In many occupational settings, clothing must be worn to protect individuals from hazards in their work environment. However, personal protective clothing (PPC) restricts heat exchange with the environment due to high thermal resistance and low water vapor permeability. As a consequence, individuals who wear PPC often work in uncompensable heat stress conditions where body heat storage continues to rise and the risk of heat injury is greatly enhanced. Tolerance time while wearing PPC is influenced by three factors: (i) initial core temperature (Tc), affected by heat acclimation, precooling, hydration, aerobic fitness, circadian rhythm, and menstrual cycle; (ii) Tc tolerated at exhaustion, influenced by state of encapsulation, hydration, and aerobic fitness; and (iii) the rate of increase in Tc from beginning to end of the heat-stress exposure, which is dependent on the clothing characteristics, thermal environment, work rate, and individual factors like body composition and economy of movement. Methods to reduce heat strain in PPC include increasing clothing permeability for air, adjusting pacing strategy, including work/rest schedules, physical training, and cooling interventions, although the additional weight and bulk of some personal cooling systems offset their intended advantage. Individuals with low body fatness who perform regular aerobic exercise have tolerance times in PPC that exceed those of their sedentary counterparts by as much as 100% due to lower resting Tc, the higher Tc tolerated at exhaustion and a slower increase in Tc during exercise. However, questions remain about the importance of activity levels, exercise intensity, cold water ingestion, and plasma volume expansion for thermotolerance. Published 2013. Compr Physiol 3:1363-1391, 2013.
Under most situations, humans control their body temperature within a narrow range seeking the aid of additional clothing to prevent excessive heat loss during exposure to the cold or the removal of clothing to assist with heat transfer during exposure to hot environments. However, there are many occupational settings, such as during firefighting, hazardous waste or explosive ordnance disposal, or in the military, where clothing must be worn to protect the individual from the hazards of their work environment. In addition, there are several sporting arenas, such as American football or auto racing, where protective clothing is worn to help prevent injury. The worker or athlete does not have the option to remove this personal protective clothing (PPC) and, as such, their ability to thermoregulate and continue to work or perform at a peak level can become compromised. In the context of the following discussion, PPC is distinct from personal protective equipment, which includes additional items carried or worn over the protective clothing to confer added protection such as body armor, air cylinders, hearing protectors, hard hats, or helmets. Respirators, however, are included as part of the encapsulating PPC terminology and this review begins with a brief discussion of the limitations involved with the use of respirators.
A detailed overview of the impact of PPC on thermoregulation andwork performance then follows with an emphasis of these effects during exposure to hot environments. In addition, the effects of physical factors, such as ambient temperature and vapor pressure, clothing design, age, body composition, sex, menstrual cycle and circadian rhythm, and physiological interventions, such as heat acclimation, aerobic fitness, hydration, work and rest schedules, pacing, and cooling will be discussed.
Tom M. McLellan, Phd, is an Exercise and Environmental Physiologist whose expertise is in understanding the limitations of working in Protective Clothing, as well as developing strategies to Optimize Physical and Cognitive Performance During Sustained Operations.
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