Protective clothing ensembles and physical employment standards.

Physical employment standards (PESs) exist for certain occupational groups that also require the use of protective clothing ensembles (PCEs) during their normal work. This review addresses whether these current PESs appropriately incorporate the physiological burden associated with wearing PCEs during respective tasks. Metabolic heat production increases due to wearing PCE; this increase is greater than that due simply to the weight of the clothing and can vary two-fold among individuals. This variation negates a simple adjustment to the PES for the effect of the clothing on metabolic rate. As a result, PES testing that only simulates the weight of the clothing and protective equipment does not adequately accommodate this effect. The physiological heat strain associated with the use of PCEs is also not addressed with current PESs. Typically the selection tests of a PES lasts less than 20 minutes whereas the requirement for use of PCE in the workplace may approach one hour before cooling strategies could be employed. One option that might be considered is to construct a heat stress test that requires new recruits and incumbents to work for a predetermined duration while exposed to a warm environmental temperature, wearing the PCE.

Physical employment standards (PESs) exist for certain occupational groups that also require the use of protective clothing ensembles (PCEs) during their normal work. This review addresses whether these current PESs appropriately incorporate the physiological burden associated with wearing PCEs during respective tasks. Metabolic heat production increases due to wearing PCE; this increase is greater than that due simply to the weight of the clothing and can vary two-fold among individuals. This variation negates a simple adjustment to the PES for the effect of the clothing on metabolic rate. As a result, PES testing that only simulates the weight of the clothing and protective equipment does not adequately accommodate this effect. The physiological heat strain associated with the use of PCEs is also not addressed with current PESs. Typically the selection tests of a PES lasts less than 20 minutes whereas the requirement for use of PCE in the workplace may approach one hour before cooling strategies could be employed. One option that D r a f t

Introduction
Physical employment standards (PESs) exist for certain public safety occupational groups such as the military (Deakin et al. 1996;Deakin et al. 2000;Todd Rogers et al. 2014), structural (Brandweer Nederland 2013;International Association of Fire Chiefs 1999;International Association of Firefighters 1999;Stevenson et al. 2009;Siddall et al. 2014) and wildland firefighters (Sharkey 1999;Petersen et al. 2010;Canadian Wildland Firefighter Fitness Testing 2012), nuclear security officers (Regulatory Document RD-363 2008) and police (Farenholtz and Rhodes 1990). These PESs typically require incumbents or new recruits to perform selection tests at least to the minimum acceptable performance level and/or perform a circuit of essential tasks of the job within a prescribed time. In some countries the PESs were developed to accommodate the females and the older worker (Jamnik et al. 2013), whereas in others the PESs were established independent of age and sex (Tipton et al. 2013). For the military, wildland firefighters and nuclear security officers the necessity to score at least to the minimum PES is a career requirement (Petersen et al. 2010; Canadian Wildland Firefighter Fitness Testing 2012; Deakin et al. 2000), whereas for other groups the PES is often used for new recruit selection only and is rarely used to reassess on an annual basis (Farenholtz and Rhodes 1990;International D r a f t personnel not involved with fire suppression activity and for police services, PESs based on tests of fitness are deemed valid for selecting and retaining candidates that can handle the physical demands of the job safely and efficiently (Deakin et al. 2000;Anderson et al. 2001;Wilkinson et al. 2008). Small additional weights totaling approximately 5 kg are carried around the waist for Canadian wildland firefighters and police PES testing to simulate the burden of a utility belt for tools and equipment (Canadian Wildland Firefighter Fitness Testing 2012; Ministry of Community Safety and Correctional Services 2014).
Certainly it seems logical to include the need to wear the PCE during PES testing if the use of the clothing is a regular requirement in the work environment. It is far less clear, however, whether the physiological effects of using a PCE are entirely evident during circuit testing that might last only 8 minutes (Dreger and Petersen 2007;vonHeimburg et al. 2013) or whether simulating the additional load-bearing penalty of the PCE while completing a task-based circuit with a pass/fail threshold of 10 minutes and 20 seconds (International Association of Fire Chiefs 1999; International Association of Firefighters 1999) is a fair representation of the burden associated with wearing the PCE. Even less obvious is the apparent assumption that PES for other occupational groups, such as police, wildland firefighters and the military (Deakin et al. D r a f t 5 overlay the use of PCE in the context of PESs it is necessary to briefly summarize the principal physiological constraints associated with wearing protective clothing. Once these issues are defined, an evaluation follows discussing current inclusion/exclusion criteria for use of a PCE during PES testing. This review then concludes with specific recommendations for additional evidence-based research that would improve the use of PES testing for various occupational groups that must wear a PCE.

