Heat Exposure Research Articles

Unveiling community adaptations to extreme heat events using mobile phone location data

The escalating frequency, duration, and intensity of extreme heat events have posed a significant threat to human society in recent decades. Understanding the dynamic patterns of human mobility under extreme heat will contribute to accurately assessing the risk of extreme heat exposure. This study leverages an emerging geospatial data source, anonymous cell phone location data, to investigate how people in different communities adapt travel behaviors responding to extreme heat events. Taking the Greater Houston Metropolitan Area as an example, we develop two indices, the Mobility Disruption Index (MDI) and the Activity Time Shift Index (ATSI), to quantify diurnal mobility changes and activity time shift patterns at the city and intra-urban scales. The results reveal that human mobility decreases significantly in the daytime of extreme heat events in Houston while the proportion of activity after 8 p.m. is increased, accompanied with a delay in travel time in the evening. Moreover, these mobility-decreasing and activity-delaying effects exhibited substantial spatial heterogeneity across census block groups. Causality analysis using the Geographical Convergent Cross Mapping (GCCM) model combined with correlation analyses indicates that people in areas with a high proportion of minorities and poverty are less able to adopt heat adaptation strategies to avoid the risk of heat exposure. These findings highlight the fact that besides the physical aspect of environmental justice on heat exposure, the inequity lies in the population's capacity and knowledge to adapt to extreme heat. This research is the first of the kind that quantifies multi-level mobility for extreme heat responses, and sheds light on a new facade to plan and implement heat mitigations and adaptation strategies beyond the traditional approaches.

Impacts of heat exposure on pregnant women, fetuses and newborns: a systematic review and meta-analysis.

Climate Change has wide-ranging and severe health impacts, especially for vulnerable groups. We systematically reviewed the literature (n=198 studies) on heat impacts on maternal, fetal, and neonatal health, conducted meta-analyses to quantify impacts, analysed periods of susceptibility, and graded certainty. Studies covered 66 countries and 23 outcomes. Our results showed increased odds of preterm birth of 1.04 (95%CI=1.03, 1.06) per 1°C increase in heat exposure and 1.26 (95%CI=1.08, 1.47) during heatwaves. Similar patterns were shown for stillbirths and congenital anomalies. Gestational diabetes mellitus odds increased by 28% (95%CI=1.05, 1.74) at higher exposures, whileodds of any obstetric complication increased by 25% (95%CI=1.09, 1.42) during heatwaves. Patterns in susceptibility windows vary by condition. The review demonstrated that escalating temperatures pose major threats to maternal and child health globally. Findings could inform research priorities and selection of heat-health indicators. Clearly more intensive action is needed to protect these vulnerable groups.

Smoldering ignition and transition to flaming in wooden mulch beds exposed to firebrands under wind

Spotting ignition by firebrands is a significant fire spread pathway at the wildland-urban interface (WUI), where mulch products are commonly used as landscaping materials. Mulch is typically organic in nature, thus it may be easily ignited into a smoldering mode by firebrands and subsequently transition to flaming, leading to direct flame contact and radiant heat exposure to siding materials of adjacent structures. This work quantified the thresholds of smoldering ignition of four common types of commercially available mulch (black mulch (BM), forest floor (FF), redwood (RW), and fir bark (FB)) exposed to heating by smoldering firebrand piles, and their propensity for smoldering-to-flaming transition under external winds (up to 1.4 m/s). We found that there was a minimum mass of firebrand pile to achieve smoldering ignition of mulch (e.g., ∼0.1 g for FF). Beyond this minimum mass, the required wind speed to trigger smoldering ignition generally decreased as the mass of the firebrand pile increased, agreeing well with theoretical analysis. After smoldering ignition, smoldering-to-flaming transition could be observed when the wind speed exceeded a critical value (e.g., ∼1 m/s for FF), which was not affected by the initial spotting process. To achieve smoldering-to-flaming transition, the glowing mulch had to reach a critical temperature of around 850 °C. Mulch samples with larger particle sizes were more likely to smolder and transition to flaming, due to increased oxygen supply through larger inter-particle pores and channels and better firebrand accumulation due to a more crevice-like geometry on the fuel surface. This work advances the fundamental understanding of the ignition and burning behavior of landscaping mulches, and thus contributes to the prevention of extreme WUI fire events.

