Abstract Armed forces typically have personal protective clothing (PPC) in place to offer protection against chemical, biological, radiological and nuclear (CBRN) agents. The regular soldier is equipped with permeable CBRN-PPC. However, depending on the operational task, these PPCs pose too much thermal strain to the wearer, which results in a higher risk of uncompensable heat stress. This study investigates the possibilities of adjustable CBRN-PPC, consisting of different layers that can be worn separately or in combination with each other. This novel concept aims to achieve optimization between protection and thermal strain during operations. Two CBRN-PPC (protective) layers were obtained from two separate manufacturers: (i) a next-to-skin (NTS) and (ii) a low-burden battle dress uniform (protective BDU). In addition to these layers, a standard (non-CBRN protective) BDU (sBDU) was also made available. The effect of combining clothing layers on the levels of protection were investigated with a Man-In-Simulant Test. Finally, a mechanistic numerical model was employed to give insight into the thermal burden of the evaluated CBRN-PPC concepts. Combining layers results in substantially higher protection that is more than the sum of the individual layers. Reducing the airflow on the protective layer closest to the skin seems to play an important role in this, since combining the NTS with the sBDU also resulted in substantially higher protection. As expected, the thermal strain posed by the different clothing layer combinations decreases as the level of protection decreases. This study has shown that the concept of adjustable protection and thermal strain through multiple layers of CBRN-PPC works. Adjustable CBRN-PPC allows for optimization of the CBRN-PPC in relation to the threat level, thermal environment, and tasks at hand in an operational setting. CBRN, Man-in-Simulant Test, personal protective equipment, thermal burden, thermal strain, whole system test Introduction Armed forces typically have personal protective clothing (PPC) in place to offer protection against chemical, biological, radiological and nuclear (CBRN) agents. Protection requirements for regular forces focus mainly on gases/vapors and liquids (North Atlantic Treaty Organization, 2011). Gas-tight systems are not considered suitable for use in most military operational environments, because they interfere with movement and induce substantial heat strain. Rather, military CBRN-PPC systems are required to be air permeable, which facilitates exchange of air from the microclimate between skin and clothing and the exterior, and vice versa. In order to preclude (hazardous) agents from penetrating the clothing, adsorbent layers are applied, often consisting of activated carbon. There are also other technologies, e.g., selective membranes. However, depending on the operational task, these permeable clothing systems pose too much thermal strain on the wearer (Havenith, 1999, 2011; McLellan et al., 2013). The increased thermal strain caused by PPC originates from (i) the resistance to heat and mass transfer (more specifically water vapor transfer) caused by the protective technologies that are employed (Havenith, 1999; McLellan et al., 2013), and (ii) increased metabolic rate, and thereby heat production, due to the weight and stiffness of the systems (Dorman and Havenith, 2009). The consequence of the reduced heat and mass transfer, as well as the increased heat production, creates a higher risk of uncompensable heat stress. The effects of heat strain and uncompensable heat stress are well documented. They also show negative effects on cognitive faculty (Hancock et al., 2007) and time to exhaustion on endurance exercise (Montain et al., 1994; Galloway and Maughan, 1997; González-Alonso et al., 1999). However, heat strain can also cause heat illnesses. Heat stroke is the most severe, even life-threatening, condition (Wexler, 2002). Currently, most armed forces have one single general purpose CBRN-PPC system in service, offering high levels of protection and corresponding high levels of thermal strain. In most cases, the level of protection is based on large-scale, cold-war scenarios, which are less likely to be encountered currently. At present, a variety of less severe scenarios are considered to be relevant. Therefore, the number of relevant scenarios has increased over the previous decade. A CBRN-PPC system offering adjustable protection and thermal strain, in which the CBRN-PPC configuration can be adjusted to the threat level and to the expected thermal stress (as a function of task, terrain, and environment), may provide an ideal solution. Such adjustable CBRN-PPC system should consist of at least two PPC layers, e.g., a next-to-skin (NTS) and a battle