Basic Concept of Heat Stress
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Transcript of Basic Concept of Heat Stress
Hendra
Iron and steel foundries
Non-ferrous foundries
Brick-firing and ceramics
plants
Glass products facilities
Rubber products factories
Electrical utilities
Bakeries
Confectioneries
Commercial kitchens
Laundries
Food canneries
Chemical plants
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Mining sites
Smelters
Steam tunnels
Fires (firefighting)
Outdoor operations
Surface mines
Agriculture sites
Construction sites
Merchant marine ships
Hazardous waste sites
Military training sites
Athletic competitions
Heat from activity
Heat from environment
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MESIN
Evaporasi
KonveksiRadiasi
Radiasi
Benda Panas
Konduksi
Konduksi Proses perpindahan panas karena terjadi kontak langsung secara
fisik
Perpindahan panas terjadi jika :
Terjadi kontak langsung
Terdapat perpedaan panas antara permukaan yang kontak
Konveksi Proses perpindahan panas melalui media cairan (uap air) dan gas.
Perpindahan panas secara konveksi dibantu oleh pergerakan udara
(angin)
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Radiasi Proses perpindahan tanpa adanya media penghantar ataupun
kontak langsung.
Panas berpindah melalui gelombang energi panas
Evaporasi Proses perpindahan panas dari tubuh (kulit) ke lingkungan seiring
dengan pengeluaran keringat
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Model of Thermal Balance
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H = M ± R ± C – E atau H + E = M ± R ± C
H = Jumlah panas tubuh (Suhu inti tubuh harus selalu konstan)
M = Panas akibat proses metabolisme
R = Panas Radian / Infra red (Berasal dari lingkungan)
C = Panas akibat konveksi (Berasal dari lingkungan)
E = Panas yang hilang karena Evaporasi
Jumlah panas tubuh merupakan jumlah panas yang dihasilkan dari proses
metabolisme ditambah (dikurangi) panas radiasi dan panas konveksi dari
lingkungan dikurangi dengan panas yang hilang melalui evaporasi
Model of Thermal Balance
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S––HEAT STORAGE RATE
If the value for S is zero, the body is in thermal
equilibrium, and heat gain is balanced by loss from the
body.
If S is positive, the body is gaining heat at the rate
indicated by the value of S.
If the value of S is negative, the body is losing heat,
and body temperature is decreasing.
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M––METABOLIC RATE
Chemical reactions occur continuously inside the body.
These serve to sustain life (basal metabolism) and meet
the demands of work (muscle metabolism).
As muscle metabolism increases to meet work demands,
the rate of energy conversion from chemical energy to
kinetic energy increases.
Because the energy conversion from chemical energy to
kinetic energy is inefficient, increased metabolism
results in increased rates of heat gain to the person.
The rate of metabolism depends directly on the rate
and type of external work demanded by the job.
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W––EXTERNAL WORK RATE
W is the amount of energy that is successfully
converted from internal chemical energy to mechanical
work on external objects.
The rate of external work depends directly on forces
applied against external resistance and distance moved.
W is usually about 10 percent of M.
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R––RADIANT HEAT EXCHANGE RATE (RADIATION)
Solid bodies of different temperatures have a net heat
flow from the hotter surface to the cooler surface by
electromagnetic radiation (primarily infrared
radiation).
The rate of heat transfer by radiation depends on the
average temperature of the surrounding solid surfaces,
skin temperature and clothing.
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C––CONVECTIVE HEAT EXCHANGE RATE
(CONVECTION)
The exchange of heat between the skin and the
surrounding air is referred to as convection.
The direction of heat flow depends on the temperature
difference between the skin and air. If air temperature
is greater than skin, C is positive and heat flows from
the air to the skin. If the air is cooler than the skin, C is
negative and heat flows from the body. The rate of
convective heat exchange depends on the magnitude of
the temperature difference, the amount of air motion,
and clothing.
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K––CONDUCTIVE HEAT EXCHANGE RATE
(CONDUCTION)
When two solid bodies are in contact, heat will flow
from the warmer body to the cooler body.
