Research On The Application Of WBGT In Hot Environment Types And The Safety Exposure Time

With the progress of social civilization, the working environment health gets moreand more attention, the hazadrd of extreme hot environment on workers health andpotential threats to safety is ugently needed to be resolved. The changing tendency ofphysiologicalparameters, the physiological thresholds and its safety exposure time arethe main factors to evaluate a worker’s heat stain. Aconvenientand accurate evaluationindice to evaluate heat strain is the main research field that many researchers have beenalways focusing on. This paper aims to verify the application and reliability of WBGTin Chinese working groups by a comparable experiment of the heat strain between hotdry environment and hot wet environment in the climate chamber.By the analysis of the body heat balance, it’s found that the heat strain can beclassified as3stages, the1ststage allows worker’s sustainable work while keep thebody’s heat balance, the2ndstage allows the worker works until the reach ofphysiological threshold which is also called safety exposure time, while the3rdstagemeans the safe thresholds are exceeded and a stop for recovery is need. A evaluationstrategy has been proposed that the easily and conveniently measured while conserved“direct index” are applicated in the1stheat strain stage, the accurate and reliable butmore complicated “empirical index” are applicated in the2ndheat stain stage, and thework should be stopped when the heat strain arrives the3rdheat strain stage.3couples of experiments were made in the climate chamber, including the veryhigh metabolic rate in WBGT=26℃environment，high metabolic rate in WBGT=28℃environment and moderate metabolic rate in WBGT=30℃, each couple of experimentsinclude experiments in hot dry environment and hot wet environment by controlling therelative humidity as30%and60%. Ten male subjects participated in each experiment.The physiological parameters are compared between the hot dry environment andhot wet environment by two way ANOVA analysis, it is found that the rectaltemperature in hot dry environment is significant higher than that in hot wetenvironment in the WBGT=30℃experiments, the sweat rate in hot dry environment issignificant higher than that in hot wet environment in the WBGT=26℃experiments,and the skin temperature in hot dry environment is significant higher than that in hotwet environment in the all experiments. The safety exposure time TTaredetermined by the time that the rectal tmeprature reached38℃while the safety exposure time THRare determined by the time that the heart rate reached(180-0.65age)bpm. A survival analysis method is applied to compare the difference ofsafety exposure time between hot dry environment and hot wet environment, it is foundthat the TTof hot wet environment is significant higher than that of hot dryenvirnment in the WBGT=28℃and WBGT=30℃experiments, while no significantdifference of the THRis found between the two environment types. The above resultreveal that even the WBGT level, metabolic rate and subject are the same, the sameWBGT value can not represent the same heat strain and safety exposure time indifferent environment types.The possible reason for the significant difference is analyzed as that the weights ofblack globe temperature and wet bulb temperature are constant in the calculation ofWBGT value, which leads to the overestimation of wet bulb temperature andunderestimation of black globe temperature in the hot dry environment, while theoverestimation of black globe temperature and underestimation of wet bulb temperaturein the hot wet environment. The result also illustrates that the subjects cannot worksustainably while keep heat balance under the "poor" environment rated by GB/T17244-1998, therefore, the safety exposure time are determined for the “poor”environment to prevent the breakthrough of thresholds. Judged by the results, thesubjects are not better in heat tolerance than the European and American experimentsubjects for ISO7243and ACGIH.