Heat waves induce milkweed resistance to a specialist herbivore via increased toxicity and reduced nutrient content.

Over the last decade, a large effort has been made to understand how extreme climate events disrupt species interactions. Yet, it is unclear how these events affect plants and herbivores directly, via metabolic changes, and indirectly, via their subsequent altered interaction. We exposed common milkweed (Asclepias syriaca) and monarch caterpillars (Danaus plexippus) to control (26:14°C, day:night) or heat wave (HW) conditions (36:24°C, day:night) for 4 days and then moved each organism to a new control or HW partner to disentangle the direct and indirect effects of heat exposure on each organism. We found that the HW directly benefited plants in terms of growth and defence expression (increased latex exudation and total cardenolides) and insect her'bivores through faster larval development. Conversely, indirect HW effects caused both plant latex and total cardenolides to decrease after subsequent herbivory. Nonetheless, increasing trends of more toxic cardenolides and lower leaf nutritional quality after herbivory by HW caterpillars likely led to reduced plant damage compared to controls. Our findings reveal that indirect impacts of HWs may play a greater role in shaping plant-herbivore interactions via changes in key physiological traits, providing valuable understanding of how ecological interactions may proceed in a changing world.

Effects of cold and hot temperatures on the renal function of people with chronic disease.

This study investigated the effects of hot and cold temperature on the renal function of people with chronic diseases, such as diabetes, hypertension, and chronic kidney disease, using large-scale clinical data. We used retrospective cohort data from the Clinical Data Warehouse of the Seoul St Mary's Hospital, which contains clinical, diagnostic, laboratory, and other information about all patients who have visited the hospital since 1997. We obtained climate data from the Automated Synoptic Observing System of the Korea Meteorological Administration. The heat index was used as a measuring tool to evaluate heat exposure by indexing the actual heat that individuals feel according to temperature and humidity. The estimated glomerular filtration rate (eGFR) was calculated using the Chronic Kidney Disease Epidemiology Collaboration equation. To investigate changes in renal function trends with heat index, this study used generalized additive mixed models. Renal function decreased linearly with increasing heat index after approximately 25°C, which was considered the flexion point of temperature. A linear decrease in the eGFR was observed with the effects of 0 to 5 lag days. Although there was a correlation observed between the decrease in eGFR and temperatures below -10°C, the results did not indicate statistical significance. The results of our study provide scientific evidence that high temperatures affect the renal function of people with chronic diseases. These results can help prevent heat-related morbidity by identifying those who are more likely to develop renal disease and experience worsening renal function.

Investigating effects of sintering mean residence time on engineering properties of coal ash‐based lightweight aggregate

AbstractThis study examines the effect of sintering mean residence time (MRT) on the engineering properties and morphological structure of lightweight aggregates (LWA) manufactured from waste coal combustion ash (W‐CCA). A thermodynamics‐based framework was used to tune LWA manufacturing processes. A minimum 35% liquid phase (by mass) and a lower bound viscosity of 100 Pa·s of molten material were found necessary to successfully produce LWA. Using W‐CCA's chemical composition and FactSage thermodynamic modeling, the sintering temperature for LWA was set to 1075°C. Green spherical pellets made using a pelletizer were sintered at MRT of 5.1, 14.8, and 25.9 min to test LWA physical‐mechanical properties including, unit weight, specific gravity, water absorption, and compressive strength. Results indicated that unit weight ranged from 752 to 800 kg/m3, providing desirable lightweight properties. Oven dry specific gravity ranged from 1.24 to 1.43, complying with ENI13055 specification for LWA. Water absorption capacity decreased as MRT extended from 5.1 to 14.8 min and then increased as MRT proceeded from 14.8 to 25.9 min, achieving an average absorption capacity of 26% ± 1.5%. The inverse effect was noticed for compressive strength as LWA sintered with an MRT of 14.8 min demonstrated the highest compressive strength of 14.7 ± 1.8 MPa. The initial decrease in water absorption and increase in compressive strength are attributed to the efficient sintering of the LWA's internal structure. The inverse effect observed as MRT proceeded from 14.8 to 25.9 min was believed to be a result of thermal cracking due to extensive heat exposure. MRT of 14.8 min is recommended for LWA production using the coal ash utilized in this study for its buoyancy characteristics, sufficient absorption capability, and higher compressive strength.

