Specifying and Mitigating Thermal Stress Effects on Cognition During Personal Protective Equipment Use

Introduction

The pandemic has hit us hard. Virtually no one in the world is exempted from its larger effects. As we struggle to resolve the challenges it has posed, we need to ask salutary questions about the ways in which our own science has, or has not, contributed to both extant and proposed solutions. HF/E has much to offer, and true to our declared aspiration to “improve the quality of human life” we are, arguably, morally obliged to contribute, to the best of our abilities. In this spirit, the present work addresses two particular, interacting challenges, one primarily physical and one primarily cognitive, that face those whose job it is to fight the pandemic “on the front lines.” Specifically, these are the thermal and performance issues associated with extended and repeated usage of varying forms of personal protective equipment (PPE).

Layers of Protection

Nearly half a century ago, William “Bill” Haddon (1973) provided a synopsis of his keynote address to our Society concerning his, even then, well-known formulations concerning the principles of protection from harm. These principles are as relevant today as they were then, and in the present circumstances perhaps even more so. In this work, and other associated publications (e.g., Haddon, 1970), he described how it was possible to erect a series of barriers between entities which would prevent the transfer of damaging “energy.” In the case of living systems, this damage was in the form of injury and fatality (see also Reason, 1990). These strategic barriers, which are as applicable to infection as other forms of energy transmission, have become codified in the “hierarchy of controls.” Here, priority is given to strategies such as preventing the accumulation of energy in the first place and thus eliminating, or at least ameliorating, the hazard at source. For the pandemic, that opportunity is now well past, although forensic investigations of this initial brisance of defenses are surely underway. The most common strategy that has been engaged in the present circumstances is that of “social distancing.” This is evidently equivalent to the idea of providing some form of engineered barriers to transmission. “Distancing” and indeed any form of barrier, seeks to generate a separation in space and/or time between the individual and the source of harm. To whatever degree data can address it, distancing appears, at least to a degree, to have enjoyed some success. Administrative controls and general warnings also have their place, although the efficacy of warnings is less impactful than might be hoped (Hancock et al., in press). What is of direct concern here is essentially the last line of physical defense which is constituted by PPE, for, in an evident way, medical personnel cannot easily “distance” themselves and so, most often, have to be in direct contact with their patients. It is to assess and communicate our current state of understanding with respect to two specific elements of this necessary PPE usage that the remainder of this short work is occupied.

Select Challenges of Personal Protective Equipment

There are many challenges associated with operating efficiently when encased in varying levels of PPE (Szczecińska & Łężak, 2000). These ensembles range in sophistication from the strictest requirements of spacesuits for EVA activity (e.g., Braddock et al., 2019), to less restrictive but still vital support ensembles for fire-fighters, enmeshed in the midst of forest conflagrations (e.g., Sköldström, 1987). Each of these specific instances falls under a more general concern for the functionality of all forms of protective clothing (see Havenith & Heus, 2004). At the same time that we use PPE to divorce individuals from sources of harm, the jobs which necessitate PPE are becoming ever more reliant on interacting with sophisticated technology. At present, many of these technologies, such as smartphones and laptop computers, are external to the PPE ensemble itself. This results in a number of both physical and cognitive ergonomic challenges in respect of information assimilation in the first place (Hancock, Sawyer et al., 2015) and subsequently PPE-mediated and enabled effector actions (Bishu et al., 2006). This brief work cannot deal with all of these concerns, so the present focus is on one specific interactive challenge which faces virtually all who are encased in full PPE.

In a very real way, it is the primary and essential purpose of PPE which generates the first of these particular challenges—heat stress. Thus, the design of the equipment is motivated by the aspiration to “cut the person off” and isolate them from sources of threat such as the virus. But this very act creates an often-impermeable barrier between the person in the PPE and the outside world. Many forms of interaction are inhibited by this isolation but one of the most important is thermal exchange (see Havenith et al., 2011). The principles of thermoregulation are well known and have now been documented for a period of even centuries (e.g., Blagden, 1775; Cooper, 2008). Critical regulation is achieved through evaporative, convective, and conductive channels, with heat exchange via respiration being an essential and dynamic feature (for a more detailed treatment, see Parsons, 2019). Here, masks, visors, faceguards, and even full exclusion helmets act as inhibitors to this moment-by-moment interchange. Nor is convection or conduction easily achieved in these circumstances. In terms of materials placed over the mouth, nose, and eye areas, we witness additional ergonomic challenges such as the “fogging” of glasses, screens, or transparent visors. In the ICU, success often depends upon fast and accurate reading of multiple displays which represent the patient’s immediate state (see Hancock et al., 2015). Even minor errors can prove fatal. Thus, as the individual works at these critical tasks they, depending upon the degree of exclusion and isolation in their PPE equipment, are taking on an ever-ascending thermal load. The question arises, what level of thermal stress can they tolerate before they begin to make critical performance mistakes? For it is known that these thermal effects and associated general stress effects have significant impacts on perceptual and response capacities (see, e.g., Hancock & Weaver, 2005; Hancock, 1993). It is here that our HF/E knowledge can prove highly informative. Although this information is here directed primarily at the current pandemic response, it is equally relevant to all who use PPE in their everyday tasks.