Protective Clothing and Metabolic Rate
The characteristics of the PCE not only have a major impact on heat transfer between the individual wearing the clothing and the external environment but also have a large influence on the wearer's metabolic rate (M ሶ ). The clothing (and other protective equipment like respirators and a self-contained breathing apparatus (SCBA)) constitutes additional weight (from approximately 5 to 25 kg) that has to be carried and thus causes an increase in M ሶ and consequently in heat production (Goldman 1969;Smolander et al. 1984). However, more than half of the observed increase in M ሶ due to clothing can be attributed to other factors, such as increased friction of movement and hobbling effects of the clothing, rather than solely to the added weight of the PCE (Teitlebaum and Goldman 1972;Duggan 1986;Patton 1995;Dorman and Havenith 2009). In addition, protective boots, for example, can have an impact on M ሶ that is greater than that due simply to their weight because of their effect on movement efficiency (see this issue Taylor et al. 2016).
For example, Teitlebaum and Goldman (1972) observed an increase in M ሶ while wearing a 5-layer PCE that was 16% greater than the energy cost associated with wearing a single layer uniform while carrying the additional weight of the PCE around the waist in a weight belt. These differences were attributed to increased friction due to the interaction of the layers of clothing.
D r a f t 6 Similarly, Duggan (1986) examined the effect of various combinations of the PCE on the energy cost of bench stepping. When corrected for the weight of the clothing, the oxygen uptake ൫V ሶ O ଶ ൯, as a measure of energy cost, was greater by an average of 9% in the 4-layer ensemble compared to the single layer control condition, which equated to approximately 3% per additional layer above the base condition. Therefore, when estimating the energy cost of work in protective clothing, it is important to consider both the weight and the number of layers in the ensemble.
Dorman and Havenith (2007a;c;2009) also demonstrated that the increase in M ሶ through the use of a PCE during stepping, walking or throughout an obstacle course was attributed to more than just the additional weight of the clothing. As depicted in Figure 1, some of the multilayer PCE's tested increased M ሶ by greater than 20% compared with the baseline single layer uniform, despite only increasing the weight of the clothing by about 5 kg or 7% of body mass. Dorman and Havenith (2009)  Another avenue through which PCE affects metabolic rate is through the faster and higher increase in body temperature it causes. Details of the mechanisms of this increase will be D r a f t discussed later, but one impact of the higher body temperature is an extra increase in metabolic rate (Q 10 effect) of around 7% per ºC body temperature increase (Kampmann and Bröde, 2015).
The Q 10 effect is independent to the numbers provided above by Dorman and Havenith (2009), where these latter values were obtained while ensuring body temperature showed only minimal increases.

Insert Figures 1 and 2 about here.
Collectively, it is clear that the impact of the PCE on M ሶ is much greater than simply the load-carriage effect of the additional weight of the clothing. These data would argue strongly, therefore, that current PES that only simulate the weight of the PCE during testing underestimate the impact of the clothing on metabolic demand by 15% or more. Interestingly, V ሶ O ଶ averaged 38 mL·kg -1 ·min -1 or approximately 75% V ሶ O ଶ୫ୟ୶ for both men and women who completed and passed the task-based PES circuit used by many fire services in North America during recruit testing (Williams-Bell et al. 2009). If the true effect of wearing the clothing, rather than simply wearing a weighted vest, was actually 15% higher than these measured values, then the true metabolic demand of this task-based circuit would approach 45 mL·kg -1 ·min -1 or almost 90% V ሶ O ଶ୫ୟ୶ for the participants that were evaluated (Williams-Bell et al. 2009). Interestingly, this value of 45 mL·kg -1 ·min -1 was similar to the oxygen cost of carrying equipment up high-rise stairs while wearing full turnout gear with SCBA, which was the most physically demanding activity identified in the original task analysis and characterization of the physical demands of firefighting activities used to support early fitness screening protocols (Gledhill and Jamnik 1992).