Impacts of Chronic Heat Stress on Lymphocytic Proliferation and Phagocytic Assay in Sahiwal and Tharparkar Cattle

High temperatures cause many immune and physiological alterations in livestock, making them more vulnerable to a wide range of diseases. Lymphocyte proliferation assay and neutrophil phagocytic assay are frequently employed to evaluate cell-mediated immunity. The present study used in vitro culture of blood polymorphonuclear cells after animals were exposed to in vivo heat stress to examine the differing effects of heat stress on defensive responses in Sahiwal and Tharparkar calves. The objectives of this research were to contrast the thermo-adaptability of Sahiwal and Tharparkar breeds based on their immune responses. In the present study, ten male cattle aged between 1 to 1.5 years were selected and divided into two groups with five animals, each of Sahiwal and Tharparkar cattle. Animals were maintained inside a psychrometric chamber under the following conditions: 7 days acclimatization period at a thermo-neutral zone, 38°C temperature exposure for 6 hours up to 49 days, followed by seven days recovery period. Blood was collected once a week on the following days: -7, 0, 7, 14, 21, 28, 35, 42, 49 and 56. Physiological responses such as rectal temperature and respiration rate were measured daily. THI was calculated by temperature and relative humidity (RH). Following their isolation from blood, the polymorphonuclear cells were cultured at 38°C. Then, using NBT and MTT tests, respectively, phagocytosis and lymphocyte proliferation were assessed. The entire mean THI was significantly (p< 0.5) higher during heat exposure period (88.41± 1.54) when compared to the control period (64.75 ± 0.97). There is a significant high lymphocytic proliferation in Tharparkar Group when compared to Sahiwal Group cattle. Additionally, the level of PA significantly decreases during the heat exposure period when compared to control period in both the breeds and in Sahiwal group I there is a significant reduction in LPA during heat stress. The current study's findings imply that during the heat stress period, Tharparkar showed greater tolerance to heat stress than Sahiwal group.

Maintaining mitochondrial NAD+ homeostasis is key for heat-induced skeletal muscle injury prevention despite presence of intracellular cation alterations.

Mitochondrial dysfunction is implicated in heat-induced skeletal muscle (SKM) injury and its underlying mechanisms remain unclear. Evidence suggests that cellular ions and molecules, including divalent cations and adenine nucleotides, are involved in the regulation of mitochondrial function. In this study, we examined Ca2+, Mg2+, and NAD+ levels in mouse C2C12 myoblasts and SKM in response to heat exposure. During heat exposure, mitochondrial Ca2+ levels increased significantly, whereas cytosolic Ca2+ levels remained unaltered. The mitochondrial Ca2+ levels in the SKM of heat-exposed mice were 28% higher compared to control mice. No changes in cytosolic Ca2+ were detected between the two groups. Following heat exposure, cytosolic and mitochondrial Mg2+ levels were reduced by 47% and 23% in C2C12 myoblasts, and by 51% and 44% in mouse SKMs, respectively. In addition, heat exposure decreased mitochondrial NAD+ levels by 32% and 26% in C2C12 myoblasts and mouse SKMs, respectively. Treatment with the NAD+ precursor nicotinamide riboside (NR) partially prevented heat-induced depletion of NAD+. Additionally, NR significantly reduced heat-increased mitochondrial fission, mitochondrial depolarization, and apoptosis in C2C12 myoblasts and mouse SKMs. No effects of NR on heat-induced changes in intracellular Ca2+ and Mg2+ levels were observed. This study provides in vitro and in vivo evidence that acute heat stress causes alterations in mitochondrial Ca2+, Mg2+, and NAD+ homeostasis. Our results suggest mitochondrial NAD+ homeostasis as a therapeutic target for the prevention of heat-induced SKM injury.

