Risk factors for heat illness among British soldiers in the hot Collective Training Environment

Sex, gender or occupational psychology: what matters most to preventing heat-related illnesses and improving outcomes for women in ground close combat?

Since the advent of women in ground close combat (WGCC) roles, the impact on women of the attendant risk of heat stress and heat illness has been considered. Much emphasis has been placed on sex differences in thermal physiology. This article considers the application of evidence of sex-associated thermoregulatory variation to the occupational and environmental setting of WGCC, and weighs the relative importance of physiological differences arising from biological sex, and behaviour associated with gender normatives. Quantifying the risk of heat illness to WGCC should draw on data from their real-world occupational context.

Next Generation Sequencing of Genotype Variants and Genetic Association between Heat Shock Proteins HSPA1B Single Nucleotide Polymorphism at the g.31829044 Locus and Heat Tolerance: A Pilot Quasi-Experimental Study

Heat tolerance and exertional heat stroke (EHS) are rare health conditions that have been described and characterised but have never been genetically solved. Knowledge of the role of single nucleotide polymorphisms (SNPs) in heat shock proteins (HSPs) genes and their associations with heat tolerance and EHS is limited. This pilot study aimed to identify SNP in HSPA1B, HSP90AA2 and DNAJA1 genes and their associations with heat tolerance and EHS history in a quasi-experimental design. Participants comprised Australian Defence Force members (ADF) who had a history of EHS and the general population. Genomic DNA samples were extracted from the venous blood samples of 48 participants, sequenced and analysed for SNP. Forty-four per cent (44%) of the participants were heat intolerant, and 29% had a history of EHS. Among participants with a history of EHS, there was an association between heat tolerance and HSPA1B SNP at the g.31829044 locus. However, there were no associations between HSPA1B and HSP90AA2 SNP and heat tolerance. All participants had the same distribution for the DNAJA1 SNP. In conclusion, the findings indicate an association between the HSPA1B genetic variant at the g.31829044 locus and heat tolerance among ADF participants with a history of EHS. Further research with a larger number of military participants will shed more light on the associations between HSP genes and heat tolerance.

Occupational heat strain in outdoor workers: A comprehensive review and meta-analysis

The present comprehensive review (i) summarizes the current knowledge on the impacts of occupational heat stress on outdoor workers, (ii) provides a historical background on this issue, (iii) presents a meta-analysis of published data, (iv) explores inter-individual and intra-individual factors, (v) discusses the available heat mitigation strategies, (vi) estimates physical work capacity, labour productivity, and metabolic rate for the year 2030, and (vii) provides an overview of existing policy and legal frameworks on occupational heat exposure. Meta-analytic findings from 38 field studies that involved monitoring 2,409 outdoor workers across 41 jobs in 21 countries suggest that occupational heat stress increases the core (r = 0.44) and skin (r = 0.44) temperatures, as well as the heart rate (r = 0.38) and urine specific gravity (r = 0.13) of outdoor workers (all p < 0.05). Moreover, it diminishes the capacity of outdoor workers for manual labour (r = -0.82; p < 0.001) and is responsible for more than two thirds of the reduction in their metabolic rate. Importantly, our analysis shows that physical work capacity is projected to be highly affected by the ongoing anthropogenic global warming. Nevertheless, the metabolic rate and, therefore, labour productivity are projected to remain at levels higher than the workers' physical work capacity, indicating that people will continue to work more intensely than they should to meet their financial obligations for food and shelter. In this respect, complementary measures targeting self-pacing, hydration, work-rest regimes, ventilated garments, and mechanization can be adopted to protect outdoor workers.

Perceptual strain in a compensable hot environment: Accuracy and clinical correlates

