LECTURE 7 THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS: 1.0 METABOLIC HEAT BALANCE IN HUMAN BODY 2.0 SOME TERMINOLOGIES

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1 LECTURE 7 THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS: 1.0 METABOLIC HEAT BALANCE IN HUMAN BODY 2.0 SOME TERMINOLOGIES 2.1 Mean Skin temperature and Cre Bdy Temperature 2.2 Hypthermia 2.3 Hyperthermia 3.0 EFFECTS OF HEAT AND HUMIDITY 3.1 Heat Lss Greater than Heat Generated 3.2 Heat Lss Lwer than Heat Generated 3.3 Effect f High Wet Bulb Temperature 3.4 Heat Hazards Heat cramps Heat exhaustin Heat strke 3.5 Effect f Air Velcity 3.6 Fall f Miners Wrking Efficiency REFERENCES Page 1 f 10

2 1.0 METABOLIC HEAT BALANCE IN HUMAN BODY The fd taken by human beings serves as fuel fr their bdy. The xidatin f fd is an exthermic reactin. The energy released in the prcess is called metablic energy. This metablic energy prduced by xidatin f fd is used fr wrking against gravity, grwth f bdy (accumulates in the bdy) and rest is transferred t the surrundings. The energy released depends upn the xygen cnsumed in the xidatin f fd. Mre is the amunt f xygen cnsumed, mre is the energy prduced. When we wrk hard, the breathing rate increases. This increases cnsumptin f xygen. That s why as the amunt f wrk lad increases, the amunt f metablic heat prduced by human beings increases. The metablic heat prduced can be transferred t the surrunding by cnvectin, radiatin, respiratry exchange (breathing) and evapratin. The heat balance in a human bdy can be best explained by Fig.1 In the frm f equatin we can write the energy/heat flw fr a human bdy as fllw: metablic energy (M) = metablic heat + wrk aginst gravity(w) + stred energy(a c ) Or M = W + A c + (Rad + Cn + Evp + Br) As per Fig.1, heat flw due t radiatin and cnvectin can be reverse als. It happens when the skin temperature (discussed later) f human bdy is less than the dry bulb temperature (DBT) f the mine air. In this case, human bdy mainly remves heat by evapratin f sweats prduced. Heat remval thrugh breathing is less than 5% f the metablic energy prduced and hence neglected. Page 2 f 10

3 Fig. 1 Heat balance in human bdy (after McPhersn, 1993) 2.0 SOME TERMINOLOGIES 2.1 Mean Skin Temperature and Cre Bdy Temperature Human bdy can be thught f divided int tw-parts (interir t exterir) namely - central cre bdy and the skin. Bld vessels separate these tw. Human bdy has t be maintained at 36.9 fr prper physilgical functins. The heat t maintain this temperature is prvided by xidatin f fd. This temperature is called temperature f the central cre bdy (T c). Hwever, T c may vary ±1 depending up n the atmspheric cnditins and individual respnse t the atmspheric cnditins. Hence, we can say that the nrmal central cre bdy temperature f human being lies in the range f Bld acts as cling agent fr skin. In general, mean skin temperature (T sk) is lwer than T c. At 34, the skin des nt feel any sensatin f htness r cld with the envirnment. T sk depends n many factrs including dry bulb and wet bulb temperature, rate f perspiratin, types f clthing, etc. 2.2 Hypthermia Page 3 f 10

4 A state when the rate f heat prductin in the bdy is lwer than the heat remved t the surrunding leading t reductin f the central cre bdy temperature i.e., belw Hyperthermia A state when the rate f metablic heat prductin is mre than the rate f metablic heat remval, leading t increase in the central cre bdy temperature i.e., abve EFFECTS OF HEAT AND HUMIDITY 3.1 Heat Lss Greater than Heat Generated When the rate f heat lss t the surrundings (H lss) is mre than the rate f metablic heat generatin (H gen), the effects bserved can be shwn as in Fig. 2. Heart rate slws dwn, bld vessels cntract, shivering may happen Fall in central cre bdy temperature Hypthermia DEATH Fig. 2 Physilgical effects bserved when H lss is mre than H gen Page 4 f 10

5 3.2 Heat Lss Lwer than Heat Generated This is mre cmmn in the mine atmsphere. This cnditin t a miner/wrker may arise due t the cmbined effect f wet-bulb and dry-bulb temperatures, psychrmetric prperties f the mine air and velcity f air-current. Besides these factrs, the effects bserved n an individual are als dependent n time duratin f expsure t such cnditin. This cnditin generally arises when the dry bulb temperature f the air increases beynd a certain limit such that heat remval frm the bdy is reduced. If the dry-bulb temperature f air/atmsphere ges n increasing, heat remval frm bdy thrugh radiatin and cnvectin may reverse the directins (heat may be added t the human bdy frm the atmsphere thrugh the prcess f radiatin and cnvectin). In such cases, the metablic heat frm the bdy is remved thrugh evapratin f sweat. Sweat, prduced by the sweat glands, adds misture t the surrunding air. If the humidity f the air is already high, the evapratin f sweat is reduced and therefre the central cre bdy temperature f an individual rises rapidly. Even if air is mderately humid, sweat glands reduce their capacity t prduce sweat with time. A time cmes when n mre sweat is prduced due t fatigue f the sweat glands. If expsure t such envirnmental cnditins cntinues, it may lead t heat strke ultimately leading t the death f the persn. Table 1 lists the physilgical effects n a miner/individual with the rise in temperature f air and human bdy. Table 1 Effect f increase in the dry bulb temperature Dry bulb Bdy Physilgical effects temperature ( C) temperature ( C) --- Nrmal bld circulatin, n bservable effect, 25 heat remval frm bdy mainly thrugh cnvectin and radiatin --- Heat remval rate increases, slight rise in central cre bdy temperature, vas-mtr cntrl f bdy increases bld circulatin Bdy starts sweating, heat remval mainly thrugh evapratin Bdy temperature equals dry bulb temperature, heat transfer thrugh cnvectin and radiatin Page 5 f 10

