critical environmental limits for older adults

The present study examined heat stress vulnerability of apparently healthy older vs. young adults and characterized critical environmental limits for older adults in an indoor setting at rest (Rest) and during minimal activity associated with activities of daily living. Critical environmental limits are combinations of ambient temperature and humidity above which heat balance cannot be maintained (i.e., becomes uncompensable) for a given metabolic heat production. Here we exposed fifty-one young (23±4 yrs) and 49 older (71±6 yrs) adults to progressive heat stress across a wide range of environments in an environmental chamber during Minimal Activity (young and older subjects) and Rest (older adults only). Heat compensability curves were shifted leftward for older adults indicating age-dependent heat vulnerablity (p < 0.01). During Minimal Activity, critical environmental limits were lower in older compared to young adults (p < 0.0001) and lower than those at Rest (p < 0.0001). These data document heat vulnerability of apparently healthy older adults and to define critical environmental limits for indoor settings in older adults at rest and during activities of daily living, and can be used to develop evidence-based recommendations to minimize the deleterious impacts of extreme heat events in this population.

Assessing the Effect of Extreme Weather on Population Health Using Consumer-Grade Wearables in Rural Burkina Faso: Observational Panel Study

BACKGROUND

Extreme weather, including heat and extreme rainfall, is projected to increase owing to climate change, which can have adverse impacts on human health. In particular, rural populations in sub-Saharan Africa are at risk because of a high burden of climate-sensitive diseases and low adaptive capacities. However, there is a lack of data on the regions that are anticipated to be most exposed to climate change. Improved public health surveillance is essential for better decision-making and health prioritization and to identify risk groups and suitable adaptation measures. Digital technologies such as consumer-grade wearable devices (wearables) may generate objective measurements to guide data-driven decision-making.

OBJECTIVE

The main objective of this observational study was to examine the impact of weather exposure on population health in rural Burkina Faso using wearables. Specifically, this study aimed to assess the relationship between individual daily activity (steps), sleep duration, and heart rate (HR), as estimated by wearables, and exposure to heat and heavy rainfall.

METHODS

Overall, 143 participants from the Nouna health and demographic surveillance system in Burkina Faso wore the Withings Pulse HR wearable 24/7 for 11 months. We collected continuous weather data using 5 weather stations throughout the study region. The heat index and wet-bulb globe temperature (WBGT) were calculated as measures of heat. We used linear mixed-effects models to quantify the relationship between exposure to heat and rainfall and the wearable parameters. Participants kept activity journals and completed a questionnaire on their perception of and adaptation to heat and other weather exposure.

RESULTS

Sleep duration decreased significantly (P<.001) with higher heat exposure, with approximately 15 minutes shorter sleep duration during heat stress nights with a heat index value of ≥25 °C. Many participants (55/137, 40.1%) reported that heat affected them the most at night. During the day, most participants (133/137, 97.1%) engaged in outdoor physical work such as farming, housework, or fetching water. During the rainy season, when WBGT was highest, daily activity was highest and increased when the daily maximum WBGT surpassed 30 °C during the rainiest month. In the hottest month, daily activity decreased per degree increase in WBGT for values >30 °C. Nighttime HR showed no significant correlation with heat exposure. Daytime HR data were insufficient for analysis. We found no negative health impact associated with heavy rainfall. With increasing rainfall, sleep duration increased, average nightly HR decreased, and activity decreased.

CONCLUSIONS

During the study period, participants were frequently exposed to heat and heavy rainfall. Heat was particularly associated with impaired sleep and daily activity. Essential tasks such as harvesting, fetching water, and caring for livestock expose this population to weather that likely has an adverse impact on their health. Further research is essential to guide interventions safeguarding vulnerable communities.

Relatively minor influence of individual characteristics on critical wet-bulb globe temperature (WBGT) limits during light activity in young adults (PSU HEAT Project)

Critical environmental limits are temperature-humidity thresholds above which heat balance cannot be maintained for a given metabolic heat production. This study examined the association between individual characteristics [sex, body surface area (AD), aerobic capacity (V̇o2max), and body mass (mb)] and critical environmental limits in young adults at low metabolic rates. Forty-four (20 M/24 F; 23 ± 4 yr) subjects were exposed to progressive heat stress in an environmental chamber at two low net metabolic rates (Mnet); minimal activity (MinAct; Mnet = ∼160 W) and light ambulation (LightAmb; Mnet = ∼260 W). In two hot-dry (HD; ≤25% rh) environments, ambient water vapor pressure (Pa = 12 or 16 mmHg) was held constant and dry-bulb temperature (Tdb) was systematically increased. In two warm-humid (WH; ≥50% rh) environments, Tdb was held constant at 34°C or 36°C, and Pa was systematically increased. The critical wet-bulb globe temperature (WBGTcrit) was determined for each condition. During MinAct, after entry of Mnet into the forward stepwise linear regression model, no individual characteristics were entered into the model for WH (R2adj = 0.01, P = 0.27) or HD environments (R2adj = -0.01, P = 0.44). During LightAmb, only mb was entered into the model for WH environments (R2adj = 0.44, P < 0.001), whereas only V̇o2max was entered for HD environments (R2adj = 0.22; P = 0.002). These data demonstrate negligible importance of individual characteristics on WBGTcrit during low-intensity nonweight-bearing (MinAct) activity with a modest impact of mb and V̇o2max during weight-bearing (LightAmb) activity in extreme thermal environments.NEW & NOTEWORTHY Our laboratory has recently published a series of papers establishing the upper ambient temperature-humidity thresholds for maintaining heat balance, termed critical environmental limits, in young adults. However, no studies have investigated the relative influence of individual characteristics, such as sex, body size, and aerobic fitness, on those environmental limits. Here, we demonstrate the contributions of sex, body mass, body surface area, and maximal aerobic capacity on critical wet-bulb globe temperature (WBGT) limits in young adults.

