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Evaluation of clothing comfort using human trials

The ASTM F 2668 standard is available to evaluate clothing comfort in a controlled laboratory environment, but clothing comfort in a controlled laboratory environment can be carried out per an investigator’s discretion. Additionally, field trials can be carried out according to investigators’ requirements and objectives. The ASTM F 2668 standard is believed to be appropriate for the evaluation of a majority of protective clothing ensembles, especially where wearers need to walk or perform similar activities; this practice utilizes a treadmill for the wearers’ exercise protocol. In certain situations, where a protective clothing ensemble is designed to be worn when the user is performing specialized functions (eg, sitting or standing with only arm movement), alternative exercise equipment (eg, arm cycle-ergonometers) or protocol should be considered for use in determining clothing comfort. Here, it is necessary to remember that it is ethically unacceptable to involve human beings for evaluating comfort in a thermal environment that firefighters face in an actual fire hazard. Thus, all researchers should use a temperate environmental chamber to evaluate the comfort provided by thermal protective clothing. During all human trials in a laboratory- controlled environment, a physician should be readily available; for human trials in a field environment, an ambulance service should be readily available in addition to a physician.

According to the ASTM F 2668 standard, first, a group of human subjects need to be selected by investigators as research participants. All the selected human subjects should be healthy, relatively fit, and must be medically screened (medical history and physical exam) to exclude those for whom the combined stress of exercise and hyperthermia could be dangerous. The selected human subjects should be without a history of heat injury/illness, chronic respiratory illness, orthopedic problems that might be exacerbated by exercise, and/or skin disorders (including sunburn) than can affect sweating or metabolic heat generation. The selected human subjects should not have a fever and/or sleep deprivation, and they should not be taking medications (in particular, antiinflammatories and antihistamines); in the case of female subjects, they should not be pregnant. Additionally, the anthropological characteristics of the selected participants should be: age = 18-40 years, weight = 65-100 kg for males and 50-90 kg for females, height = 1.7-1.95 for males and 1.6-1.85 for females. The subjects must maintain a normal exercise routine so that their fitness levels do not change during the trials.

After selecting participants based on the above preliminary criteria, they should be informed about the purpose of the study, any known risks, and their right to terminate participation without penalty. At this point, the participants will be asked to enter an environmental chamber, wearing identical clothing; this chamber must be large enough to accommodate a treadmill, a research participant, and at least two other people at the same time. The air temperature, air velocity, and air humidity of the chamber needs to be set as per the test requirements; and the test chamber must provide uniform conditions, both spatially (air temperature ±1°C; relative humidity ±5%; air velocity ±50%) and temporally (air temperature ±0.5°C; relative humidity ±5%; air velocity ±20%). The treadmill must accommodate all participants (small or large in size) safely and comfortably within a minimum running surface of 1.8 m x 1.6 m and should have a calibrated analog scale or digital indicator of speed (2-20 km/h in increments of 0.2 km/h) and angle of inclination (0-20%). In the test chamber, the clothed participants will rest for a determined time to adjust to the ambient atmosphere; subsequently, they will run on the treadmill for a baseline physiological assessment. During the baseline assessment, each participant’s maximum oxygen consumption (VO2max) or maximal aerobic power will be carried out to the nearest mL/kg/min using the ISO 8996 standard. Only individuals with the VO2max between 35 and 65 mL/kg/min will be screened as possible participants. At least eight possible participants who commit to multiple trial sessions at a rate of no more than once per day will be selected to evaluate clothing comfort. After selecting human subjects based on the above criteria, the selected subjects will participate in a 5-10-day exercise-heat acclimatization process. For this, the subjects should refrain from drinking alcohol and participating in heavy/vigorous exercise for at least 12 h before the adjustment period in the ambient environment each day. This acclimatization and familiarization process ensures that the thermoregulatory status of the subjects will not change during human trials and will provide the subjects with a better chance of completing the trials and of recovering faster to normal conditions after the trials.