Heat strain monitoring indexes are important to prevent exertional heat illness (EHI) and uncover risk factors. Two indexes are the Physiological Strain Index (PSI) and a subjective PSI analogue, the Perceptual Strain Index (PeSI). The PeSI is a feasible alternative to PSI in field conditions, although the validity has been variable in previous research. However, the PeSI has been rarely examined at a low heat strain with compensable heat stress, such as during a heat tolerance test (HTT). This study evaluated the discrepancy between the maximal PeSI and maximal PSI achieved during a HTT and determined their association with EHI risk factors, including history of EHI, percent body fat (%BF), relative VO_2max, fatigue and sleep status (n = 121; 47 without prior EHI, 74 with prior EHI). The PSI was calculated using the change in rectal temperature (T_re) and heart rate (HR) and PeSI was calculated based on the formula containing thermal sensation (TS), a T_re analogue, and rate of perceived exertion (RPE), a HR analogue. Significant associations were identified between PSI and PeSI and between PSI_HR and PeSI_HR in the total sample and between PSI and PeSI in the EHI group. Bland-Altman analyses indicated PeSI underestimated PSI in the total sample, PSI_HR was greater than PeSI_HR, and that PSI_core and PeSI_core were not significantly different, but values varied widely at different heat strains. This indicates the use of RPE underestimates HR and that the accuracy of TS to predict T_re may be subpar. This study also demonstrated that participants with higher %BF have a decreased perception of heat strain and that post-fatigue, sleep status and a prior EHI may increase the perception of heat strain. Overall, these results suggest that PeSI is a poor surrogate for PSI in a compensable heat stress environment at low heat strain.

Individual Anthropometric, Aerobic Capacity and Demographic Characteristics as Predictors of Heat Intolerance in Military Populations

Background and objectives: The Australian Defence Force (ADF) engages in combat-related activities in hot climatic conditions, which exposes ADF members to the threat of exertional heat illness (EHI). After an episode of EHI, the heat tolerance test (HTT) is conducted to determine heat tolerance. Heat intolerance is the inability to maintain thermal balance while exercising in a hot environment. This study investigated the predictive roles of individual characteristics (age, gender, aerobic capacity (VO₂max) and body composition) on physiological responses to the HTT in a group comprising ADF personnel and civilian volunteers. Materials and Methods: A quasi-experimental design was used and 52 (38 males and 14 females) participants were recruited from the ADF and the general population for the HTT. Heat intolerance was defined following the standard criteria for the HTT (temperature and heart rate). Data were analysed using inferential statistics. Results: The mean age of the participants was 31.1 ± 11.6 years, and 44% (23 people: 19 males and 4 females) of the participants were heat intolerant. Independent samples T-test showed that body mass index (p = 0.011) and body fat% (p = 0.034) of heat-intolerant participants were significantly higher than their heat-tolerant counterparts. Body surface area to mass ratio (p = 0.005) and aerobic capacity (p = 0.001) were significantly lower in heat-intolerant participants. Regression analyses showed that age, gender, aerobic capacity and body fat% were significant (p < 0.001) predictors of heat tolerance outcomes, with R² values ranging from 0.505 to 0.636. Conclusions: This study showed that aerobic capacity, body fat%, age and gender are predictors of heat intolerance among military and non-military populations. However, there may be a need for future studies to consider identifying other indicators such as clinical biomarkers of heat intolerance, which could be used to develop a more reliable HTT protocol.

Crowe M. Epidemiology of Exertional Heat Illness in the Military: A Systematic Review of Observational Studies

Exertional heat illness (EHI) is an occupational hazard among military personnel. This systematic review describes the incidence, risk factors, clinical manifestations, and biomarkers of EHI in the military. Six databases from inception to 28 May 2020 were systematically reviewed using the PRISMA guidelines. Forty-one articles met the inclusion criteria and the incidence of EHI ranged from 0.2 to 10.5 per 1000 person years, while the prevalence rates ranged from 0.3% to 9.3%. Intrinsic risk factors influencing EHI were gender, physical fitness, obesity, previous history of heat illness, and motivation, while the extrinsic factors included hot environmental conditions and service unit. Evidence suggests that loss of consciousness, absence of sweating and confusion were the common clinical features of exertional heat stroke (EHS). The mean core temperature ranged from 40 to 41.6 °C, while elevated levels of creatine phosphokinase, liver enzymes, and creatinine were common biochemical markers of EHS. The findings of the review suggest a variation in the incidence of EHI among military populations possibly due to the varying definitions used. Although some risk factors of EHI were identified, more analytical studies are needed to investigate the association between EHI and other important factors such as acclimatisation and occlusive clothing.