Task by Thermal Load Limits

One of the first points to note about thermal loads in PPE is that they derive not only from heat exchange isolation per se, but that they accrue primarily from the inability of the bodily heat generated by both physical and cognitive work to escape. One shortfall in HF/E research is that we still possess insufficient understanding of these conjoint effects of coincident physical and cognitive work (see Marras & Hancock, 2014). Yet, this being said, we do possess extensive understanding of heat effects on cognitive performance, which has its modern foundations now more than half a century ago (Hancock, 1981, 1982; Wing, 1965). This early research (for a review, see Hancock & Vasmatzidis, 1998) exploded the myth of the physiological tolerance theory. This latter proposition suggested that conditions which an individual could thermophysiologically tolerate would not significantly affect their cognitive and psychomotor task performance. The body of empirical data subsequently showed, in contrast, that differing forms of cognitive performance were actually differentially sensitive to ascending heat loads (Ramsey & Morrissey, 1978). Original standards (National Institute for Occupational Safety and Health, 1972), which had been developed from Wing’s (1965) review, were revised in light of more detailed evaluation of the emerging comprehension (see Hancock, 1982; Parsons, 1995). These latter tolerance limits, described in Figure 1, have been supported and validated by subsequent meta-analyses of the overall area across the years (see Hancock, Ross et al., 2007; Pilcher et al., 2002), as well as a series of experimental investigations (see, e.g., Vasmatzidis et al., 2002). Figure 1, which employs a logarithmic time base, illustrates that the more attention-demanding and cognitively loading a task becomes, the more vulnerable it is to deterioration in adverse thermal conditions. It most especially features the vulnerability of vigilance-type tasks to heat stress (Hancock, 1986a, 2013, 2017) as they are to stress in general (Hancock & Warm, 1989). We can, with equal facility, assert that performance failure under any driving form of stress (e.g., noise, vibration) also tends to show these systematic vulnerabilities. Thus, the case of heat load demonstrates that stress effects interact with the attention-demanding nature of the task being performed. Consistent with earlier experimental observations, experience with either the stress itself or the task to be performed under that stress or both in combination provides some indemnification or protection against these deleterious exposure effects (Hancock, 1986b). Each of these processes (i.e., stress resistance and task skill) can be seen as forms of acclimation and manifest expressions of adaptability to the current environment and the task to hand.

OPEN IN VIEWER

Figure 1

Specification of limits of performance tolerance in time/temperature combinations, each expressed on logarithmic scale. Pure physiological collapse (E) is preceded by failure in simple mental transformations (D) which are less vulnerable than psychomotor performance (C). Dual-task performance proves more susceptible to heat (B), yet prolonged monitoring of displays for rare signals (vigilance), line A, proves the most vulnerable to the deleterious effects of incipient heat stress (illustration from Hancock & Vasmatzidis, 1998); reproduced with permission.

Recent work allows us to be more precise about these specifications, not only about absolute tolerance limits but about the probability of and degree of performance failure as a function of the time and the temperature of exposure (Figure 2). What is illustrated here are formally modeled isodecrement expressions of what can be expected in the way of performance degradation across time as a function of the ambient surround, this being a realm in which models have always played a significant role now for an extended period of time (Hancock, 1980). These thermal specifications are applicable whether the environment is an open situation such as an office (see Zhang et al., 2019) or whether it is composed of a microclimate inside a PPE ensemble. Clearly, there is an invidious trade-off here. The more open the PPE is to the external world, the more one can regulate the associated thermal load. Yet at the same time, the more open the PPE, the greater the probability of viral infection (see Havenith, 1999). It is a riddle that, at present, must be resolved by work–rest planning and organizing breaks to occur, where possible, in beneficial thermal conditions and in circumstances as remote as feasible from the viral threat.

OPEN IN VIEWER

Figure 2

Degree of anticipated task capacity diminution versus the time/temperature combinations experienced. The graphic representation provides implied guidelines as to the acceptable levels of exposure (from López-Sánchez & Hancock, 2018).