dress uniform (BDU) both layers with CBRN protection integrated, besides the standard, non-CBRN-protective BDU. By changing the combination of layers, the protection of the PPC, and the thermal strain it poses onto the user, can be balanced. In fact, four clothing configurations can be created, i.e., (i) NTS as only layer, (ii) protective BDU (pBDU) only, (iii) NTS combined with pBDU, and (iv) NTS combined with the standard BDU (sBDU). These four different clothing configurations are given in Fig. 1. The NTS only (configuration 1) is likely to provide the lowest thermal burden but also the lowest level of protection, whereas the configuration of NTS and pBDU (configuration 3) poses the highest levels of protection as well as the highest level of thermal burden. Therefore, our study aimed to explore the concept of adjustable PPC, focusing on the effect of combining clothing layers on protection and thermal strain. Figure 1. View largeDownload slide The different clothing configurations, as indicated. These configurations were evaluated for the products of manufacturers A and B. Figure 1. View largeDownload slide The different clothing configurations, as indicated. These configurations were evaluated for the products of manufacturers A and B. Previous numerical simulations performed by our group suggest that combinations of layers have added value in CBRN-PPC, as the protection levels tend to increase to a larger extent than the resistance to heat and mass transfer (Ambesi et al., 2013). It is hypothesized that by adding an additional clothing layer that fits more tightly around the body the airflow through the layer closest to the skin is strongly reduced since (i) the outer layer reduces the effect of wind speed, and (ii) the microclimate between the tight-fitting clothing layer and the skin is small thereby providing reduced possibility for air to flow causing an increased air permeability resistance. This dynamic behavior between penetration airflow and fabric layers further reduces the transmission and potential deposition on the skin. In addition, the human body loses heat through combined heat and mass transfer, which not only depends on diffusion of water vapor molecules, but also on the convective heat transfer processes around the body (Ambesi et al., 2013). Results of numerical studies suggested a considerable increase in protection when combining multiple layers of clothing. These results were, although to a limited extent, confirmed in laboratory experiments on cylinders covered with protective material and exposed to aerosols and a vapor simulant (methyl salicylate, MeS) (TNO, unpublished data). The relationship between penetration of airflow and the combination of fabric layers cannot be mimicked in the standard material tests that are performed on CBRN-PPC materials. In those tests (North Atlantic Treaty Organization, 2011; Test Operations Procedure, 2013), the contaminated air is forced through the material and immediately collected underneath (the so-called ‘convective flow test’). Previous studies performed at our laboratory showed that in a real-life, 3-dimensional simulation, a majority of the air coming through the material will actually exit again through the back side (Brasser, 2004; Sobera and Kleijn, 2008; Ambesi et al., 2013). In these tests, a cylinder donned with CBRN-PPC material was used. Airflow was projected onto the cylinder, but could also flow around the PPC material or exit the microclimate as described above, resulting in a more realistic condition compared to the standard material tests. This led to substantially higher levels of protection of such air-permeable systems than can be expected from the convective flow material tests. The effect of single and combined CBRN-PPC layers is explored in this study. In addition, the effect of combining protective and non-protective (regular) clothing layers on the levels of protection to the human body has been investigated. These evaluations were performed with the so-called Man-In-Simulant Test (MIST) that has been described in the literature and has been validated for many clothing systems (Duncan and Dickson, 2003; North Atlantic Treaty Organization, 2011; American Society for Testing and Materials, 2012; Ormond, 2012). Finally, a mechanistic numerical model was employed to give insight into the thermal burden of the evaluated CBRN-PPC concepts. Methods Clothing concepts Upon our written request, two manufacturers kindly provided clothing systems for this study. Most were in the prototype phase of development. All systems consisted of a tight-fitting, NTS garment, and a (wider fitted) pBDU, both equipped with an activated carbon technology. Details on these systems are given in Table 1. In addition, a standard, non-CBRN pBDU (sBDU) was