Influence of aerobic fitness on gastrointestinal barrier integrity and microbial translocation following a fixed-intensity military exertional heat stress test

PURPOSE

Exertional-heat stress adversely disrupts gastrointestinal (GI) barrier integrity, whereby subsequent microbial translocation (MT) can result in potentially serious health consequences. To date, the influence of aerobic fitness on GI barrier integrity and MT following exertional-heat stress is poorly characterised.

METHOD

Ten untrained (UT; VO_2max = 45 ± 3 ml·kg^-1·min^-1) and ten highly trained (HT; VO_2max = 64 ± 4 ml·kg^-1·min^-1) males completed an ecologically valid (military) 80-min fixed-intensity exertional-heat stress test (EHST). Venous blood was drawn immediately pre- and post-EHST. GI barrier integrity was assessed using the serum dual-sugar absorption test (DSAT) and plasma Intestinal Fatty-Acid Binding Protein (I-FABP). MT was assessed using plasma Bacteroides/total 16S DNA.

RESULTS

UT experienced greater thermoregulatory, cardiovascular and perceptual strain (p < 0.05) than HT during the EHST. Serum DSAT responses were similar between the two groups (p = 0.59), although Δ I-FABP was greater (p = 0.04) in the UT (1.14 ± 1.36 ng·ml^-1) versus HT (0.20 ± 0.29 ng·ml^-1) group. Bacteroides/Total 16S DNA ratio was unchanged (Δ; -0.04 ± 0.18) following the EHST in the HT group, but increased (Δ; 0.19 ± 0.25) in the UT group (p = 0.05). Weekly aerobic training hours had a weak, negative correlation with Δ I-FABP and Bacteroides/total 16S DNA responses.

CONCLUSION

When exercising at the same absolute workload, UT individuals are more susceptible to small intestinal epithelial injury and MT than HT individuals. These responses appear partially attributable to greater thermoregulatory, cardiovascular, and perceptual strain.

Individual risk factors associated with exertional heat illness: A systematic review

NEW FINDINGS

What is the topic of this review? Exertional heat illness (EHI) remains a persistent problem for athletes and individuals. This threat remains despite numerous athletic position statements and occupational guidance policies. This review explores primary evidence that demonstrates a direct association between 'known' risk factors and EHI. What advances does it highlight? Primary evidence to support 'known' risk factors associated with EHI is not comprehensive. Furthermore, it is not evident that single individual factors predispose individuals to greater risk. In fact, the evidence indicates that EHI can manifest in non-hostile compensable environments when a combination of risk factors is prevalent.

ABSTRACT

Despite the widespread knowledge of exertional heat illness (EHI) and clear guidance for its prevention, the incidence of EHI remains high. We carried out a systematic review of available literature evaluating the scientific evidence underpinning the risk factors associated with EHI. Medline, PsycINFO, SportDiscus and Embase were searched from inception to January 2019 with no date limitation, with supplementary searches also being performed. Search terms included permutations of risk and heat illness, with only studies in English included. Study selection, data extraction and quality assessment, using the QUALSYST tool, were performed by two independent reviewers. Of 8898 articles identified by the searches, 42 were included in the systematic review as primary evidence demonstrating a link between a risk factor and EHI. The quality scores ranged from 57.50 to 100%, and studies were generally considered to be of strong quality. The majority of risks attributable to EHI were categorized as those associated with lifestyle factors. The findings from the systematic review suggest complex manifestation of EHI through multiple risk factors rather than any one factor in isolation. Further research is needed to explore the accumulation of risk factors to help in development of effective preventative measures.

Heat Adaptation in Military Personnel: Mitigating Risk, Maximizing Performance

The study of heat adaptation in military personnel offers generalizable insights into a variety of sporting, recreational and occupational populations. Conversely, certain characteristics of military employment have few parallels in civilian life, such as the imperative to achieve mission objectives during deployed operations, the opportunity to undergo training and selection for elite units or the requirement to fulfill essential duties under prolonged thermal stress. In such settings, achieving peak individual performance can be critical to organizational success. Short-notice deployment to a hot operational or training environment, exposure to high intensity exercise and undertaking ceremonial duties during extreme weather may challenge the ability to protect personnel from excessive thermal strain, especially where heat adaptation is incomplete. Graded and progressive acclimatization can reduce morbidity substantially and impact on mortality rates, yet individual variation in adaptation has the potential to undermine empirical approaches. Incapacity under heat stress can present the military with medical, occupational and logistic challenges requiring dynamic risk stratification during initial and subsequent heat stress. Using data from large studies of military personnel observing traditional and more contemporary acclimatization practices, this review article (1) characterizes the physical challenges that military training and deployed operations present (2) considers how heat adaptation has been used to augment military performance under thermal stress and (3) identifies potential solutions to optimize the risk-performance paradigm, including those with broader relevance to other populations exposed to heat stress.