The rate of heat transfer depends on the difference in
temperatures between the skin and the solid surface,
the thermal conductivity of the solid body that the
person contacts, and clothing that may separate the
person from the solid surface.
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CRESP––RATE OF CONVECTIVE HEAT EXCHANGE BY
RESPIRATION
The fact that air is moved in and out of the lungs,
which have a large surface area, means there is an
opportunity to gain or lose heat.
The rate of heat exchange depends on the air
temperature and volume of air inhaled.
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ERESP––RATE OF EVAPORATIVE HEAT LOSS BY
RESPIRATION
The large surface area of the lungs provides an
opportunity to lose heat by evaporation.
The rate of heat exchange depends on the air humidity
and volume of air inhaled.
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E––RATE OF EVAPORATIVE HEAT LOSS
Sweat on the skin surface will absorb heat from the skin
when evaporating into the air.
The process of evaporation cools the skin and in turn
the body.
The rate of evaporative heat loss depends on the
amount of sweating, air movement, ambient humidity,
and clothing.
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Because W, K, Cresp and Eresp are small relative to the
other routes of heat exchange in industrial
applications, they are usually ignored. When
calculating heat storage, Equation 1 becomes Equation
2 as a general statement of heat balance.
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Excessive heating or cooling of a small portion of the skin can
occur when it comes in contact with a hot or cold surface. The
contact can be either intentional or incidental. Injury occurs when
there is sufficient heat gain to cause a burn or heat loss to cause
the tissue to freeze (or at least become very cold for a period of
time). In these cases, the local storage rate (Slocal) becomes
important.
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where K is conductive heat transfer between the skin and an object,
and D is the rate of heat transfer to or from the local area by
conduction through the local tissue and by the heat supplied or
removed via local blood flow.
Climatic conditions of the environment Climatic conditions are widely used to describe the degree of
stress, as seen in casual descriptions by air temperature, relative humidity, and wind chill.
Work demands In heat stress, metabolic rate can add 10 to 100 times more
heat to the body than radiation and convection combined. In cold stress, metabolic rate affects heat balance on the same order as radiation and convection losses.
Clothing.. Clothing has three characteristics: insulation, permeability, and
ventilation.
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The deep organs, especially heart,
lungs, and other vital organs
The arms, legs, and the tissues close to
the skin are referred to as the
periphery
The core and the periphery compete
with each other for blood supply
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The body does, however, maintain a constant
temperature at its centre, that is, in the interior of the
brain, the heart and abdominal organs. This constant
temperature is known as the core temperature, and
fluctuates very slightly at around 37OC. Maintaining this
core temperature is necessary for the normal function
of important vital organs.
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Schematic Diagram of the
Human Temperature
Regulation System
(Grandjean 1986)
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So much sweat is lost that
dehydration results
the body cannot cool itself by
sweating, and the core temperature
rises
Salt loss causes heat cramps
So much of the blood flow goes to
the skin that other organs cannot
function properly
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When body core temperature rises
Blood flow to skin increases
Sweating increases
Heart rate increases to move blood – and heat - to the
skin
When this works well
Core temperature drops or stabilizes at a safe level
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Thermal stress is a combination of environmental,
work, and clothing factors, heat stress is a combination
that tends to increase body temperature, heart rate,
and sweating. These physiological adaptations are
collectively known as heat strain. (FIH-Chapter 12)
Heat stress is the burden or load of heat which must be
dissipated if the body is to remain in thermal
equilibrium. It is represented by the sum of the
metabolic rate (minus external work) and gain or loss
by convection and radiation.
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Heat strain is the change resulting from heat stress. It
may be an adaptive response e.g. increased sweat
rate, increased heart rate or body temperature, or a
harmful effect such as heat exhaustion or heat stroke.
The buildup in the body of heat
generated by the muscles during work
coming from warm and hot environments
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Heat stress is the net heat load on the body with
contributions from both metabolic heat
production, and external environmental factors
including temperature, relative humidity, radiant
heat transfer and air movement, as they are
affected by clothing.