included. Table 1. Textile properties of the different systems. In all cases, CBRN protection is being offered on the basis of a woven fabric containing activated carbon. Manufacturer A A B B C Layer pBDU NTS pBDU NTS sBDU CBRN protection Yes Yes Yes Yes No Thickness (mm) 1.1 0.8 0.6 0.4 0.4 Fabric weight (g m−2) 471 364 289 133 500 Air permeability (L m−2 s−1 @ 200 Pa) 554 474 758 1750 300 Evaporative resistance (Ret)* (m2 Pa W−1) 13.7 6.4 5.8 3.0 3.0 Thermal resistance (Rct)* (m2 K W−1) 0.038 0.015 0.024 0.012 0.010 Manufacturer A A B B C Layer pBDU NTS pBDU NTS sBDU CBRN protection Yes Yes Yes Yes No Thickness (mm) 1.1 0.8 0.6 0.4 0.4 Fabric weight (g m−2) 471 364 289 133 500 Air permeability (L m−2 s−1 @ 200 Pa) 554 474 758 1750 300 Evaporative resistance (Ret)* (m2 Pa W−1) 13.7 6.4 5.8 3.0 3.0 Thermal resistance (Rct)* (m2 K W−1) 0.038 0.015 0.024 0.012 0.010 *According to (International Organisation for Standardisation, 1993). View Large For each system, the following configurations were evaluated: (i) NTS only, (ii) pBDU only, (iii) NTS with pBDU, and (iv) NTS with sBDU. These four different configurations are visualized in Fig. 1. All systems were worn in combination with a gas mask (FM12, Avon, Melksham, UK), gloves (Paul Boyé, Le Vernet, France), butyl rubber overboots (Airboss, Bromont, Quebec, Canada), standard military socks, and the cotton T-shirt and shorts. For the NTS without hood (manufacturer B), a standard hood was used (SR3, Remploy, Merseyside, UK). Finally, a belt was used in conjunction with each NTS to reduce the substantial leakage through the interface between the shirt and pants of the garment, as they were not designed to be worn as the only layer of clothing, and the interface between shirt and pants was therefore not optimal for single-layer use. The belt reduced leakage of contaminated air from the exterior to the microclimate underneath the clothing. In total, two systems and four configurations resulted in (2 × 4 =) 8 combinations that were tested for protection using the MIST method. The aim was to test all combinations three times; however, due to the limited number of clothing samples received, this was not always possible (details provided in the Results section). Participants Ten male participants took part in this study (age: 28.4 ± 13.4 years, height: 1.8 ± 0.1 m, and weight: 74.1 ± 8.2 kg). All participants were included on the basis of a favorable outcome on a health questionnaire. In addition, the participants were selected so that their clothing sizes optimally matched with the sizes of the prototypes of the clothing concepts. The participants were requested not to use any deodorants, after-shaves, perfumes, or hair styling gels, as these may cause false positive readouts on the applied dosimeters. Finally, the experimental protocol was reviewed and approved by the TNO Ethics Committee and was in accordance with the Helsinki Declaration of 1975, as revised in 2013. Protocol A typical MIST protocol was used, complying to NATO’s AEP-38 (North Atlantic Treaty Organization, 2011). In brief, after arrival, the participants undressed down to their underwear and were equipped with dosimeters on their skin (details provided in the Dosimeters section). After this, they donned the PPC configuration. A proper fit of the respirator was verified through measurement of the protection factor (PF) by means of a PortaCount (TSI, Shoreview, MN, USA). This measurement determines the ratio between particles in the ambient air outside the mask and the inside of the gasmask. A proper fit was defined as a PF ≥2000. Consequently, the participants entered the MIST chamber, in which they carried out an exercise protocol (Table 2). After having completed the 60-min protocol, they exited the chamber and undressed. Afterwards, the dosimeters were collected and stored so that their status relative to the measurement would remain unchanged. Table 2. The tasks carried out as part of the exercise protocol according to NATO’s AEP-38 (North Atlantic Treaty Organization, 2011). Task A Move weights. Two 2.5 kg weights are repeatedly lifted from a shelf (at waist level) and placed on the floor. A standing position is then assumed. The weights are retrieved and returned to the shelf. The right side of the participant faces the wind. B Sit (facing wind). C Jumping jacks. One jumping jack is performed about every 2 s, and every few seconds, the participant rotates position so that a different aspect is facing the wind. D Sit (back to wind). E Walk on treadmill. The participant walks at a rate of 1.3–1.8 m s−1 facing the wind. F Sit (facing wind). G Climb ladder. The test participant repeatedly climbs 2–3 steps of a stepladder and touches the ceiling of the vapor test chamber (alternating hands), then returns to the