After a preliminary acclimatization and familiarization trial of the selected participants, they will complete several actual test trials. Before the test trials, the participants must avoid moderate to high-level exercise 24 h prior to testing as well as stimulants or diuretics (eg, cigarettes and caffeine) 12 h prior to testing; a large meal should be avoided at least 3 h prior to the testing, with food consumption stopped 2 h prior to testing. In addition, all individuals should be properly hydrated prior to testing (eg, 500 mL approximately 2h before bed the night prior, then approximately 1-1.5 L approximately 1-2 h before the trial). Then, the selected participants will be asked to walk on the treadmill at the predetermined (as determined during baseline assessment) speed and grade (Fig. 5.25). Individuals will be tested at the same time of day to avoid influence of a diurnal variation in body temperature. During the actual test trials, their physiological parameters, such as maximum heart rate response, maximum oxygen consumption, core body temperature (esophageal, rectal, and intestinal) and mean skin temperature (forehead, right scapula, left upper chest, upper right arm, lower left arm, left hand, right anterior thigh, and left calf), and exposure time will be measured after a fixed interval by using the metabolic measurement cart, thermistors, heart rate meter, etc; the core and skin temperatures can be measured according to the ISO 9886 standard. The “whole body sweat rate” of the participants will be determined by subtracting the measured postdry nude body mass (if urination occurs during testing, the mass of urine shall be added to the postbody mass; if fluid consumption occurs during the testing, the mass of the fluid must be subtracted from the postbody mass) from predry nude body mass; and this rate will be used to assess the level of dehydration caused by the thermal stress of the test conditions. Occasionally, the subjective evaluations of the participants’ comfort will be recorded after certain intervals using the Likert or rating scale; in order to avoid any influence on their subjective ratings, the participants will not be informed about the details of the study, such as selection of clothing and air temperature. The termination of the test should result either by voluntary withdrawal, signs of impending heat illness (eg, disorientation, chills, or nausea), if heart rate reaches 90% of the measured maximum during baseline assessment, core temperature exceeds 39°C, or a skin temperature measurement at any location exceeds 38°C, or any combination of the above. Here, participants must attempt to last as long as possible to allow the physiological response to become sufficiently evident; a maximum time of 2 h

A participant on a treadmill during testing in the work physiology laboratory of University of Alberta

Fig. 5.25 A participant on a treadmill during testing in the work physiology laboratory of University of Alberta.

should be allotted for this test. A recovery period will be provided for participants upon completion of each test condition; at the end of the recovery period, the core temperature and heart rate should be below 38°C and 100 beats/min, respectively.

During the trials, the measured physiological parameters and/or subjective rating can represent the heat stress conditions for selected research participants. Thereafter, the association between different types of clothing, clothing attributes (property, size fittings), and heat stress conditions will be identified through statistical analysis; here, a clothing causing high/quick heat stress will be interpreted as clothing with low comfort. This analysis and interpretation may be used in the research and development of advanced ensembles designed to reduce heat stress on the wearer, thereby reducing the potential injury associated with wearing protective clothing ensembles. In this context, it is necessary to remember that extensive precautions, financial investment, and test designs are necessary during and after the experiments, as this type of test involves human subjects. Additionally, many ethical issues are involved to recruit research participants, who can exercise up to their volitional exhaustion. This is the reason that very few studies are available on the evaluation of comfort provided by clothing using human trials [165,384,466,467].

Assessment of clothing comfort using human trials: Huck and McCullough [384] examined the comfort provided by thermal protective clothing along with SCBA. They confirmed that the clothing, in combination with SCBA, contributed greatly to the physiological load of firefighters. Mountain et al. [165] studied the comfort associated with thermal protective clothing by evaluating wearers’ physiological parameters at their volitional exhaustion after exercise. They identified that thermal protective clothing possesses a significant role on the wearers’ body temperature depending upon exercise intensity and ambient temperatures. Here, the core body temperatures of clothed wearers increased very rapidly in comparison to unclothed wearers during exercise at a certain intensity in a constant ambient temperature; this situation resulted in a quick heat stress on clothed wearers. Barker [468] investigated the sensorial comfort of protective clothing using human trials. His research demonstrated that clothing comfort is a complex function of fabric properties and clothing design, as well as conditions of use. The study concluded that there is a need to select fabric materials carefully in order to achieve a high level of clothing comfort.

