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.


IntroductIon
We commend Corbett et al for their recent article 'Sex differences in response to exercise heat stress in the context of the military environment' 1 which was both enjoyable and apposite. Exposure to heat stress is likely to increase with climate change, and ground close combat (GCC) roles have recently been opened to women as well as men. Moreover, our understanding of how physiology, behaviour and occupational requirements may contribute to incapacitation caused by body temperature elevation (heat illness) has increased significantly in recent years. 2 3 Corbett et al described expected differences in body composition, physical fitness and sex hormones between male and female Service personnel (SP) exposed to exercise in the heat. In evaluating the potential impact of these differences on the deployment to hot environments of GCC troops, which may now include female SP, the authors rightly alluded to the dual military objectives of maximising performance while minimising risk of heat illness. 3

SettIng Sex In the context of other rISk fActorS for heAt IllneSS
In hot, temperate or even colder military environments, formal and dynamic risk assessments are required wherever Commanders identify the potential for significant heat stress. 2 Risk may arise from increased metabolic heat production, as with physical exertion, or reduced heat loss to the external environment, including to microenvironments created by clothing or personal protective equipment (PPE). Thus, the question arises of whether any bearing of sex on these variables merits the inclusion of sex as an independent modifier of risk-for degraded performance (through reduced tolerance to heat stress) or heat illness, with a common assumption being that the former associates with increased risk for the latter. Extrapolating purely from the physiological evidence available, there are several potentially relevant hypotheses why women in GCC (WGCC), relative to men, may have an increased risk of heat illness. These include participation in higher intensity physical activities, increased risk of syncope under exercise-heat stress from relative vasomotor instability, especially where acclimatisation is incomplete, and increased risk of exertional hyponatraemia (with an attendant threat from heat stroke, as perceptual cues to curtail physical activity become impaired with worsening metabolic disturbance).
do phySIologIcAl Sex dIfferenceS trAnSlAte Into reAl-world dIfferenceS In rAteS of heAt IllneSS?
One would be likely to surmise that the physiological factors conveying 'thermoregulatory disadvantage' described by Corbett et al place women at increased risk of physiological failure in the GCC context. However, our recent systematic review and meta-analysis of sex-associated rates of heat illness found a more than two-fold higher incident rate ratio of heat illness among men than women. 4 This striking finding ran contrary to our a priori hypothesis which was based on the physiological evidence and in line with much of that summarised by Corbett et al. The increased rate in men was proven to be remarkably consistent across a range of occupations, age and heat illness severity. These data question the 'realworld' significance of small, physiological sex differences identified in laboratory settings, isolated from the occupational, recreational and sporting settings in which heat illness arises. Indeed, in the literature highlighting potential disadvantages arising from typical female characteristics (eg, increased body fat and decreased aerobic exercise capacity relative to men 5 6 ) their influence on body temperature responses is actually downplayed, (to within 1%-4% of individual variability to exercise in the heat), with work-rate playing a much greater role.
Even where sex differences would be expected to make an overriding impact on tolerance and risk of illness under heat stress, we would caution that the evidence be carefully selected and matched or judiciously extrapolated to the precise question being asked of female SP. For example, data from GCC-based basic military training indicate that individuals typically perform moderate physical activity ⇒ Following the entry of women into ground close combat roles, the impact on women of the attendant risk of heat illness has been increasingly considered. ⇒ Emphasis on sex differences in thermal physiology must be interpreted in the context of likely occupational and environmental situations of heat stress. ⇒ Sex-associated thermoregulatory variation may be relatively unimportant compared with genderassociated differences in behaviour. ⇒ Quantifying the risk of heat illness to women in ground close combat should draw on data from their realworld context of these roles. We were also interested in the discussion of hormonal influences on heat illness risk and agree that estradiol may confer protection against certain forms of heat illness in premenopausal women, while the physicality (and psychological stress) of GCC training may erode temperature variability between sexes. Since the prevalence of oligo or amenorrhoea in military trainees is commonly reported as 40%-60%, 8 9 it can be anticipated that progesterone-induced temperature fluctuations (induced by a healthy, sustained luteal phase) will be greatly diminished. 10 Moreover, uptake of hormonal contraception can be expected to be high (some 67% of female cadets at the Royal Military Academy Sandhurst (RMAS) used hormonal contraception between 2017 and 2019) which may therefore negate the effect of endogenous sex steroid changes. Like the Ex ICE MAIDEN team, many women undertaking arduous training choose long-acting reversible contraceptives or take several packs uninterrupted in order to induce amenorrhoea. 11 Such longterm flattening of sex steroid variability would be expected to mitigate any effect of sex hormone variability on temperature. Hence, many laboratory studies of female sex steroids and thermoregulation may be rendered irrelevant to WGCC by the choices individuals are likely to make in the occupational context.