Heat strain refers to the acute (short-term) or
chronic (long-term) consequences of exposure to
environmental heat stress on a person’s physical
and mental states.
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Heat flow through the body, beginning with heating the body core by metabolism, the transfer
of heat by blood flow to the skin, heat gain or loss to the skin from the environment by
radiation and convection, heat loss by sweat evaporation, and cooler blood returning to the
core.Skin blood flow (sbf) to promote heat transfer is proportional to metabolic rate (M)
divided by the difference between core and skin temperatures (∆Tc-s).
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Heat Stress
Worker
- Acclimatization
- Hydration
- Clothing
- Medical Condition
Environment
•Air temperature
•Airflow
•Humidity
•Radian heat (e.g. sun, kiln) Work
• Workload
• Work rate
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Work safe British of Columbia)
(Occupational Safety and Health Service Department of Labour Wellington New Zealand)
There are six main factors:
Air temperature is how hot or cold the air around us is. It is what
we measure with a thermometer. It will have a direct warming or
cooling effect on a person. In situations with a high radiant heat
level, air temperature alone is not a good indicator of the thermal
environment.
Humidity is the moisture content of the air. Relative humidity is
the moisture content expressed as a percentage, with 100% being
total saturation for that temperature. The warmer the air, the
more moisture is able to be carried in the air.
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(Occupational Safety and Health Service Department of Labour Wellington New Zealand)
There are six main factors:
Radiant heat is emitted from anything that is hot. Radiant heat
will in time heat the air, but people will absorb heat far more
quickly. Radiant heat will affect people anywhere there is direct
sunlight, or where a person is close to a process that emits heat.
Air movement in most situations will cool a person. This will
provide some relief to people in a hot situation, but extra chill to
people in a cold situation. In hot environments increasing the air
speed can be used as a control measure. In cold environments, a
wind chill factor can make a person considerably colder than if
there was no wind.
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(Occupational Safety and Health Service Department of Labour Wellington New Zealand)
There are six main factors:
Physical activity will increase the generation of heat in the body.
In a cold environment, physical activity can help to warm a person.
In a warm or hot environment, physical activity can increase the
load of heat on a person. A high level of physical activity on a hot
day can place a worker at risk of heat strain, where the heat of
the day alone would not cause a problem.
Clothing aids or prevents heat transfer from our bodies to the
surrounding environment. In a cold environment, a person should
wear clothing that will prevent as much heat transfer as possible.
Ideal clothing in a hot environment will allow a worker to freely
dissipate heat. Clothing can also be used to shield a person from
factors such as radiant heat or a high wind speed.
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(Occupational Safety and Health Service Department of Labour Wellington New Zealand)
Other Factor (personal factors)
Weight
Overweight people are more at risk of harm in both hot and cold
environments. This is due to an imbalance in heat transfer.
Health
There are a number of medical conditions that increase the risk of harm
to people working in an extreme hot or cold environment.
Level of fitness
A physically fit person will acclimatise better and generally cope with
heat or cold stress better than an unfitperson.
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(Occupational Safety and Health Service Department of Labour Wellington New Zealand)
Other personal factors
Age
As a person reaches middle age (45+), lifestyle health issues can start to
emerge. These can make people more susceptible to harm caused by
extreme hot or cold environments.
Use of prescribed substances
Use of some prescribed medications will adversely affect people
working in extreme hot or cold environments.
Use of non-prescribed substances
Use of many non-prescribed substances such as alcohol or cannabis will
adversely affect people working in extreme hot or cold environments.
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Thermal comfort
What an individual feels in an ideal thermal environment. They are
not conscious of being either too hot or too cold.
Thermal discomfort
A person experiencing thermal discomfort feels either too hot or
too cold. They can often be very uncomfortable, but the body’s
temperature control mechanisms are working adequately, and
there is low risk of harm.
Heat or cold stress and strain
This is where harm or serious harm can occur to a person as a
result of working in a temperature extreme situation.
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