floor and squats to touch the floor with both hands. The back of the test participant faces the wind. H Sit (back to wind). The participant looks side-to-side then at ceiling and floor every 15 s. Task A Move weights. Two 2.5 kg weights are repeatedly lifted from a shelf (at waist level) and placed on the floor. A standing position is then assumed. The weights are retrieved and returned to the shelf. The right side of the participant faces the wind. B Sit (facing wind). C Jumping jacks. One jumping jack is performed about every 2 s, and every few seconds, the participant rotates position so that a different aspect is facing the wind. D Sit (back to wind). E Walk on treadmill. The participant walks at a rate of 1.3–1.8 m s−1 facing the wind. F Sit (facing wind). G Climb ladder. The test participant repeatedly climbs 2–3 steps of a stepladder and touches the ceiling of the vapor test chamber (alternating hands), then returns to the floor and squats to touch the floor with both hands. The back of the test participant faces the wind. H Sit (back to wind). The participant looks side-to-side then at ceiling and floor every 15 s. The tasks were carried out in a random order and took 3.75 min each. Each task was executed twice; the total protocol lasted 60 min. View Large Test chamber The test chamber consisted of an 18-m long, closed-circuit wind tunnel. Temperature, wind speed, and concentration of MeS could be controlled in the chamber. MeS was used as challenge agent, since it simulates sulfur mustard with respect to the CBRN-protective technologies employed in the evaluated PPC. This simulant for sulfur mustard is relatively non-toxic for the participants in the exposure levels used in this study. The MeS vapor was generated in a controlled evaporator mixer behind the exposure section of the chamber, so that the whole length of the tunnel was used to mix the vapor with the test chamber’s atmosphere. The challenge concentration was monitored online by a gas chromatograph equipped with a flame ionization detector. The vapor concentration MeS in the test chamber was kept stable at 82 ± 4 mg m−3, which resulted in an exposure dosage experienced by the participants of 4929 ± 248 mg min m−3. The applied wind speed in the chamber was 4.5 ± 0.5 m s−1 with an ambient temperature of 25.1 ± 1.2°C and a relative humidity of 53 ± 5%. Dosimeters Passive adsorbing samplers (PADs) were used as dosimeters. These PADs are sensitive to MeS and were attached to the participants’ skin in 41 different locations (Fig. 2). The PADs consist of a pouch that is MeS permeable, with a total adsorbing surface area of 1.8 × 2.0 cm2. The passive adsorbent used was 80 mg of Tenax TA 60/80 mesh, which was fabricated in our own laboratory. Figure 2. View largeDownload slide Sampler locations (North Atlantic Treaty Organization, 2011). Figure 2. View largeDownload slide Sampler locations (North Atlantic Treaty Organization, 2011). During the experiment, the MeS that entered the suit—either via openings in the suit or permeation through the material—was adsorbed onto the Tenax of the PADs underneath the clothing. The adsorbed quantity of MeS on the PADs was analyzed after the experiment by means of thermal desorption gas chromatography (see also Ormond, 2012). Thermal strain modeling In order to create an initial understanding of the thermal strain of the different clothing concepts and their layer combinations, simulations were carried out using a mechanistic model on thermal strain (referred to as SCOPE Light version 1.1.0-b) as a function of (i) thermal environment, (ii) task, (iii) clothing, and (iv) individual characteristics. The model was used in multiple publications regarding the effect of clothing on thermal strain (Lotens, 1993; Havenith, 1997; DenHartog, 2002; Heus et al., 2014). The input parameters to the model are given in Table 1 (clothing properties) and Table 3 (other characteristics). The conditions in Table 3 were chosen to give a good basis for differentiating between the different conditions. It should be noted that the conditions used for thermal strain modeling do not reflect the conditions of the MIST. In addition to the parameters stated in Tables 1 and 3, the SCOPE Light model also requires specification of the fit via determination of the different air layers in the system (microclimates) to allow for accurate simulation of the ventilation and convective heat loss effects. Of all clothing systems and their configurations (under garment plus over garment), the air layers were manually determined by measuring the circumferences on the various body parts, according to Lotens’ method (Lotens and Havenith, 1991; Daanen et al., 2005). Thus, all clothing configurations were simulated for thermal strain with the design and fit of the