Understanding the physiological and biological response to ambient heat exposure in pregnancy: protocol for a systematic review and meta-analysis

IntroductionClimate change increases not only the frequency, intensity and duration of extreme heat events but also annual temperatures globally, resulting in many negative health effects, including harmful effects on pregnancy...

Hiding the evidence: Preliminary investigation of heat-induced alterations to pre-existing saw mark trauma

Dismemberment and subsequent burning are common methods employed in an attempt to conceal or destroy evidence. While kerf characteristics can be utilised to identify tool(s) used for dismemberment, further research is necessary to assess the effect of burning on these characteristics. In this study, a back (tenon) saw (13 teeth per inch) was used to manually inflict trauma on Ovis aries de-fleshed femur bones (n = 18). Three different cut marks (shallow false start, incomplete cut and complete transection) were made on the mid-shaft of each bone. Subsequently, the bones were burned for 20 minutes in a muffle furnace. Three burn temperatures were assessed: 400 °C, 600 °C and 800 °C. Saw mark characteristics of each cut type were assessed and compared pre- and post-burning. All pre-existing trauma was recognisable post-burning; however, metric and morphological alterations were apparent. An increase in kerf width was observed at 600 °C in false start lesions and 800 °C in incomplete cuts. Breakaway spur thickness decreased post-burning (at 400 °C and 800 °C) but length was not significantly affected. Mean inter-striation distance decreased post burning at all temperature groups. Saw marks were distinguishable from heat-related fractures across all temperature groups. One false start lesion was obliterated at 800 °C. Exit chipping, pull-out striae as well as striation regularity appeared to be more enhanced after heat exposure. These alterations indicate a temperature-dependent impact on these characteristics. Further research is necessary to assess the role of burn duration.

Assessment of Heat Exposure and Health Outcomes in Rural Populations of Western Kenya by Using Wearable Devices: Observational Case Study.

Climate change increasingly impacts health, particularly of rural populations in sub-Saharan Africa due to their limited resources for adaptation. Understanding these impacts remains a challenge, as continuous monitoring of vital signs in such populations is limited. Wearable devices (wearables) present a viable approach to studying these impacts on human health in real time. The aim of this study was to assess the feasibility and effectiveness of consumer-grade wearables in measuring the health impacts of weather exposure on physiological responses (including activity, heart rate, body shell temperature, and sleep) of rural populations in western Kenya and to identify the health impacts associated with the weather exposures. We conducted an observational case study in western Kenya by utilizing wearables over a 3-week period to continuously monitor various health metrics such as step count, sleep patterns, heart rate, and body shell temperature. Additionally, a local weather station provided detailed data on environmental conditions such as rainfall and heat, with measurements taken every 15 minutes. Our cohort comprised 83 participants (42 women and 41 men), with an average age of 33 years. We observed a positive correlation between step count and maximum wet bulb globe temperature (estimate 0.06, SE 0.02; P=.008). Although there was a negative correlation between minimum nighttime temperatures and heat index with sleep duration, these were not statistically significant. No significant correlations were found in other applied models. A cautionary heat index level was recorded on 194 (95.1%) of 204 days. Heavy rainfall (>20 mm/day) occurred on 16 (7.8%) out of 204 days. Despite 10 (21%) out of 47 devices failing, data completeness was high for sleep and step count (mean 82.6%, SD 21.3% and mean 86.1%, SD 18.9%, respectively), but low for heart rate (mean 7%, SD 14%), with adult women showing significantly higher data completeness for heart rate than men (2-sided t test: P=.003; Mann-Whitney U test: P=.001). Body shell temperature data achieved 36.2% (SD 24.5%) completeness. Our study provides a nuanced understanding of the health impacts of weather exposures in rural Kenya. Our study's application of wearables reveals a significant correlation between physical activity levels and high temperature stress, contrasting with other studies suggesting decreased activity in hotter conditions. This discrepancy invites further investigation into the unique socioenvironmental dynamics at play, particularly in sub-Saharan African contexts. Moreover, the nonsignificant trends observed in sleep disruption due to heat expose the need for localized climate change mitigation strategies, considering the vital role of sleep in health. These findings emphasize the need for context-specific research to inform policy and practice in regions susceptible to the adverse health effects of climate change.