D r a f t
Ultimately, the relevant question is whether the additional effect of the clothing on the metabolic cost of movement necessitates an adjustment to the use of this task-based PES for recruit selection. The answer should consider the individual variation associated with this increased metabolic cost of movement. For example, if the additional metabolic cost was constant for all individuals then either the task-based pass/fail completion criterion could remain as it is without wearing the PCE or the completion criterion time could be adjusted proportionately to accommodate for the use of the clothing during testing. However, studies have shown that the additional metabolic cost of the clothing can vary among individuals by at least two-fold (Teitlebaum and Goldman 1972;Dorman and Havenith 2009), possibly related to different movement strategies and efficiencies (Dorman and Havenith 2007d). Thus, failure to not recognize this additional, highly individual, effect of the clothing during recruit testing or to simply apply the same adjustment to the pass/fail criterion for all participants would appear inappropriate. Additional research is needed to clarify those factors that create the variation among individuals in this additional metabolic penalty of wearing the PCE. Further, since some PESs were established to accommodate women and older incumbent personnel (Jamnik et al. 2013), research should focus on these subgroups. Even with gender-and age-free PESs, there is considerable individual variation with the performance of critical job tasks (Tipton et al. 2013).
Certainly the impact of anthropometric factors on the fit of the clothing would seem relevant to consider as size and fit of clothing can have substantial effects on heat transfer through the PCE (Chen et al. 2004;Ueda et al. 2006;Wang et al. 2012 ).  observed for different work wear that tight clothing fit showed a 6-31% lower insulation than loose fit. In addition, effects of fit on clothing friction and bulk can be expected, though no detailed research on this has taken place yet to our knowledge.

Protective Clothing and Breathing Apparatus
To confer protection from airborne hazards in the work environment, many occupations require the use of a breathing apparatus that either filters the inspired air to remove contaminants or require workers to breathe from a SCBA. Regardless of the type of respirator used there is an increase in both inspiratory and expiratory breathing resistance, which increases as flow rates increase to match metabolic demands (Butcher et al. 2006;Muza et al. 2002). At high flow rates, as required during heavy work, the increased breathing resistance could lead to respiratory muscle fatigue (Butcher et al. 2007). The use of the SCBA also decreases maximal exercise flow rates and V ሶ O ଶ୫ୟ୶ with one study reporting a 15% reduction in aerobic fitness solely due to the requirement to breathe through the regulator of the SCBA (Eves et al. 2005). The use of a multilayered PCE together with the SCBA harness strapped around the chest further impedes the worker's ventilatory function accounting for approximately 20% of the total ventilatory impairment (Muza et al. 1996).
One might expect that occupational groups would include the requirement to breathe through a respirator during PES testing if their daily work environment requires the use of a respirator as part of their PCE. This does not appear to consistently be the case however.
Certainly in its present format the Candidate Physical Ability Test (International Association of Fire Chiefs 1999; International Association of Firefighters 1999) for firefighters only simulates the weight of the PCE, which includes the SCBA, but there is no requirement to breathe from the respirator while performing the testing. In contrast, the PES developed for incumbent (but not recruit) Canadian military firefighters (Deakin et al. 1996;Todd Rogers et al. 2014 , it would be logical to ask whether a candidate that barely meets the PES testing without the requirement to breathe from the SCBA would meet the PES determined while carrying and breathing from the SCBA. Certainly additional research that highlights this issue would be a valuable addition to PES testing for occupational groups that require the use of a breathing apparatus as part of their PCE.