real garments used in the MIST protocol. Table 3. Input parameters on climate, activity, and individual characteristics. Physical Activity Thermal environment Person Type Walking Wind speed 1 m s−1 Age 28 Years Speed 6 km h−1 Radiation 0 W m−2 Body Mass 74 Kg Distance 6 Km Relative Humidity 50% Body Height 1.8 M Slope 1% Air Temperature 30°C Fat 15% External weight 25 Kg Acclimatization 100% Terrain Light brush Physical Activity Thermal environment Person Type Walking Wind speed 1 m s−1 Age 28 Years Speed 6 km h−1 Radiation 0 W m−2 Body Mass 74 Kg Distance 6 Km Relative Humidity 50% Body Height 1.8 M Slope 1% Air Temperature 30°C Fat 15% External weight 25 Kg Acclimatization 100% Terrain Light brush The values presented here were used as standard input values with the clothing characteristics varying between the simulations. View Large Protection factor The PF was derived from several steps of data processing, detailed below according to internationally employed standards (North Atlantic Treaty Organization, 2011; American Society for Testing and Materials, 2012). The uptake speed of the PADs is limited, as they are covered by a polyethylene foil to simulate skin uptake rates (Ormond, 2012). The rate of uptake of the samplers for the current set of experiments was measured at 26 ml min−1. The conversion of the measured quantity into a dosage was: skin dose(mg.min/m3)=mass on sampler(ng)uptake rate sampler(ml/mn) (1) Since the actual test conditions (in terms of ambient MeS concentration and exact duration) always differed slightly from the desired settings, the measured penetrated dosages were normalized to the desired challenge dosage. The desired challenge dosage was 5000 mg min m−3, and the average measured dosage was 4,929 ± 248 mg min m−3. The coefficient of variation of the difference between the actual and desired dosage was 5%. The normalization of the measured penetrated dose was carried out as follows: normalized skin dose=measured penetrated dose×desired challenge doseactual challenge dose (2) The test results were reported as PF offered by the PPC system. The PF was determined by the ratio between the desired MeS dosage outside of the clothing and the normalized MeS dosage as measured on the skin, according to the following equation: protection factor=desired challenge dosenormalized skin dose (3) The PF was obtained for each of the 41 PAD locations. From these data, the following parameters were derived: (i) systemic PF and a (ii) local PF, both according to NATO’s AEP-38 (North Atlantic Treaty Organization, 2011). The systemic PF is a weighted average that takes into account the relative surface area of the body part on which the PAD(s) is/are located and the sensitivity to a highly toxic chemical warfare agent (VX) of that body part. The local PF is the single lowest PF measured during a given trial. Statistics All data is given as means ± standard deviation. The data processing was carried out with MATLAB R2012a. Analysis of variance (ANOVA) was used for statistical analysis using a between-participant approach, with a Bonferroni post-hoc test upon statistical significant effects (P < 0.05). Statistical analysis was carried out with IBM SPSS Statistics 22. Given the low number of trials per condition (detailed under Results), the value of the statistical analysis is limited. However, the consistency of the statistical effects does support a generic effect of combining clothing layers on the level of protection. Results MISTs are typically carried out for PPC systems with a relatively low air permeability. Since the CBRN-PPC systems evaluated here have a much higher air permeability compared to classic CBRN-PPC systems, a number of pilot experiments were performed in order to find out which wind speed had to be used to reliably establish a difference in PFs between the various clothing configurations. At low wind speeds (<<4 m s−1), the PF of the NTS was already near the maximum measurable value of PF = 5000, which implies that an increase in the protection afforded by additional clothing layers would neither be detectable nor relevant. By increasing the wind speed to 4–5 m s−1, the PF of the NTS was reduced to PF = 100, providing sufficient ‘headroom’ to observe improvement of the PF by combining several PPC layers. The number of repetitions (or trials) per condition was limited, given that new/unused clothing was used for each trial and the supplied number of new/unused clothing items was constant. Table 4 gives the number of trials carried out per condition, as well as all results obtained from the MIST evaluation. Table 4. Results for the protection factor as obtained per manufacturer and condition as indicated. Manufacturer Configuration Number of trials Wind speed (m s−1) Local protection factor