dIfferentIAtIng Sex And gender
Importantly, while Corbett et al refer to gender-free physical employment standards in their article, the literature they discuss primarily derives from biological differences in sex (ie, genetically driven, dimorphic variation) and omits to address directly the role played by gender (referring here to a behavioural normative, eg, male or female, within a military occupational setting). Gender may be a particularly powerful determinant of behaviour within military culture and could help to explain increased rates of heat illness in civilian as well as military populations. 4 We were delighted to see that Corbett et al's discussion of thermoregulation opened with analysis of differences in behaviour under heat stress, but the data available to them was limited and typically did not refer to gender. The two small studies cited supported a muted assertion that women might demonstrate greater perception of a given thermal stimulus than men. While interesting, these findings do not pertain to behaviour and are likely to be sex differences. In their section dedicated to heat illness, the authors cited the qualitative study by Khare et al, 12 one of several of which we are aware to suggest women may demonstrate more circumspect, protective behavioural responses to heat stress than men when given opportunity to do so. [13][14][15] Indeed, women have been shown to consistently show greater risk awareness than men, although the gap appears to be narrowing with time. 16 As Corbett et al point out, there is a paucity of ecologically valid data of sex differences and physical performance in the GCC role and more data are required, particularly to understand the influence of behavioural thermoregulation and cognition under heat stress.

conSIderIng the mIlItAry context of heAt IllneSS rISk
We would suggest that the behaviour of an individual within their environment and culture may have a stronger bearing on outcomes under heat stress than sex differences per se. Therefore, the psychology of military heat stress and incapacitation demand greater attention from studies and reviews. The differences in gender-associated rates of heat illness we described 4 highlighted the importance of behaviour, but the implementation of our findings to mitigate heat illness for all SP will by no means limited to questions of sex difference. Improvements in understanding of tbehaviour may dwarf the impact, on heat illness rates, of including physiological females in GCC roles. For example, individual motivation to accomplish a mission, training or selection activity is frequently cited as a leading factor contributing to heat illness from physical exertion. 17 18 In line with experience from the British Army, heat illness rates peak in GCC personnel and decline greatly as occupational focus shifts away from Combat to Support and Service Support functions. 19 Investigating the values, priorities and resilience of military units and the individuals serving in them, while systematically exploring how such factors influence the approach to their roles may shed the greatest light on performance issues and risk for heat illness. Any differences between men and women would best be explored within this wider enquiry.
On this theme, we look to a retrospective study published recently in BMJ Military Health, describing the circumstances of heat illness and other forms of exertional incapacity occurring in the GCC-based model of training employed at RMAS. In this important work by Everest, 20 female officer cadets (OCs) represented approximately 15% of the commissioning courses studied. They accounted for 11% of suspected heat illness, with half of cases being reclassified as non-heat related on subsequent review. In addition to highlighting the scope for disorders other than heat illness to manifest under heat stress (and perhaps thus helping to explain why female SP are proportionately overrepresented in reporting of heat illness deemed to be milder in nature, but underrepresented for more severe heat stroke), this work also identifies risk of heat illness with lean, fit, motivated OCs. As we have previously emphasised, a performancerisk paradigm may operate in military personnel and other special populations such as elite athletes, whereby increased heat tolerance may on occasion predispose to more severe illness. 3 21 Whether 'typical' female characteristics associated by Corbett et al with lower physical performance and heat tolerance could actually be protective against occupationally related heat stroke should be considered in future investigations conducted in military settings.

SummAry
Two important conclusions may be drawn. First, behavioural factors are of likely central importance to population-level personal view risk to heat illness and are likely to confer a greater component of this risk than thermophysiological nuance. Second, to address any heat illness risk differential in a military setting, delineating intrinsic gender normative from extrinsic behavioural influence is of central importance. 22 Such extrinsic influences are peculiar to the military and include the imperatives driven by training, commanders or operations. Individual autonomy of response to extrinsic factors in GCC roles, for example, the opportunity for self-pacing, is perhaps under-recognised. We propose heat and occupation-associated behaviours are likely to be of greater importance than subtleties in physiology, although behavioural responses to extrinsic military influences have not been studied. As with others, 23 we have argued that the challenge of operating under heat stress is best met by fostering physiological adaptation through leaning into, rather than away from, exercise in the heat. Such adaptation should be undertaken through regular structured exposures in a safe, supportive and vigilant training system. As women take on all roles across Defence, the importance of surveillance for variation in rates of heat and other exercise-related illnesses by accurate and timely reporting must be emphasised. By incorporating 'in the context of the military environment' in the title of their article, Corbett et al correctly emphasise the consideration of differences between real world, lived experiences of male and female SP. It is important to report outcomes from relevant settings and future research directions must focus on the most pertinent risk factors for our SP. twitter N Taylor @icemaidennat contributors All authors developed the ideas and structure of the manuscript jointly. RMG and MS drafted the manuscript. DRW and NT reviewed, edited and agreed the manuscript.
funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.