Performance of Iron Ore Tailings/Cement Composite at High Temperatures: Inconbustibility

Objective: This study aims to evaluate the performance of an iron ore tailings/cement composite at high temperatures, used in brick manufacturing. Specifically, it investigates the non-combustibility of the material and its structural integrity after exposure to intense heat, with an emphasis on compressive strength and the economic and sustainable viability of this composite. Theoretical Framework: The research is based on concepts of sustainability in civil construction and the reuse of industrial waste. The literature highlights the synergy between iron ore tailings and Portland cement, resulting in materials with good durability and non-combustibility. Previous studies indicate a reduction in environmental impact and the economic feasibility of using these composites as substitutes for traditional materials, in addition to the importance of maintaining structural strength under extreme conditions. Method: The adopted methodology involves the manufacture of test specimens using CP V-ARI cement and iron ore tailings, molded and cured according to Brazilian standards. Non-combustibility tests are conducted in a muffle furnace at 750°C, measuring temperature variation, smoke emission, and mass loss. Additionally, compressive strength tests are conducted before and after heat exposure to assess material degradation. Results and Discussion: The results indicate that the composite exhibits satisfactory non-combustibility properties, with an average mass loss of 3.1%, well below the permissible limit of 50%. However, a significant loss of 67.5% in compressive strength was observed after exposure to high temperatures. The discussion contextualizes these results, highlighting the need to optimize the composition to minimize strength loss, although non-combustibility remains a positive aspect. Research Implications: Practical implications include the potential use of the composite in environments subject to high temperatures, as a passive fire protection measure. Theoretically, the research contributes to the understanding of the behavior of cementitious composites with industrial waste under extreme conditions, suggesting directions for future studies and applications in civil engineering and sustainable construction. Originality/Value: This study makes an original contribution by exploring the specific combination of iron ore tailings with cement at high temperatures, providing new insights into its applicability and limitations. The relevance of the research lies in the potential reduction of environmental impact and the promotion of sustainable practices in civil construction, as well as enhancing fire safety in buildings.

Urbanization-induced warming amplifies population exposure to compound heatwaves but narrows exposure inequality between global North and South cities

Urban populations face heightened extreme heat risks attributed to urban heat islands and high population densities. Although previous studies have examined global urban population exposure to heatwaves, the influence of urbanization-induced warming is still not quantified. Here, leveraging satellite-derived near-surface air temperature data, we assess the impacts of urbanization-induced warming on heat exposure in 1028 cities worldwide. Additionally, we investigate its role in shaping disparities in heat exposure between global North and South cities. Our findings reveal that urbanization-amplified compound heatwaves exacerbate heat exposure risk in more than 90% of cities, and that this amplification is stronger in high urbanization areas. Moreover, our analysis highlights the potential for overestimating disparities between global North and South cities if urbanization-induced warming is overlooked. The inequality of higher heat exposure in the global South cities than in the global North cities will be narrowed in real scenarios due to more intense urbanization-induced warming in the global North cities. We emphasize the pivotal role of urbanization-induced heatwave intensification in heat exposure assessments and call for its inclusion in future population vulnerability evaluations to extreme heat.