Systemic protection factor Mean Mean Median Max Min Std Mean Median Max Min Std A NTS 3 4.1 12 12 12 12 0 74 79 79 63 9 B NTS 3 4.0 12 12 12 12 0 81 81 87 75 8 A pBDU 2 5.1 34 30 52 20 17 154 161 165 137 15 B pBDU 3 4.1 79 23 195 19 100 182 182 195 169 13 A NTS + pBDU 2 5.3 27 26 34 22 6 155 149 168 148 12 B NTS + pBDU 3 4.0 12 12 12 12 0 74 79 79 63 9 A NTS + sBDU 3 5.1 13 13 15 12 2 50 42 66 42 14 B NTS + sBDU 3 4.1 34 30 52 20 17 154 161 165 137 15 Manufacturer Configuration Number of trials Wind speed (m s−1) Local protection factor Systemic protection factor Mean Mean Median Max Min Std Mean Median Max Min Std A NTS 3 4.1 12 12 12 12 0 74 79 79 63 9 B NTS 3 4.0 12 12 12 12 0 81 81 87 75 8 A pBDU 2 5.1 34 30 52 20 17 154 161 165 137 15 B pBDU 3 4.1 79 23 195 19 100 182 182 195 169 13 A NTS + pBDU 2 5.3 27 26 34 22 6 155 149 168 148 12 B NTS + pBDU 3 4.0 12 12 12 12 0 74 79 79 63 9 A NTS + sBDU 3 5.1 13 13 15 12 2 50 42 66 42 14 B NTS + sBDU 3 4.1 34 30 52 20 17 154 161 165 137 15 View Large Local protection factor Fig. 3 and Table 4 show the calculated local PFs. For manufacturer A, differences in PF between the NTS and both configurations with double-layer protection were observed (NTS + pBDU (p = 0.003), and NTS + sBDU (p = 0.009)). The single pBDU layer showed similar differences to both systems with double layers [NTS + pBDU (p = 0.005) and NTS + sBDU (p = 0.016)]. These results indicate that combining NTS with a BDU layer substantially increases the PF. In fact, the PF increases by a factor of 2.6 ± 0.3 for a double-layer configuration (PF = 30.2 ± 3.9) compared to a single-layer configuration (PF = 12 ± 0.1) for manufacturer A. For the prototypes from manufacturer B, the combined-layer configurations (PF = 78.1 ± 1.6) increase the PF relative to the single NTS (PF = 13) by a factor of 5.9 ± 0.1. However, these effects for manufacturer B did not reach statistical significance. Figure 3. View largeDownload slide Local protection factors (PFs) averaged over the different trials, for the different clothing configurations as indicated. The error bar indicates one standard deviation. Figure 3. View largeDownload slide Local protection factors (PFs) averaged over the different trials, for the different clothing configurations as indicated. The error bar indicates one standard deviation. Figure 4. View largeDownload slide Systemic protection factors (PFs) averaged over the different trials, for the different clothing configurations, as indicated. The error bar indicates one standard deviation. Figure 4. View largeDownload slide Systemic protection factors (PFs) averaged over the different trials, for the different clothing configurations, as indicated. The error bar indicates one standard deviation. Systemic protection factor The results for the calculated systemic PFs are given in Fig. 4 and Table 4. For the clothing configurations from manufacturer A, differences between the single NTS and both double layers were found [NTS + pBDU (P = 0.008) and NTS + sBDU (P = 0.049)]. For the single pBDU layer, differences were found for the combined pBDU (P = 0.016). For the configurations with combined layers, the PFs were a factor 7.5 ± 2.4 higher, compared to the single-layer configuration (manufacturer A). No statistical differences were observed for the configurations of manufacturer B (P = 0.143). Interestingly, the data for all configurations from the three manufacturers shows a lack of difference among the systems with two protective layers (NTS + pBDU) and the systems with the protective NTS plus sBDU (NTS + sBDU). Thermal strain simulations The thermal strain of the CBRN-PPC configurations, as predicted from the SCOPE Light simulations, is presented in Fig. 5 as the core temperature after 60 min of exercise. The exercise is described in more detail in the Methods section, and amounted to a metabolic rate of 718 W. The predicted core temperature after 1 hour of simulated exercise is given in Fig. 5. Given that a core temperature of 39.0°C is generally accepted as a threshold above which the likelihood for heat-related illness substantially increases (International Organisation for Standardisation, 2004), one can observe that this threshold is reached in many cases. Figure 5. View largeDownload slide Simulated core temperature after 60 min of exercise for the different clothing configurations. Figure 5. View largeDownload slide Simulated core temperature after 60 min of exercise for the different clothing configurations. Skin temperature and sweat rate differences reflected the differences in predicted core temperature. As expected, both single-layer systems performed better than both double-layer systems, for all manufacturers. Finally, the NTS with sBDU resulted in a higher thermal burden compared to the NTS with