Negative Alcohol and Tobacco Consumption Behaviors in an Australian Army Combat Brigade

Introduction

Western militaries have consumption rates of alcohol and tobacco which are higher than the associated civilian populations, and are concerned about the negative effects on health, wellbeing, and military performance that such high rates may have.

Materials and Methods

A survey questionnaire which collected nonidentifiable data was distributed to all accessible members of an Australian Army Combat Brigade over the annual induction training and prefield exercise period between January and March 2019. The survey was composed of personal and demographic questions, followed by questions concerning the personal consumption habits of alcohol and tobacco. 1,606 complete and valid surveys were returned, representing 55.3% of the Brigade’s posted strength. Ethical clearance for this project was provided by the Australian Defence Force Joint Health Command Low Risk Ethics Panel (18-012).

Results

Almost one quarter of the Combat Brigade exceeded the Australian daily risk guideline for alcohol consumption, and over three quarters exceeded the occasion risk guideline; with 6.1% of soldiers drinking alcohol daily. 24.3% identified as tobacco smokers, with 15.9% smoking daily. Smoking rates and volumes were higher when deployed on field training exercises and on international training and operational deployments; as was the consumption of roll-your-own cigarettes. Overall, the main determinants of the negative consumption behaviors were gender (male) and age (under 25).

Conclusions

This study confirmed that the members of an Australian Army Combat Brigade have higher negative health consumption behaviors concerning alcohol and tobacco than the general civilian population. The results also identified a cohort of Senior Non-Commissioned Officers and Warrant Officers (E5–E9) between the ages of 35 and 44 who had high negative consumption behaviors which could result in poor health outcomes. This could be a suitable cohort for a targeted campaign to reduce tobacco and alcohol consumption and to assist to make healthy life changes.

Health Risks and Interventions in Exertional Heat Stress

BACKGROUND

With climate change, heat waves are expected to become more frequent in the near future. Already, on average more than 25 000 "heat deaths" are estimated to occur in Europe every year. However, heat stress and heat illnesses arise not just when ambient temperatures are high. Physical exertion increases heat production within the organism many times over; if not enough heat is lost, there is a risk of exertional heat stress. This review article discusses contributing factors, at-risk groups, and the diagnosis and treatment of heat illnesses.

METHODS

A selective literature search was carried out on PubMed. Current guidelines and expert recommendations were also included.

RESULTS

Apart from muscular heat production (>70% of converted energy), there are other factors that singly or in combination can give rise to heat stress: clothing, climate/acclimatization, and individual factors. Through its insulating properties, clothing reduces the evaporation of sweat (the most effective physiological cooling mechanism). A sudden heat wave, or changing the climate zone (as with air travel), increases the risk of a heat-related health event. Overweight, low fitness level, acute infections, illness, dehydration, and other factors also reduce heat tolerance. In addition to children, older people are particularly at risk because of their reduced physiological adaptability, (multi-)morbidity, and intake of prescription drugs. A heat illness can progress suddenly to life-threatening heat stroke. Successful treatment depends on rapid diagnosis and cooling the body down as quickly as possible. The aim is to reduce core body temperature to <40 °C within 30 minutes.

CONCLUSION

Immediately effective cooling interventions are the only causal treatment for heat stroke. Time once lost cannot be made up. Prevention (acclimatization, reduced exposure, etc.) and terminating the heat stress in good time (e.g., stopping work) are better than any cure.

An exertional heat illness triage tool for a jungle training environment

This article introduces a practical triage tool designed to assist commanders, jungle training instructors (JTIs) and medical personnel to identify Defence Personnel (DP) with suspected exertional heat illness (EHI). The challenges of managing suspected EHI in a jungle training environment and the potential advantages to stratifying the urgency of evacuation are discussed. This tool has been designed to be an adjunct to the existing MOD mandated heat illness recognition and first aid training.