Functional role of metabolic suppression in avian thermoregulation in the heat

Hypometabolism arising from active metabolic suppression occurs in several contexts among endotherms, particularly during heterothermic states such as torpor. However, observed Q10 ≈ 1 for avian resting metabolic rate within the thermoneutral zone, values far below the Q10 = 2–3 expected on the basis of Arrhenius effects, suggests hypometabolism also plays a role in birds’ thermoregulation at environmental temperatures approaching or exceeding normothermic body temperature (Tb). We evaluated the occurrence of hypometabolism during heat exposure among birds by re-analysing literature data to quantify changes in Tb and resting metabolic rate (RMR) near the upper boundary of the thermoneutral zone, at air temperatures (Tair) between the inflection above which Tb increases above normothermic levels (Tb.inf) and the upper critical limit of thermoneutrality (Tuc). Among the ∼55 % of species in which Tuc – Tb.inf > 0, Q10 < 2–3 occurred in nine of 10 orders for which suitable data exist, indicating that hypometabolism during heat exposure is widespread across the avian phylogeny. Values of Q10 < 2–3 were not restricted to small body mass, as previously proposed. Our findings support the idea that metabolic suppression reduces avian metabolic heat production and hence evaporative cooling requirements during heat exposure, with reductions of 20–30 % in RMR in some species. Moreover, these findings add to evidence that hypometabolism is an important component of heat tolerance among endotherms such as birds and tropical arboreal mammals.

Meat quality and safety issues during high temperatures and cutting-edge technologies to mitigate the scenario.

Climate change, driven by the natural process of global warming, is a worldwide issue of significant concern because of its adverse effects on livestock output. The increasing trend of environmental temperature surging has drastically affected meat production and meat product quality, hence result in economic losses for the worldwide livestock business. Due to the increasing greenhouse gas emissions, the situation would get prolonged, and heat exposure-related stress is expected to worsen. Heat exposure causes metabolic and physiological disruptions in livestock. Ruminants and monogastric animals are very sensitive to heat stress due to their rate of metabolism, development, and higher production levels. Before slaughter, intense hot weather triggers muscle glycogen breakdown, producing pale, mushy, and exudative meat with less water-holding capacity. Animals exposed to prolonged high temperatures experience a decrease in their muscle glycogen reserves, producing dry, dark, and complex meat with elevated final pH and increased water-holding capacity. Furthermore, heat stress also causes oxidative stresses, especially secondary metabolites from lipid oxidation, severely affects the functionality of proteins, oxidation of proteins, decreasing shelf life, and food safety by promoting exfoliation and bacterial growth. Addressing the heat-related issues to retain the sustainability of the meat sector is an essential task that deserves an inclusive and comprehensive approach. Considering the intensity of the heat stress effects, this review has been designed primarily to examine the consequences of hot environment temperatures and related stresses on the quality and safety of meat and secondarily focus on cutting edge technology to reduce or alleviate the situational impact.

Ground Urban Heat Island: Strengthening the Connection Between Spaceborne Thermal Observations and Urban Heat Risk Management.

As urbanization progresses under a changing climate, urban populations face increasing threats from chronically higher heat exposures and more frequent extreme heat events. Understanding the complex urban thermal exposure patterns becomes crucial for effective heat risk management. The spatial advantage of satellite thermal observations positions surface urban heat islands (SUHI) as a primary measure for such applications at the city scale. However, satellite-inherent biases pose considerable uncertainties. To improve the representation of human-relevant heat exposure, this study proposes a simple but effective satellite-based measure- ground urban heat island (GUHI), focusing solely on radiant temperatures from urban ground elements. Leveraging ECOSTRESS land surface temperature product and radiation-based statistical downscaling, diurnally representative GUHIs were evaluated over NYC. The findings reveal that overall GUHI is consistently warmer than SUHI diurnally. However, GUHI exhibits complex spatial contrasts with SUHI, primarily influenced by vegetation coverage. Various indicators associated with urban structures and materials were examined, showing important but dissimilar roles in shaping the spatial dynamics of GUHI and SUHI. This study highlights the value of satellite thermal observations compared to air temperature while addressing uncertainties in widely adopted practices of using them. By improving the depiction of human-related urban heat patterns from Earth observations, this research offers valuable insight and more reliable measures to address the urgent requirements for urban heat risk management globally.