pBDU. The sBDU’s higher resistance for heat and mass transfer, compared to the pBDU, likely caused this difference. Fig. 6 combines the calculated systemic PF and the predicted thermal strain for the various configurations. The Pearson’s correlation coefficient between systemic PF and thermal strain is r2 = 0.30. This indicates that about 30% of the variance in the (predicted) thermal strain data is explained by the variance in the PF. Figure 6. View largeDownload slide Relationship between predicted thermal strain and measured/calculated systemic protection factor, with Pearson’s r2 = 0.30. Figure 6. View largeDownload slide Relationship between predicted thermal strain and measured/calculated systemic protection factor, with Pearson’s r2 = 0.30. Discussion This manuscript presents the evaluation of the effect of combining clothing layers on the protection against gases/vapors. Moreover, it addresses the protective performance of combined layers in relation to thermal strain. The concept of adjustable protection provides the user with the opportunity to adjust the PF to the actual chemical threat level and, concomitantly to adjust levels of thermal strain. The PF’s potential for adjustability has been experimentally demonstrated in the MIST set-up. Although thermal strain was not experimentally monitored, but rather simulated with a numerical model, the results indicate that the NTS and pBDU by themselves will not cause an unacceptable increase in core temperature during a 60-min exercise period. The required level of protection is dictated by the (presumed) threat levels, which are quantified by the challenge dosage (i.e., expected agent concentration multiplied by the exposure time). In addition, to obtain PFs, toxicological threshold values for dermal exposure need to be known. Dividing the challenge dosage by the toxicological threshold gives the minimum required PF. The PF therefore indicates with which factor the PPC needs to reduce the challenge. NATO gives 700 mg min m−3 as the vapor dosage that should be expected in a low-intensity conflict (North Atlantic Treaty Organization, 2012). For example, sulfur mustard is a dermally highly toxic agent, with a local dermal toxicology threshold of 50 mg min m−3 (North Atlantic Treaty Organization, 2003). At this threshold, only negligible health effects are expected. PPC should therefore offer a PF of at least 14 (= 700/50). Table 4 indicates that most systems achieve a PF just short of 14. However, for less toxic vapors, the single-layer configurations will most likely provide sufficient dermal protection. The local PF is simply the lowest PF measured over all measurement sites. This value is strongly affected by suboptimal fit of the PPC, causing local leakage through interfaces, which emphasizes the importance of adequate design for a CBRN-PPC. The systemic PF is much larger than the local PF, since it is a weighted average of all measuring sites. The systemic PF indicates that the material itself is most likely not the limiting factor. As expected, higher levels of protection were provided by the pBDU. Interestingly, the PF of manufacturer B’s pBDU is lower than that of the corresponding NTS. Combining the two layers provides PFs that are much higher than those of the individual clothing layers, which demonstrates the potential of adjustable protection concept. Even a combination of NTS and a sBDU (non-CBRN protective) provides very high PF values. These trends were observed for both the local and the systemic PFs. The results indicate that combining the NTS with the sBDU substantially improves the PF in comparison with the NTS alone. This is most likely caused by the reduction of airflow by the outer layer (i.e., the sBDU). In this case, the wind speed of the exterior air is reduced when passing through the outer layer, therewith reducing the wind speed before it intercepts with the NTS. The overall result is a decreased likelihood of agent penetration. As the sBDU had lower air permeability than the pBDUs (Table 1, 300 versus > 500), it showed a greater decrease in wind speed under the garment, as had been predicted by previous simulations (Brasser, 2004; Sobera and Kleijn, 2008; Ambesi et al., 2013). Besides the PF, as has been measured in the present study, the duration of the protection is also relevant. The protective layers are all equipped with an activated carbon adsorbent. At some point in time, the adsorbent may become saturated and will thus not adsorb additional toxic agent. This point of saturation is referred to as the ‘breakthrough time’, which is typically assessed by material tests (e.g., Test Operations Procedure, 2013). Although the actual breakthrough time was not assessed in the present study, it was verified that the clothing layers used did