Protective Clothing and Heat Storage
Protective clothing is designed to confer protection for individuals from the hazards of their workplace, which might include fire, smoke, chemical spills, biological agents, falling objects, explosives and projectiles. To obtain the desired level of protection, therefore, the clothing may be relatively thick and/or have low air and water vapour permeability that limits the transfer of heat, liquid and gas from the environment to the worker. At the same time, however, the clothing restricts the transfer of metabolic heat and water vapour produced by the evaporation of sweat, from the body to the environment. As a consequence, the rate of body heat storage (S ሶ ) will be greater when the PCE is used. The effect of wearing PCE on work performance can be substantial with reductions being 50% or greater compared with the wearing of normal work clothing (McLellan 1993;McLellan et al. 1993). Thickness and vapour permeability characteristics of specific PCE's are provided in detail by McLellan et al. (2013). Military biological and chemical protective clothing, for example, is almost twice as thick as the business attire established as the reference clothing and water vapour permeability is reduced by 35% (McLellan 2008).
The heat balance equation, shown below, represents the relationship between avenues for heat exchange between the body and the environment. The impact of clothing insulation (I T ) and D r a f t 11 clothing water vapour resistance (R eT ) on dry (radiation, convection and conduction) and wet (evaporation from skin) heat transfer are also depicted in the heat balance equation.
The rate of heat production (M ሶ ) will always represent a source of heat gain whereas wet heat transfer through evaporation at the skin ((P sk -P a ) · R eT -1) or through respiration (E ሶ resp ) will generally represent an avenue of heat loss. Dry heat transfer depends on the temperature gradient between the ambient environment (T a ), the clothing and the skin (T sk ) and can represent either an avenue of heat loss (if skin temperature exceeds the clothing and ambient temperatures) or heat gain (if ambient and clothing temperatures exceed skin temperature). In some special cases, e.g.
of impermeable clothing, condensation of moisture may take place in the clothing and calculations become more complex. The reader is referred to specialist literature for this (Havenith et al. 2008;Havenith et al. 2013). Convective heat transfer through respiration (C resp ) is dependent on the temperature gradient between inspired and expired air and flow rates.
Under conditions where the requirement to dissipate metabolic heat from the body (E req ) exceeds the capacity of the environment to transfer this heat (E max ), uncompensable heat stress (UHS) is created where body heat storage and temperature continue to rise to individual limits of tolerance (Cheung et al. 2000;McLellan et al. 2013). The characteristics of the clothing and surrounding environment (temperature, vapour pressure, air speed, radiation) and the temperature and vapour pressure within the clothing determine E max , whereas M ሶ and the temperature gradient between the skin and the environment are the primary determinants of E req .
The relationship between M ሶ and ambient temperature and vapour pressure on tolerance limits is shown in Figure 3  becomes proportional to the vapour pressure gradient between the PCE and the environment allowing a balance to be achieved (McLellan et al. 1996).

Insert Figure 3 about here.
The curves shown in Figure 3 could also be used to explain the influence of changing thermal characteristics, or R eT , of the PCE on tolerance. For example, if the clothing becomes thinner (I T decreases) or less resistant to water vapour transfer (R eT decreases) the curve would shift to the right. In contrast, with more layers or additional thickness of the PCE or an increased resistance to water vapour transfer the curve would shift to the left. With totally impermeable clothing, such as used by hazmat workers (Beckett et al. 1986;Paull and Rosenthal 1987), the curve would be shifted far to the left, and the differentiation due to ambient relative humidity would be lost. Similarly, even within a given occupational group, such as firefighters, different countries may adopt different strategies for containing structural fires which may increase Arguably, the risk of becoming a heat casualty due to the continued rise in body temperature during UHS is the greatest concern when individuals don the PCE, especially in hot environments or when M ሶ is high ( Figure 3). For structural firefighters this risk is evident in less than 60 min while conducting heavy work in their PCE and exposed to ambient temperatures at or above 25°C (Selkirk and McLellan 2004). Yet the physical demand analyses that were used to generate the current PES for structural firefighters did not consider the impact of the heat strain of wearing PCE on the ability to conduct the job-related tasks in a safe and efficient manner (Jamnik et al. 2013 include an assessment of individual heat tolerance as discussed in more detail below.