Associations Between Heat Exposure in Early Pregnancy and Anemia, Inflammation and Iron Status Among Women in Rural Pakistan

Associations Between Heat Exposure in Early Pregnancy and Anemia, Inflammation and Iron Status Among Women in Rural Pakistan

Long-term temporal and spatial divergence patterns of urban heat risk in the Beijing–Tianjin–Hebei urban agglomeration

As urbanization accelerates and global warming intensifies, urban heat risks are increasingly becoming a critical issue and addressing this challenge to foster more harmonious and livable urban environments is essential. In our study, multisource remote sensing and point-of-interest (POI) data were applied to assess the urban heat risks in the Beijing-Tianjin-Hebei (BTH) urban agglomeration from 2001 to 2020. The results indicated that the heat hazard, exposure, and sensitivity were notably greater in the southeastern of the BTH. Conversely, the heat adaptability was lower in the southeast, leading to a severe high heat risk in this region. From 2001 to 2020, the heat risk index (HRI) values for these cities generally displayed an upward trend, peaking in 2015. Among the cities, Handan had the highest heat risk, followed by Baoding and Beijing, with HRIs of 0.32, 0.32, and 0.31, respectively. In urban areas, commercial zones had the highest average HRI at 0.3, while public management zones had the lowest at 0.28. These findings highlight significant variations in heat risk, indicating the need for targeted urban planning and heat mitigation strategies in the most affected areas of the BTH.

Measurement of microclimates in a warming world: problems and solutions.

As the world warms, it will be tempting to relate the biological responses of terrestrial animals to air temperature. But air temperature typically plays a lesser role in the heat exchange of those animals than does radiant heat. Under radiant load, animals can gain heat even when body surface temperature exceeds air temperature. However, animals can buffer the impacts of radiant heat exposure: burrows and other refuges may block solar radiant heat fully, but trees and agricultural shelters provide only partial relief. For animals that can do so effectively, evaporative cooling will be used to dissipate body heat. Evaporative cooling is dependent directly on the water vapour pressure difference between the body surface and immediate surroundings, but only indirectly on relative humidity. High relative humidity at high air temperature implies a high water vapour pressure, but evaporation into air with 100% relative humidity is not impossible. Evaporation is enhanced by wind, but the wind speed reported by meteorological services is not that experienced by animals; instead, the wind, air temperature, humidity and radiation experienced is that of the animal's microclimate. In this Commentary, we discuss how microclimate should be quantified to ensure accurate assessment of an animal's thermal environment. We propose that the microclimate metric of dry heat load to which the biological responses of animals should be related is black-globe temperature measured on or near the animal, and not air temperature. Finally, when analysing those responses, the metric of humidity should be water vapour pressure, not relative humidity.

Extreme Temperatures, Birth Outcomes, and Social Inequalities: Evidence from South China

Abstract The impacts of climate change on health are a critical public health issue, but the association between extreme temperatures and birth outcomes remains poorly understood. This paper links over 1 million birth records from Dongguan, China, between 2004 and 2013, to meteorological data. We investigate the relationship between extreme temperatures and birth outcomes and explore the heterogeneity among different demographic and socioeconomic factors, including maternal migrant status, education level, and mode of delivery. We find that one percentage increase in the number of days exposed to extreme heat during pregnancy is associated with a reduction in birth weight of 2.31 g and a 2% increase in odds of low birth weight (LBW), while exposure to extreme cold temperatures is associated with a reduction in birth weight (0.66 g) and an increase in the risk of LBW (1%). The association between extreme high temperatures and adverse birth outcomes is stronger for groups with disadvantaged social status. Specifically, the migrant group (for extreme heat exposure, local residents, −0.37 g; intraprovincial migrants, −2.75 g; out-of-province migrants, −2.49 g), the less-educated group (for extreme heat exposure, middle school or below, −2.47 g; high school or above, −1.66 g), and the group with vaginal birth [for extreme heat exposure, cesarean sections (C-sections), −1.56 g; vaginal birth, −2.62 g] are more sensitive to extreme weather conditions. Our study provides further evidence about the association of extreme temperatures with birth outcomes and for vulnerable groups of pregnant women.