not reach the breakthrough time during the 60-min MIST procedure. In fact, the breakthrough time depends upon the total amount of adsorbent available (Linders, 1999; Ormond and Barker, 2014). Thus, combining layers does not only result in an increase in PF, it likely also substantially prolongs the breakthrough time and therefore the protection duration. The number of repetitions of the different conditions per manufacturer was limited. In fact, for manufacturer B, only one measurement could be carried out per condition. Therefore, the results obtained with prototypes delivered by manufacturer B have not been included in the statistical analysis. For the other manufacturers, three measurements per condition could be carried out (Table 4). The statistical power (Cohen, 1988) of the ANOVA carried out for manufacturers A and C was 0.98 and 0.40, respectively. A power of 0.80 or higher is usually considered acceptable. This justifies the statement that the generic effect observed in the present results is valuable to consider (see also the Statistics section). In addition, insufficient power for manufacturer C indicates that MIST protocol for the studying effect sizes as observed in the present study are optimally carried out with a larger total sample size than in the present study. Given the results of the present study, G * Power (version 18.104.22.168; Faul et al., 2007) was used to estimate the required sample size at 7 and 17, respectively, for manufacturers A and B. Input and results are given in Table 5. This information can be used to design future studies involving the MIST protocol. Table 5. Input and result of the determination of the total sample size. Manufacturer Observed power η2 Effect size Degrees of freedom Groups Total sample size Present study Minimally required A 0.979 0.876 2.66 3 4 10 7 B 0.401 0.474 0.95 3 4 12 17 Manufacturer Observed power η2 Effect size Degrees of freedom Groups Total sample size Present study Minimally required A 0.979 0.876 2.66 3 4 10 7 B 0.401 0.474 0.95 3 4 12 17 View Large Conclusions A concept is presented in which the protection against chemical warfare agents has been made adjustable, by using multiple clothing layers in a CBRN-PPC system. As expected, the thermal strain posed by the different clothing layer combinations decreases as the level of protection decreases. Such a concept allows the selection of a combination of layers that not only faces up to the required threat level (protection), but also takes the operational environment and task into account. For instance, in a situation of a perceived low threat level, the use of just the NTS layer would suffice for protection. Of course, the NTS will always be combined with some type of uniform, together providing a low level of thermal burden. At a higher threat level, the NTS can be combined with a pBDU. This combination of clothing layers will provide a higher level of protection compared to a single-layer configuration, but also will induce more thermal burden. This study has shown that the concept of adjustable protection and thermal strain through multiple layers of CBRN-PPC works. In view of the circumstances under which modern-day military operations take place, this may be a suitable alternative for the current, in-service CBRN-PPC, which manifests high PFs and concomitant high thermal strain. Total protection time should also be taken into consideration in future studies. In addition, a CBRN-PPC system with multiple layers also introduces a more complex decision-making process for military planners and commanders. In such a process, threat, task, environment, and personal characteristics should be integrated. A decision support system might advise decision-makers. However, such support systems are not yet available. Acknowledgements Funding for this project was provided by The Netherlands Ministry of Defense, under R&T program . The authors declare no conflict of interest relating to the material presented in this Article. Its contents, including any opinions and/or conclusions expressed, are solely those of the authors. The authors gratefully acknowledge the efforts of Toni Bellanca, Ruud Busker, Saskia de Kant, Ilse Tuinman, Pim Rensink, ProQares, The Netherlands Ministry of Defence, and suppliers of prototypes of CBRN-PPC concepts. References Ambesi D, Bouma R, den Hartog Eet al. ( 2013) Predicting the chemical protection factor of CBRN protective garments. J Occup Environ Hyg ; 10: 270– 6. Google Scholar CrossRef Search ADS PubMed American Society for Testing and Materials. 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Annals of Work Exposures and Health (formerly Annals Of Occupational Hygiene) – Oxford University Press
Published: Mar 1, 2018
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