Similarly, Wu, Zhang, and Li [469] studied the comfort provided by different types of T-shirts made of 10 kinds of hygroscopic fibers: cotton, wool, lyocell, model, soybean, bamboo, and their blends. The experimental results showed that the thermal wet comfort of 10 T-shirts varied due to the fiber types, which mainly influenced the heat and moisture transfer during exercise. It was found that the natural hygroscopic fibers (eg, cotton and wool) are damper and more thermal than the other fibers (eg, regenerated cellulosic fibers, bamboo, soybean, and model). Additionally, model fiber is particularly adhesive to wearers’ bodies, bamboo has a significant coolness to the touch, and lyocell shows a higher thermal sensation than the other regenerated cel- lulosic fibers, with moderate dampness and stickiness. The comfort sensations of different body parts of the wearers were also very different during exercise. It seems that there is a need to carefully design T-shirts to provide better comfort to wearers.

Hostler et al. [470] compared the comfort provided by thermal protective clothing and regular clothing based on physiological parameters (eg, hydration and maximum oxygen consumption) of firefighters during their volitional exhaustion. It was established that thermal protective clothing possesses lower comfort than regular clothing because it causes dehydration to firefighters; this dehydration helps to quickly bring firefighters to their maximum oxygen consumption level. This situation results in high heat stress for firefighters during a certain duration of work.

Marszalek, Bartkowiak, and Lezak [119] reported that impermeable protective clothing generally possesses lower comfort than permeable clothing. They explained that impermeable clothing did not allow the transfer of sweat-vapor and metabolic heat from firefighters’ bodies, which ultimately causes a greater heat stress to wearers. Nevertheless, clothing comfort is directly dependent upon firefighters’ physiological attributes—intensity of sweating, sweat accumulation in different parts of the body, and moisture absorbency of the fabric materials used to manufacture the clothing. Recently, Wang et al. [457] determined the comfort provided by various types of clothing (thermal protective clothing, military clothing, high visibility clothing) on firefighters’ physiological parameters (maximum oxygen consumption, blood pressure) using human trial methods. They recognized that thermal protective clothing possesses lower comfortability than military or high visibility clothing, which causes a great heat stress on firefighters’ bodies. Additionally, various attributes (property, size fittings) of thermal protective clothing cause different levels of comfort. For example, thick, weighty, highly thermal-insulated and evaporative-resistant, tightly fitted, and/or minimal closure/vent-based thermal protective clothing do not allow for the transfer of metabolic heat and sweat-vapor from firefighters’ bodies to their ambient environments, which causes more heat stress on firefighters than thin, light-weight, less thermal-insulated and evaporative-resistant, perfectly fitted, and/or optimum closure/vent-based thermal protective clothing. It seems that the former types of thermal protective clothing possess lower comfortability than the latter. In this regard, many researchers suggested using the supplementary cooling systems (low pressure convective hand cooling, ice and phase change material cooling vests, liquid circulating garments [LCG]; immersion cooling systems [water immersion of extremities—hand, feet, and cranial]; fanning systems [whole body fanning]; and, head cooling systems) in combination with thermal protective clothing. These innovative approaches can enhance the comfort features of thermal protective clothing to reduce firefighters’ heat stress [471,472].

Yoo and Kim [473] analyzed the comfort provided by layered structure cold-weather protective clothing. Two five-layer, 100% polyester-based clothing ensembles (one underwear, three fleece insulated shirts, and one expanded polytetrafluoroethylene membrane laminated jacket) were constructed using different array types (an all- separated type and a combined type) in which the final insulating layer was mechanically attached to the jacket without an air gap. This study showed that the combined ensembles produced significantly less sweat, displayed 31.3% improved vapor permeability, and 25% lower sweat accumulation compared to the separated ensembles. Following exercise, the all-separated ensembles displayed up to a 74% greater cooling rate of skin temperature compared to the combined ensembles. Research participants also noted a tendency of warmer, drier, and less clammy conditions in the combined clothing ensembles. It was concluded that the layered structure clothing design can provide better protection to wearers.

Zavec, Wissler, and Mekjavic [474] evaluated the comfort provided by military clothing during a 3-h hike across a 160 m high ridge. The comfort values of the soldiers measured during the trial were oxygen consumption, skin core temperature, regional thermal fluxes, clothing temperature, and environmental conditions. Subjective assessments of thermal comfort were made at regular intervals during the hike. It was found that military clothing provided a different degree of thermal and evaporative resistance in different body parts. Thermal resistance determined from the field data was generally larger during the first 45 min of hiking than during the rest of the hike, which can reasonably be attributed to accumulation of sweat in the garments. Additionally, many other factors (weight of the soldier and his load, walking speed, and the angle of inclination of the terrain) along with the clothing also affected the maximum oxygen consumption of the soldiers during hiking.

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