Yet even these work–rest opportunities come with concerns. The donning and doffing of PPE can itself be a cumbersome and frustrating process. With limited PPE availability, reuse becomes likely. Yet here, extra care must be taken during the doffing process since the subsequent efficacy of the PPE assemblage depends upon the integrity of the ensemble that is now to be reworn. And all this during an interval that is putative designed to be a full “rest.” Our science knows much about the sorts of issues that are relevant to protective clothing (see, e.g., Havenith & Heus, 2004). This understanding very much goes to show we are dealing with a “systems” question in which the answer to one issue impacts the nature of the next question and so on (Carayon et al., 2015). And there remains the dedication and motivation of those critical workers involved. Few in the ICU will take even mandatory rest breaks if they believe there is an opportunity to help their patient in extremis. Thus, we also have to convey our understanding about dimensions such as “work as planned” versus “work as practiced” (see, e.g., Hollnagel et al., 2006); otherwise we risk the potential of becoming irrelevant and ignored. In a short communication such as the present one, we cannot hope to provide a comprehensive understanding of the systems issues intertwined with even this one restricted realm of concern. Nor, sadly, are we able to provide a full exposition of the systematic applications of our science’s understanding to the wide variety of other challenges that now attend the present crisis.

Despite constraints on space here, it is possible to provide some limited observations upon the near-, middle- and far-term strategies that can help those presently engaged in this critical battle. In the short term, medical personnel can use a tool, readily to hand, that is, thermometers. We already use these to screen individuals for the potential presence of indicative fever. Thermometers, most especially those that provide convenient, almost instantaneous readings, can be employed to address the problem of incipient heat stress–related cognitive debilitation. No additional technology is required, but simply to use this current equipment for self-monitoring. Yet, what do such readings effectively mean? How do we directly convey the degree of performance decrement? It is here that medium-term interventions can, quite easily, be enacted. The absolute limitations (Figure 1) and differing degrees of decrement experienced (Figure 2) cry out for a portable application (APP) that warns individuals as to their effective performance capacity changes. Our science can contribute in terms of both the creation and the display-based presentation of such information. It is a step that is strongly advocated here. In terms of the further-off horizon, we need to create protective ensembles that selectively, either permit or prevent the transmission of energy and physical material between the external world and the protected person. It requires of us a greater focus on both human–equipment interaction and a more careful design of operational environments. Thus, for example, can we design spaces in which thermal exchange is readily achieved, perhaps via the process of conduction, which nevertheless continues to exclude the opportunity for viral transmission? If we are able to innovate in areas such as race-car driver thermal protection (see, e.g., Talukdar et al., 2016), as well as the even more challenging realm of thermal control in space-based activities (e.g., Nyberg et al., 2000), it should not be beyond us to provide enhanced opportunities to defeat the thermal stress versus protection trade-off by innovations informed by our own science (see, e.g., de Almeida et al., 2012). Notwithstanding the brevity of this present paper, the observations and works cited here can point toward practical and achievable lines of defense which the collective corpus of HF/E science can support. While the present work is focused upon the proximal issues which have been identified, we nevertheless need to keep our eyes on even greater, related problems which loom over the whole of our civilized world (Hancock, 2019a). It is with these even greater considerations of these that I conclude.

Summary and Conclusion

The pandemic has had global impacts that promise to reverberate for even years to come. However, it is not only with concerns for PPE usage by medical personnel that the effects of heat stress on performance are relevant (see López-Sánchez & Hancock, 2019). It is, therefore, a potentially even more dangerous, supraordinate issue that is considered as a finale to this present work. As noted, the pandemic has hit hard. It has again demonstrated how relatively fragile the skein of civilized society actually is (see Hancock, 2019a). But COVID-19 will seem as but a stroll along the primrose path compared to the unmuted effects of global warming. Contemporary politicians, however able or inept they each individually prove to be, can still today hold out a public expectation of the return to something approximating our prior “normality”; although it remains more than likely, even in the present instance, we shall have to exhibit some degree of societal resilience (Hoffman & Hancock, 2017). Yet the societal step changes that will be involved in the excesses of global warming will not provide these comforting thoughts of a recovered normality. Excessive thermal load, now expressed on a global scale, will push the cybernetic opportunities for stability maintenance beyond any opportunity for adaptive recovery (Hancock, 2019b). COVID-19 should therefore be viewed as a warning, or even a dress rehearsal in miniature for the existential thermal threats that loom ever more imminently. The evident empirical question is what HF/E can and should do about these incipient, existential challenges, set alongside one that currently confronts us?

Key Points