Thermal Limits of Men in Moderate to Heavy Work in Tropical Farming

The farmers in tropical climate are exposed to high heat during the summer months. The study examined the physiological strains of farmers (N=26) to six combined exposures of work and high heat, with moderate and heavy work (26 to 50%, and 51 to 75% VO(2max)) and three ambient conditions, i.e., 34.4 to 42.2 degrees C WBGT (inside) in an environmental chamber. While the cardio-respiratory responses and Tcr were predominantly influenced by the work severity (p<0.001), the environmental warmth greatly influenced the sweating response (p<0.001). The importance was placed on the segmental Tsk as the first rank indicator of the bodily heat strain. Both the environmental warmth and work severity had independent discernable effects on the dynamic equilibrium of the central and peripheral mechanism to regulate the body temperature. The segmental and compartmental (core, muscle, fat and skin) heat balance analysis indicated the span of convergence of the segmental core and muscle temperatures to the divergence of skin and fat temperatures (CORE-SHELL) as a quantitative estimate of the segmental gradient for heat transfer. The summation of heat exchange across the compartments and segments yielded the transient change in Tcr (0.06 to 0.12 degrees C/min), with significant difference between the moderate and heavy work. The Tcr of 39 degrees C was taken as the limit of tolerance for the farmers, and by defining the criteria limit of Tcr of approximately 2.5 degrees C gradient from the basal Tcr and the rate of change in Tcr, the tolerance times were estimated. Corollary to the development of ISO 7933 standard (PHS index), the predictions of tolerance times from the transient change in Tcr or the exponential relationship with the WBGT (tolerance time, min = 1,841 e (-0.103 WBGT)) were useful to suggest the protective limit for men at work in extremely hot environment. The simplicity of prediction lies in using WBGT as a criterion. The exponential equation estimated the tolerance time of 55 min at 34 degrees C WBGT, and up to 38 degrees C WBGT, the decrease in tolerance time was 4 to 5 min per degree increase in environmental warmth. Beyond 38 degrees C WBGT, the estimated tolerance time decreased by 2 to 3 min per degree increase in WBGT. Further optimization and validation of the knowledge for men and women farmers in different age groups will have application in managing heat illnesses and disorders in tropical farming.