Protective Clothing and Aerobic Fitness
Aerobic fitness is a key factor in understanding the individual variation to thermoregulation (Havenith and van Middendorp 1990;Havenith et al. 1995;Jay 2014). In addition, studies have shown that endurance trained men and women, who are typically leaner than their untrained counterparts, can tolerate larger increases in body temperature during UHS (Cheung and McLellan 1998a;Selkirk and McLellan 2001;Selkirk et al. 2008). Even during passive heating at rest endurance trained can tolerate greater increases in core temperature (Morrison et al. 2006). Regular aerobic exercise is accompanied by an expanded plasma volume, which confers greater protection from gut endotoxin leakage as thermal strain rises above 38.0ºC during UHS for the endurance trained (Selkirk et al. 2008). In addition, a given absolute metabolic rate and thermal strain represents a lower relative strain for the endurance trained, D r a f t leading to lower heart rates and less redistribution of blood flow away from the gut (Selkirk et al. 2008), as well as lower neuroendocrine responses (Wright et al. 2010;Wright et al. 2012 (Deakin et al. 2000). Certainly with the more conservative limit used to establish guidelines for the Toronto Fire Service, heat casualty rates would be reduced. However, 30% of the sedentary participants were still unable to tolerate increases in core temperature to 38.5°C.

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Approximately half of these individuals would also have been unable to meet the PES due to maximal fitness levels below 40 mL·kg -1 ·min -1 . Therefore, it is conceivable that 15% of new firefighter recruits who meet the PES would be unable to perform their tasks while wearing the PCE for extended periods before succumbing to heat strain. Certainly the constraints of wearing PCE would not permit these recruits to perform their duties in a safe and efficient manner.
It would seem reasonable to expect that the fitness level of the new recruit should ensure not only their ability to conduct work-related tasks but also, just as importantly, their ability to tolerate the heat strain associated with wearing the PCE required by their employment. If the ability to tolerate a certain level of thermal strain in PCE became a requirement for the PES then how would it be evaluated? Current PES task-based circuits that last 8-10 minutes (Deakin et al. 1996 Selkirk, 2006).
Some of the public safety occupational groups offer 'de facto' accommodation, such as familiarization to the PES testing and 6-week physical training programs, which have increased success rates during testing for recruits (Jamnik et al. 2013). Similar short-term aerobic training programs and/or acclimation to the hot-wet microenvironment of the PCE have not proven overly successful for improving heat tolerance while the clothing is worn (McLellan and Aoyagi 1996;Cheung and McLellan 1998b;Cheung and McLellan 1999). Since there is a rapid plasma volume expansion during the first few days of an aerobic exercise program (Green et al. 1987), it is likely that other factors, which require longer periods of adaptation, are important to account D r a f t 20 for the differences in heat tolerance between endurance trained and untrained (Selkirk et al. 2008).