6 reverses the directin, heat remval frm bdy thrugh evapratin nly Heart beat rises abve 140 beats /min, fatal Uncnsciusness, cma, may lead t death Sudden death 3.3 Effect f High Wet Bulb Temperature We have already learnt in psychrmetry that the difference between the wet bulb temperature and dry bulb temperature decide the relative humidity f the air. The lesser is the difference, the mre is the relative humidity. We knw that air at high relative humidity hampers rate f evapratin. At higher wet bulb temperature, the reductin in evapratin f sweat causes bdy temperature t rise and in extreme case if the rise in temperature exceeds 2, heart beat becmes faster, which n cntinuatin may lead t uncnsciusness r even death. Table 2 lists the rise in bdy temperature with wet-bulb temperature. Table 2 Increase in bdy temperature with wet bulb temperature (after, Mishra, 1986) Wet-bulb temperature in K Rise in bdy temperature in K Heat Hazards Heat cramps General infrmatin It happens when an individual is expsed fr a perid f lnger duratin t temperature mderately higher than the nrmal envirnmental temperature. Heavy perspiratin frm bdy leading t salt lss frm the bdy and muscle cramps. Sign and symptms Page 6 f 10

7 Severe muscle cramps Exhaustin Dizziness Care prcedures Patient shuld be taken t cl place Salted water shuld be given t the patient Massage shuld be dne t get relief frm cramps Mist twels/ice packs shuld be applied n frehead Heat exhaustin General infrmatin It is caused because f expsure t excessive heat while individual is at wrk. It is like a frm f shck triggered by lss f fluid and salt frm the bdy. In wrst cnditin, it may develp t heat strke. Sign and symptms Rapid and shallw breathing Cld and clammy skin Dizziness Heavy perspiratin Weak pulse Care prcedures Patient shuld be mved t cl place and allwed t rest Clthes shuld be remved Fan patient s skin t reduce the temperature Salted water shuld be given If necessary, xygen shuld be supplied Heat strke General infrmatin It happens when the bdy is unable t remve heat because f failure f individual s temperature-regulating mechanisms like, sweat glands, vas- Page 7 f 10

8 mtr cntrl, etc. It is mainly due t expsure t high temperature and humidity, especially when bdy temperature exceeds 41. Heat strke is fatal in nature and immediate cling f bdy shuld be dne and dctr shuld be called. Signs and symptms Deep breaths fllwed by shallw breathing Rapid strng pulse fllwed by rapid weak pulse Dry, ht skin Dilated pupils, skin turns blue Lss f cnsciusness Care prcedures Patient shuld be immediately cled by any pssible means Clthes shuld be remved and persn shuld be kept in dark rm Treatment fr shck shuld be dne by administering xygen Ice packs, if available, shuld be applied near pressure pints Persn shuld be immediately taken t dctrs 3.5 Effect f Air Velcity Higher air velcity aids in evaprative cling f the bdy. Generally air velcity upt 1 m/s prvides gd cmfrt cnditins Air velcity greater than 2 m/s prduces dust and causes discmfrt t the miners Higher air velcity f the rder f 3 m/s + High dry bulb temperature (322 K) + Lw relative humidity = Causes burning sensatin + rise f bdy temperature = This may lead t heat strke 3.6 Fall f Miners Wrking Efficiency With the rise in temperature and relative humidity f the mine atmsphere, a decrement in miners efficiencies is bserved. Initially efficiency decreases gently and becmes steeper when the wet bulb temperature ges n increasing. Table 3 lists fall in wrking efficiency against wet bulb temperature and effective temperature (will be discussed later). Page 8 f 10

9 Table 3 Reductin in wrking efficiency with increase in wet-bulb temperature and effective temperature (after, Banerjee, 2003) Effective temperature ( ) Wet bulb temperature ( ) Efficiency % Fig.3 shws fall in efficiency with rise in wet bulb temperature at different velcities f air current. Fig.3 Wrking efficiency vs. wet-bulb temperature (after Misra, 1986) REFERENCES Banerjee S.P. (2003); Mine Ventilatin ; Lvely Prakashan, Dhanbad, India. Hartman, H. L., Mutmansky, J. M. & Wang, Y. J. (1982); Mine Ventilatin and Air Cnditining ; Jhn Wiley & Sns, New Yrk. Page 9 f 10

10 Le Rux, W. L. (1972); Mine Ventilatin Ntes fr Beginners ; The Mine Ventilatin Sciety f Suth Africa. McPhersn, M. J. (1993); Subsurface Ventilatin and Envirnmental Engineering ; Chapman & Hall, Lndn. Misra G.B. Calcutta, India. (1986); Mine Envirnment and Ventilatin ; Oxfrd University Press, Vutukuri, V. S. & Lama, R. D. (1986); Envirnmental Engineering in Mines ; Cambridge University Press, Cambridge. Page 10 f 10