Adverse heat-health outcomes and critical environmental limits (Pennsylvania State University Human Environmental Age Thresholds project)

BACKGROUND

The earth's climate is warming and the frequency, duration, and severity of heat waves are increasing. Meanwhile, the world's population is rapidly aging. Epidemiological data demonstrate exponentially greater increases in morbidity and mortality during heat waves in adults ≥65 years. Laboratory data substantiate the mechanistic underpinnings of age-associated differences in thermoregulatory function. However, the specific combinations of environmental conditions (i.e., ambient temperature and absolute/relative humidity) above which older adults are at increased risk of heat-related morbidity and mortality are less clear.

METHODS

This review was conducted to (1) examine the recent (past 3 years) literature regarding heat-related morbidity and mortality in the elderly and discuss projections of future heat-related morbidity and mortality based on climate model data, and (2) detail the background and unique methodology of our ongoing laboratory-based projects aimed toward identifying the specific environmental conditions that result in elevated risk of heat illness in older adults, and the implications of using the data toward the development of evidence-based safety interventions in a continually-warming climate (PSU HEAT; Human Environmental Age Thresholds).

RESULTS

The recent literature demonstrates that extreme heat continues to be increasingly detrimental to the health of the elderly and that this is apparent across the world, although the specific environmental conditions above which older adults are at increased risk of heat-related morbidity and mortality remain unclear.

CONCLUSION

Characterizing the environmental conditions above which risk of heat-related illnesses increase remains critical to enact policy decisions and mitigation efforts to protect vulnerable people during extreme heat events.

Core temperature responses to compensable versus uncompensable heat stress in young adults (PSU HEAT Project)

With global warming, much attention has been paid to the upper limits of human adaptability. However, the time to reach a generally accepted core temperature criterion (40.2°C) associated with heat-related illness above (uncompensable heat stress) and just below (compensable heat stress) the upper limits for heat balance remains unclear. Forty-eight (22 men/26 women; 23 ± 4 yr) subjects were exposed to progressive heat stress in an environmental chamber during minimal activity (MinAct, 159 ± 34 W) and light ambulation (LightAmb, 260 ± 55 W) in warm-humid (WH; ∼35°C, >60% RH) and hot-dry (HD; 43°C-48°C, <25% RH) environments until heat stress became uncompensable. For each condition, we compared heat storage (S) and the change in gastrointestinal temperature (ΔTgi) over time during compensable and uncompensable heat stress. In addition, we examined whether individual characteristics or seasonality were associated with the rate of increase in Tgi. During compensable heat stress, S was higher in HD than in WH environments (P < 0.05) resulting in a greater but more variable ΔTgi (P ≥ 0.06) for both metabolic rates. There were no differences among conditions during uncompensable heat stress (all P > 0.05). There was no influence of sex, aerobic fitness, or seasonality, but a larger body size was associated with a greater ΔTgi during LightAmb in WH (P = 0.003). The slopes of the Tgi response during compensable (WH: MinAct, 0.06, LightAmb, 0.09; HD: MinAct, 0.12, LightAmb, 0.15°C/h) and uncompensable (WH: MinAct, 0.74, LightAmb, 0.87; HD: MinAct, 0.71, LightAmb, 0.93°C/h) heat stress can be used to estimate the time to reach a target core temperature from any given starting value.NEW & NOTEWORTHY This study is the first to examine heat storage and the rate of change in core temperature above (uncompensable heat stress) and just below (compensable heat stress) critical environmental limits to human heat balance. Furthermore, we examine the influence of individual subject characteristics and seasonality on the change in core temperature in warm-humid versus hot-dry environments. We provide the rate of change in core temperature, enabling projections to be made to and from any hypothetical core temperature.

