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 Core Body Temperature
2.2 Hypothermia
2.3 Hyperthermia
3.0 EFFECTS OF HEAT AND HUMIDITY
3.1 Heat Loss Greater than Heat Generated
3.2 Heat Loss Lower than Heat Generated
3.3 Effect of High Wet Bulb Temperature
3.4 Heat Hazards
3.4.1 Heat cramps
3.4.2 Heat exhaustion
3.4.3 Heat stroke
3.5 Effect of Air Velocity
3.6 Fall of Miners Working Efficiency
REFERENCES
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1.0 METABOLIC HEAT BALANCE IN HUMAN BODY
The food taken by human beings serves as fuel for their body. The oxidation of food
is an exothermic reaction. The energy released in the process is called metabolic
energy. This metabolic energy produced by oxidation of food is used for working
against gravity, growth of body (accumulates in the body) and rest is transferred to
the surroundings. The energy released depends upon the oxygen consumed in the
oxidation of food. More is the amount of oxygen consumed, more is the energy
produced. When we work hard, the breathing rate increases. This increases
consumption of oxygen. That’s why as the amount of work load increases, the
amount of metabolic heat produced by human beings increases. The metabolic heat
produced can be transferred to the surrounding by convection, radiation,
respiratory exchange (breathing) and evaporation. The heat balance in a human
body can be best explained by Fig.1
In the form of equation we can write the energy/heat flow for a human body as
follow:
𝑚𝑒𝑡𝑎𝑏𝑜𝑙𝑖𝑐 𝑒𝑛𝑒𝑟𝑔𝑦 (𝑀) = 𝑚𝑒𝑡𝑎𝑏𝑜𝑙𝑖𝑐 ℎ𝑒𝑎𝑡 + 𝑤𝑜𝑟𝑘 𝑎𝑔𝑖𝑛𝑠𝑡 𝑔𝑟𝑎𝑣𝑖𝑡𝑦(𝑊) + 𝑠𝑡𝑜𝑟𝑒𝑑 𝑒𝑛𝑒𝑟𝑔𝑦(𝐴𝑐 )
Or
𝑀 = 𝑊 + 𝐴𝑐 + (𝑅𝑎𝑑 + 𝐶𝑜𝑛 + 𝐸𝑣𝑝 + 𝐵𝑟)
As per Fig.1, heat flow due to radiation and convection can be reverse also. It
happens when the skin temperature (discussed later) of human body is less than
the dry bulb temperature (DBT) of the mine air. In this case, human body mainly
removes heat by evaporation of sweats produced. Heat removal through breathing
is less than 5% of the metabolic energy produced and hence neglected.
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Fig. 1 Heat balance in human body (after McPherson, 1993)
2.0 SOME TERMINOLOGIES
2.1 Mean Skin Temperature and Core Body Temperature
Human body can be thought of divided into two-parts (interior to exterior) namely central core body and the skin. Blood vessels separate these two. Human body has
to be maintained at 36.9℃ for proper physiological functions. The heat to maintain
this temperature is provided by oxidation of food. This temperature is called
temperature of the central core body (Tc). However, Tc may vary ±1℃ depending up
on the atmospheric conditions and individual response to the atmospheric
conditions. Hence, we can say that the normal central core body temperature of
human being lies in the range of35.9 − 37.9℃. Blood acts as cooling agent for skin. In
general, mean skin temperature (Tsk) is lower than Tc. At 34℃, the skin does not
feel any sensation of hotness or cold with the environment. Tsk depends on many
factors including dry bulb and wet bulb temperature, rate of perspiration, types of
clothing, etc.
2.2 Hypothermia
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A state when the rate of heat production in the body is lower than the heat
removed to the surrounding leading to reduction of the central core body
temperature i.e., below 36℃.
2.3 Hyperthermia
A state when the rate of metabolic heat production is more than the rate of
metabolic heat removal, leading to increase in the central core body temperature
i.e., above 37.9℃.
3.0
EFFECTS OF HEAT AND HUMIDITY
3.1 Heat Loss Greater than Heat Generated
When the rate of heat loss to the surroundings (Hloss) is more than the rate of
metabolic heat generation (Hgen), the effects observed can be shown as in Fig. 2.
Heart rate slows down, blood vessels contract, shivering may happen
Fall in central core body temperature
Hypothermia
DEATH
Fig. 2 Physiological effects observed when Hloss is more than Hgen
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3.2 Heat Loss Lower than Heat Generated
This is more common in the mine atmosphere. This condition to a miner/worker
may arise due to the combined effect of wet-bulb and dry-bulb temperatures,
psychrometric properties of the mine air and velocity of air-current. Besides these
factors, the effects observed on an individual are also dependent on ‘time duration
of exposure’ to such condition. This condition generally arises when the dry bulb
temperature of the air increases beyond a certain limit such that heat removal from
the body is reduced. If the dry-bulb temperature of air/atmosphere goes on
increasing, heat removal from body through radiation and convection may reverse
the directions (heat may be added to the human body from the atmosphere
through the process of radiation and convection). In such cases, the metabolic heat
from the body is removed through evaporation of sweat. Sweat, produced by the
sweat glands, adds moisture to the surrounding air. If the humidity of the air is
already high, the evaporation of sweat is reduced and therefore the central core
body temperature of an individual rises rapidly. Even if air is moderately humid,
sweat glands reduce their capacity to produce sweat with time. A time comes when
no more sweat is produced due to fatigue of the sweat glands. If exposure to such
environmental conditions continues, it may lead to heat stroke ultimately leading to
the death of the person. Table 1 lists the physiological effects on a miner/individual
with the rise in temperature of air and human body.