Protective Clothing Equipment, Load Carriage and Aerobic Fitness
An entire chapter within this series is devoted to the impact of load carriage for PESs and the reader is directed to this work for greater detail on this topic (Taylor et al. 2016). In addition, however, there are issues that are directly relevant when PCE is worn together with the requirement to carry additional loads. For example, although current PES testing for firefighters require candidates to carry the weight of the clothing and equipment routinely used in the workplace, it is important to realize that it is an absolute load of approximately 23 kg that is used for all candidates (Deakin et al. 1996; International Association of Fire Chiefs 1999; International Association of Firefighters 1999). This absolute load, however, represents a different relative weight-bearing penalty that is dependent on body mass. For example, this load represents an additional 23% penalty for a larger 100 kg candidate but a far greater penalty of 35% or more for smaller candidates that might weigh 65 kg or less. The smaller individual, therefore, must be more aerobically fit than their larger counterpart to meet the PES while wearing or carrying the equivalent weight of the PCE. This effect is outlined below in Table 2 where it shows that the smaller individual requires a maximal aerobic fitness closer to 50 mL·kg -D r a f t 21 absolute oxygen cost of weight-bearing activity is determined by the total weight carried, which should include the body mass together with any clothing worn and equipment carried.
Insert Table 2 about here.
Although the smaller individual must be more aerobically fit to accommodate the additional weight of the PCE and meet the PES, this increased fitness is associated with other advantages while performing their duties. As mentioned above, the higher aerobic fitness should reduce their risk of succumbing to heat injury while wearing the clothing due to higher core temperatures that can be tolerated (Cheung and McLellan 1998a;Selkirk and McLellan 2001;Selkirk et al. 2008). Further, higher levels of aerobic fitness are typically associated with reduced levels of body fatness, which will slow the rate of increase in core temperature for any given rate of heat production (Selkirk and McLellan 2001) due to the higher specific heat of lean versus adipose tissue (Gephart and Dubois 1915).
Air demand from the SCBA also will be reduced for the smaller individual allowing them to perform their duties for longer periods of time before the requirement for air resupply. This was highlighted with actual measurement of air demand from the SCBA for incumbent firefighters during a simulated high-rise ascent to perform search and rescue (Williams-Bell et al. women and 40 participants were tested. Two of these women had a body mass below 65 kg but maximal aerobic fitness levels exceeded 55 mL·kg -1 ·min -1 , whereas the values varied from a low of 42 to over 65 mL·kg -1 ·min -1 for the male participants.
The reader should be convinced that the smaller individual, regardless of sex, must possess a higher aerobic fitness to meet the minimum requirement for any PES that imposes an absolute weight-bearing penalty to represent the PCE. We do not see this as being biased or unfair but instead would argue that the smaller individual who passes the PES would actually fair better than their larger counterpart when PCE is worn. These differences would be evident with their greater thermotolerance with the heat strain of wearing the clothing, as well as a reduced air demand and work of breathing if job requirements include the use of SCBA.

Recommendations for additional evidence-based research
The discussion above identified the following research topics that would assist in future development of PESs that involve wearing a PCE: 1. An assessment of the anthropometric factors that account for the individual variation in the load-bearing penalty of wearing different PCEs, while giving special attention to sex and age in this analysis;  Rogers et al. 2014). The use of the SCBA during PES testing should also be considered since this imposes limitations on aerobic power (Eves et al. 2005). It is also critical to identify those factors that influence the individual variation in the physiological penalty associated with wearing the PCE. Although the larger individual might tolerate the load-bearing penalty of the PCE more easily (Table 2), their heat tolerance may be reduced compared with their smaller counterpart who must have a higher aerobic fitness to D r a f t accommodate the load. Thus, in order to continue to perform their duties during their career, the smaller individual might actually have to maintain a higher level of aerobic fitness. In contrast, the larger incumbent firefighter, although being able to accommodate the load-bearing penalty of the PCE, may actually be at greater risk of succumbing to heat injury while wearing the protective clothing.
We would also recommend that a unique heat-tolerance test be developed that could be used in certain jurisdictions where there is an ongoing risk of UHS when PCE is worn on a regular basis or during specialized operations as part of the job requirement. This heat-tolerance test would be assessed separately within the hybrid PES model, just as aerobic fitness is assessed independently from the applicant's ability to perform job-related tasks for some occupational groups (Jamnik et al. 2013).
There are also certain public safety occupational subgroups where PESs have not been developed, yet the physiological strain of wearing a PCE is very high, such as occurs with the use of a bomb disposal suit or impermeable chemical protective clothing. Typically the individuals that wear these PCEs are selected from the incumbent ranks of the military, police or firefighters. However, it could be argued that the additional physiological burden of wearing these specific PCEs require unique adjustments to the PESs for incumbent personnel chosen to perform the job-related tasks.

Summary and Conclusions
Many public safety occupational groups require the use of PCEs on a regular basis, yet

Conflict of Interest
The authors declare that they have no conflict of interest.  D r a f t 37