Utility of the Heat Index in defining the upper limits of thermal balance during light physical activity (PSU HEAT Project)

Extreme heat events and consequent detrimental heat-health outcomes have been increasing in recent decades and are expected to continue with future climate warming. While many indices have been created to quantify the combined atmospheric contributions to heat, few have been validated to determine how index-defined heat conditions impact human health. However, this subset of indices is likely not valid for all situations and populations nor easily understood and interpreted by health officials and the public. In this study, we compare the ability of thresholds determined from the National Weather Service's (NWS) Heat Index (HI), the Wet Bulb Globe Temperature (WBGT), and the Universal Thermal Climate Index (UTCI) to predict the compensability of human heat stress (upper limits of heat balance) measured as part of the Pennsylvania State University's Heat Environmental Age Thresholds (PSU HEAT) project. While the WBGT performed the best of the three indices for both minimal activities of daily living (MinAct; 83 W·m-2) and light ambulation (LightAmb; 133 W·m-2) in a cohort of young, healthy subjects, HI was likewise accurate in predicting heat stress compensability in MinAct conditions. HI was significantly correlated with subjects' perception of temperature and humidity as well as their body core temperature, linking perception of the ambient environment with physiological responses in MinAct conditions. Given the familiarity the public has with HI, it may be better utilized in the expansion of safeguard policies and the issuance of heat warnings during extreme heat events, especially when access to engineered cooling strategies is unavailable.

Heat exposure limits for young unacclimatized males and females at low and high humidity

Little is known about the separate and combined influences of humidity conditions, sex, and aerobic fitness on heat tolerance in unacclimatized males and females. The purpose of the current study was to describe heat tolerance, in terms of critical WBGT (WBGT_crit), in unacclimatized young males and females in hot-dry (HD) and warm-humid (WH) environments. Eighteen subjects (9 M/9F; 21 ± 2 yr) were tested during exercise at 30% V̇O_2max in a controlled environmental chamber. Progressive heat stress exposures were performed with either (1) constant dry-bulb temperature (T_db) of 34 and 36 °C and increasing ambient water vapor pressure (P_a) (P_crit trials; WH); or (2) constant P_a of 12 and 16 mmHg and increasing T_db (T_crit trials; HD). Chamber T_db and P_a, and subject esophageal temperature (T_es), were continuously monitored throughout each trial. After a 30-min equilibration period, progressive heat stress continued until subject heat balance could no longer be maintained and a clear rise in T_es was observed. Absolute WBGT_crit and WBGT_crit adjusted to a metabolic rate of 300 W (WBGT₃₀₀), and the difference between WBGT_crit and occupational exposure limits (OEL; ΔOEL) was assessed. WBGT_crit, WBGT₃₀₀, and ΔOEL were higher in WH compared to HD (p < 0.0001) for females but were the same between environments for males (p ≥ 0.21). WBGT_crit was higher in females compared to males in WH (p < 0.0001) but was similar between sexes in HD (p = 0.44). When controlling for metabolic rate, WBGT₃₀₀ and ΔOEL were higher in males compared to females in WH and HD (both p < 0.0001). When controlling for sex, V̇O_2max was not associated with WBGT₃₀₀ or ΔOEL for either sex (r ≤ 0.12, p ≥ 0.49). These findings suggest that WBGT_crit is higher in females compared to males in WH, but not HD, conditions. Additionally, the WBGT_crit is lower in females, but not males, in HD compared to WH conditions.

Critical environmental limits for young, healthy adults (PSU HEAT Project)

Critical environmental limits are those combinations of ambient temperature and humidity above which heat balance cannot be maintained for a given metabolic heat production, limiting exposure time, and placing individuals at increased risk of heat-related illness. The aim of this study was to establish those limits in young (18-34 yr) healthy adults during low-intensity activity approximating the metabolic demand of activities of daily living. Twenty-five (12 men/13 women) subjects were exposed to progressive heat stress in an environmental chamber at two rates of metabolic heat production chosen to represent minimal activity (MinAct) or light ambulation (LightAmb). Progressive heat stress was performed with either 1) constant dry-bulb temperature (T_db) and increasing ambient water vapor pressure (P_a) (P_crit trials; 36°C, 38°C, or 40°C) or 2) constant P_a and increasing T_db (T_crit trials; 12, 16, or 20 mmHg). Each subject was tested during MinAct and LightAmb in two to three experimental conditions in random order, for a total of four to six trials per participant. Higher metabolic heat production (P < 0.001) during LightAmb compared with MinAct trials resulted in significantly lower critical environmental limits across all P_crit and T_crit conditions (all P < 0.001). These data, presented graphically herein on a psychrometric chart, are the first to define critical environmental limits for young adults during activity resembling those of light household tasks or other activities of daily living and can be used to develop guidelines, policy decisions, and evidence-based alert communications to minimize the deleterious impacts of extreme heat events.NEW & NOTEWORTHY Critical environmental limits are those combinations of temperature and humidity above which heat balance cannot be maintained, placing individuals at increased risk of heat-related illness. Those limits have been investigated in young adults during exercise at 30% V̇o_2max, but not during metabolic rates that approximate those of light activities of daily living. Herein, we establish critical environmental limits for young adults at two metabolic rates that reflect activities of daily living and leisurely walking.