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Article Excerpt
EFFECTS OF UNCOMPENSABLE HEAT STRESS
The modern warfighter must be equipped to survive nuclear, biological, and chemical contamination in the battlefield. To this end, the United States military has been developing and refining Chemical Protective Equipment (CPE). Presently this equipment is used in five different Mission Oriented Protective Posture (MOPP) suit configurations depending on the level of threat, ranging from MOPP to MOPP 4. The various MOPP suits degrade the ability of the warfighter to see, speak, and hear. The rubber gloves restrict air circulation and limit the sense of touch and the ability to perform tasks requiring delicate manipulation. The wearing of full CPE can cause psychological stress (e.g., claustrophobia) in some persons as well as heat stress in most individuals (US Army, 1992). This paper focuses on tolerable heat stress limits for subjects donning various MOPP suits in different types of warm climates.
In high temperature and/or relative humidity conditions, evaporative heat loss can be restricted, a situation that is exacerbated by heavy, impermeable clothing. If the evaporative heat loss needed to maintain thermal equilibrium exceeds the maximal evaporative capacity of the environment, the body will experience uncompensable heat stress (UHS) resulting in a rise in core temperature. Performance and heat tolerance are influenced by such subjective factors as gender, age and anthropometry, and physiological manipulations such as hydration, training, fitness or heat acclimation may also significantly affect subject response to heat stress (Cheung et al., 2000).
Two definitions of heat tolerance are common in the literature: (i) heat tolerance as the highest core temperature that can be tolerated prior to exhaustion (Latzka et al., 1998; Montain et al., 1994; Sawka et al., 1992), and (ii) heat tolerance in terms of an increase in body heat storage (Webb, 1995). Tolerance time is defined as the time elapsed between the initiation of uncompensable heat stress (UHS) exposure and the attainment of one of several end-point criteria. The exact end-point criteria may vary between different labs or even within the same lab from one experiment to another (Cheung et al., 2000).
Several models using empirically derived equations or using first principles have been developed by researchers at US Army Research Institute of Environmental Medicine (USARIEM). These models include ARIEM (Pandolf et al. 1986), ARIEM_MOD (Gonzalez et al. 1997), and SCENARIO (Kraning and Gonzalez, 1997), with the latter and its revised versions for males being the ones most reported. Refinements have been suggested to improve prediction of final equilibrium rectal temperature and time histories to improve average standard deviation from the experiments (Yokata et al., 2004; Cardette and Montain, 1999). SCENARIO can be used to predict thermal responses such as rectal temperature, heart rate, sweat loss and blood flow, which can in turn be used to predict tolerance limits using relevant indices, e.g., to reach the ethical rectal temperature limit (e.g., 39.5 C). Santee et al. (1999) analyzed the effects of extreme heat on women and concluded that it is appropriate to use the male-based predictive model, SCENARIO, for female subjects as long as model parameters were consistent with actual physiological and anthropomorphic data for the female population. However, predictive models over a range of warm environments and MOPP levels for individual subjects, i.e., not averaged cases, using black-box modeling approaches, have not been reported in the literature. A survey of other issues related heat stress studies has been reported by other members of our group (Iyoho et al., 2008) and so not repeated here, with the focus here being only on the development of black box models.
The objective of the present study is to develop predictive models to determine thermal responses and heat tolerance time limits for males wearing heavy, impermeable clothing and exercising in a wide range of environments ranging from neutral to hot humid to desert like conditions. Additionally, the research seeks to characterize the effects of relevant subject parameters on the tolerance time limits. Thermal data were generated by experimental testing in an environmental chamber for a variety of environmental conditions. Using that data, two types of computational black-box models are developed, given a set of 'individual' parameters and environmental conditions, to predict: (i) HR and Tcore time histories, and (ii) heat tolerance time limits. The five input parameters considered to characterize individual differences are age, height, weight, % body fat, and maximal oxygen consumption ([VO.sub.2max]), and the other three inputs are environmental conditions represented by the ambient temperature (Tair) and relative humidity (Rh), and the clothing level (MOPP level). A methodology is then proposed to identify the relative importance of input parameters on heat tolerance time using a sensitivity analysis.
EXPERIMENTAL STUDY AND MODEL...
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