Table 1 Effect of increase in the dry bulb temperature
Dry
bulb Body
temperature
temperature
(˚C)
(˚C)
--≤ 25
25-29
29-37.5
≥ 36.9
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----36.9
Physiological effects
Normal blood circulation, no observable effect,
heat removal from body mainly through
convection and radiation
Heat removal rate increases, slight rise in central
core body temperature, vaso-motor control of
body increases blood circulation
Body starts sweating, heat removal mainly
through evaporation
Body temperature equals dry bulb temperature,
heat transfer through convection and radiation
-------
39
41
≥ 43.3
reverses the direction, heat removal from body
through evaporation only.
Heart beat rises above 140 beats /min, fatal
Unconsciousness, coma, may lead to death
Sudden death
3.3 Effect of High Wet Bulb Temperature
We have already learnt in psychrometry that the difference between the wet bulb
temperature and dry bulb temperature decide the relative humidity of the air. The
lesser is the difference, the more is the relative humidity. We know that air at high
relative humidity hampers rate of evaporation. At higher wet bulb temperature, the
reduction in evaporation of sweat causes body temperature to rise and in extreme
case if the rise in temperature exceeds 2℃, heart beat becomes faster, which on
continuation may lead to unconsciousness or even death. Table 2 lists the rise in
body temperature with wet-bulb temperature.
Table 2 Increase in body temperature with wet bulb temperature (after,
Mishra, 1986)
Wet-bulb temperature in K
≤302.15
302.65 – 304.85
305.35 – 307.65
≥307.65
3.4
Rise in body temperature in K
0.11- 0.66
0.33 – 0.77
0.66 – 1.55
1.44 -1.90
Heat Hazards
3.4.1 Heat cramps
General information
o
o
It happens when an individual is exposed for a period of longer duration to
temperature moderately higher than the normal environmental
temperature.
Heavy perspiration from body leading to salt loss from the body and muscle
cramps.
Sign and symptoms
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Severe muscle cramps
Exhaustion
Dizziness
o
o
o
Care procedures
Patient should be taken to cool place
Salted water should be given to the patient
Massage should be done to get relief from cramps
Moist towels/ice packs should be applied on forehead
o
o
o
o
3.4.2 Heat exhaustion
General information
o
It is caused because of exposure to excessive heat while individual is at
work. It is like a form of shock triggered by loss of fluid and salt from the
body. In worst condition, it may develop to heat stroke.
Sign and symptoms
o
o
o
o
o
Rapid and shallow breathing
Cold and clammy skin
Dizziness
Heavy perspiration
Weak pulse
Care procedures
Patient should be moved to cool place and allowed to rest
Clothes should be removed
Fan patient’s skin to reduce the temperature
Salted water should be given
If necessary, oxygen should be supplied
o
o
o
o
o
3.4.3 Heat stroke
General information
o
It happens when the body is unable to remove heat because of failure of
individual’s temperature-regulating mechanisms like, sweat glands, vaso-
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o
motor control, etc. It is mainly due to exposure to high temperature and
humidity, especially when body temperature exceeds 41℃.
Heat stroke is fatal in nature and immediate cooling of body should be
done and doctor should be called.
Signs and symptoms
o
o
o
o
o
Deep breaths followed by shallow breathing
Rapid strong pulse followed by rapid weak pulse
Dry, hot skin
Dilated pupils, skin turns blue
Loss of consciousness
Care procedures
o
o
o
o
o
Patient should be immediately cooled by any possible means
Clothes should be removed and person should be kept in dark room
Treatment for shock should be done by administering oxygen
Ice packs, if available, should be applied near pressure points
Person should be immediately taken to doctors
3.5 Effect of Air Velocity
o
o
o
o
Higher air velocity aids in evaporative cooling of the body.
Generally air velocity upto 1 m/s provides good comfort conditions
Air velocity greater than 2 m/s produces dust and causes discomfort to the
miners
Higher air velocity of the order of 3 m/s + High dry bulb temperature (322 K)
+ Low relative humidity =
Causes burning sensation + rise of body
temperature = This may lead to heat stroke
3.6 Fall of Miners’ Working Efficiency
With the rise in temperature and relative humidity of the mine atmosphere, a
decrement in miners’ efficiencies is observed. Initially efficiency decreases gently
and becomes steeper when the wet bulb temperature goes on increasing. Table 3
lists fall in working efficiency against wet bulb temperature and effective
temperature (will be discussed later).
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Table 3 Reduction in working efficiency with increase in wet-bulb
temperature and effective temperature (after, Banerjee, 2003)
Effective temperature (℃)
30.2
32.5
33.7
34.7
35.5
36.2
36.7
37.2
37.2
-
Wet bulb temperature (℃)
29.6
31.2
32.2
33.0
33.7
34.3
34.9
35.3
35.8
36.2
Efficiency %
90
80
70
60
50
40
30
20
10
00
Fig.3 shows fall in efficiency with rise in wet bulb temperature at different velocities
of air current.
Fig.3 Working efficiency vs. wet-bulb temperature (after Misra, 1986)
REFERENCES
Banerjee S.P. (2003); “Mine Ventilation”; Lovely Prakashan, Dhanbad, India.
Hartman, H. L., Mutmansky, J. M. & Wang, Y. J. (1982); “Mine Ventilation and Air
Conditioning”; John Wiley & Sons, New York.
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Le Roux, W. L. (1972); Mine Ventilation Notes for Beginners”; The Mine Ventilation
Society of South Africa.
McPherson, M. J. (1993); Subsurface Ventilation and Environmental Engineering”;
Chapman & Hall, London.
Misra G.B.
(1986); “Mine Environment and Ventilation”; Oxford University Press,
Calcutta, India.
Vutukuri, V. S. & Lama, R. D. (1986); “Environmental Engineering in Mines”;
Cambridge University Press, Cambridge.
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