The human body responds to heat stress through various mechanisms controlled by the hypothalamic thermoregulatory center. When core body temperature rises above 37°C, the body initiates heat loss responses like sweating and vasodilation to transfer heat to the skin where it can be dissipated. If skin temperature drops below 37°C, heat production mechanisms like shivering are activated. Prolonged heat exposure can lead to heat illness ranging from mild conditions like heat cramps and heat edema to the life-threatening heat stroke. Acclimatization over 10 days allows the body to better regulate temperature and sweat in hot environments.

1. HEAT
Dr Sanjib Kumar Das PhD

2. Temperature Ranges
Temperature is a physical
quantity expressing hot and
cold. It is measured with a
thermometer calibrated in
one or more temperature
scales. The most commonly
used scales are the Celsius,
Fahrenheit and Kelvin scale.

3. • Heat is defined as any spontaneous flow of energy
from one object to another, caused by a difference in
temperature between two objects.
• Heat from sun is the driving force of life.
• The science of heat and its relation to work is
thermodynamics.

4. Heat & Thermoregulation

5. Body Temperature
• Shell temperature:
 Temperature closer to skin
 Oral temperature
• 36.6o-37.0oC (97.9o-98.6oF)
• Core temperature:
 Most important temperature
 Temperature of “core” (organs in cranial, thoracic and
abdominal cavities)
 Rectal temperature
• 37.2o-37.6oC (99.0o-99.7oF)

6. Heat Production
• Exergonic reactions:
 Oxidation and ATP use.
• Most heat generated by brain, heart, liver
and glands at rest.
• Skeletal muscles 20-30% at rest. Can
increase 30-40 times during exercise.

7. MUSCLE
CHEMICAL ENERGY
THERMAL MECHANICAL
ENERGY ENERGY
HEAT MUSCULAR WORK
(75%) (25%)
ENERGY SYSTEM

8. If the skin temperature drops
below 37°C a variety of responses
are initiated to conserve the heat
in the body and to increase
heat production. These include:
• Vasoconstriction to decrease the flow of heat to the skin.
• Cessation of sweating.
• Shivering to increase heat production in the muscles.
• Secretion of norepinephrine, epinephrine, and thyroxine
to increase heat production
• In lower animals, the erection of the hairs and fur to
increase insulation
Temperature Regulation

9. Thermoregulatory Center
• Hypothalamus:
– hypothalamic thermostat:
• Heat-losing center
• Heat-promoting center
• Monitors temperature of blood and receives signals from
peripheral thermoreceptors.
• Peripheral thermoreceptors:
– Temperature of skin.
• Central thermoreceptors:
– Temperature of core.
– Most important located in hypothalamus
• Negative feedback loops

10. Thermoregulatory Center
• Heat-losing center:
• Activates heat losing mechanisms:
– Dilation of dermal arterioles: increase blood
flow to skin.
– Sweating.
– Increased respiration through mouth.
– Behavioral: remove clothing.
• Inhibits heat-promoting center.

11. Thermoregulatory Center
• Heat-promoting center:
• Activates heat generating mechanisms:
– SNS:
• Vasoconstriction of dermal arterioles: decrease blood flow to
skin
• Stimulates arrector pili muscles: hair stands on end
• Shivering thermogenesis: spinal reflex of alternating
contractions in antagonistic muscles
– Nonshivering thermogenesis:
• Long-term mechanism stimulating thyroid hormone release
T3 and T4.
– Inhibits heat-loss center.

12. Mechanisms of Heat Transfer
• Radiation:
– Infrared radiation.
• Conduction:
– Direct transfer of energy through physical contact.
• Convection:
– Heat loss to air around the human body.
• Evaporation:
– Energy change in water molecule from liquid to vapor.

14. Heat stress: definition
• "Heat stress" is the net (overall) heat burden on
the body from the combination of the body heat
(metabolic cost of work) generated while
working, environmental sources (air
temperature, humidity, air movement, radiation
from the sun or hot surfaces/sources) and
clothing requirements.

15. Heat strain
The overall physiological response
resulting from heat stress aimed at
dissipating excess heat from the body.
Heat index
Combines temperature and relative
humidity for a “feels like” value

16. Heat Index

17. Heat Stress Index (HSI)
HSI = Evaporation required / Maximum
possible evaporation
HSI <20%, the body is in a state of thermal comfort.
>30% but <60%, heat levels are uncomfortable,
interfere with concentration and fine motor
performance, but are tolerable.
HSI > 60%, conditions are intolerable.

18. HEAT-STRESS FACTORS
Environmental factors (externally imposed factors):
Temperature -- the higher the ambient temperature, the
greater the heat load placed on the body.
Humidity -- the higher the humidity, the more sweat
evaporation is impeded.
Air movement -- air movement promotes the evaporation
of sweat and convection of heat to the ambient air.
Radiant heat -- radiant sources, including the sun, can
place additional heat load on the body.

19. HEAT-STRESS FACTORS
Metabolic heat factor (internally generated factor):
Proportionate to the intensity of the work performed.
Clothing factor:
Different types of clothing can impede to varying
degrees the movement of air over the skin’s surface,
which helps in heat removal through convection and
evaporation.

20. Comfortable condition
Air temperature is between 20°C - 27°C
Relative humidity from 35 - 60%.

21. Key Body Temperatures

22. The relative humidity of an air-water mixture is defined as the ratio of
the partial pressure of water vapor in the mixture to the saturated
vapor pressure of water at a given temperature.
Relative humidity is expressed as a percentage and is calculated in
the following manner:
RH = p(H2O) / p*(h20) * 100
where:
• RH is the relative humidity of the mixture being considered.
• p(H2O) is the partial pressure of water vapor in the mixture.
• p*(h20) is the saturation vapor pressure of water at the
temperature of the mixture.
RELATIVE HUMIDITY

23. How does the human body react to hot
environments?
Healthy human body maintains its internal temperature around
37°C.
Variations, < 1°C, occur during the day, level of physical activity or
emotional state.
A change of body temperature >1°C occurs during illness or when
environmental conditions exceed the body's ability to cope with
extreme temperatures.

24. Problems and Symptoms Caused by Hot Temperatures
Temperature Range
(°C)
Effects
20 - 27°C Comfort Zone Maximum efficiency
as temperature
increases...
Discomfort:
•Increased irritability
•Loss of concentration
•Loss of efficiency in mental
tasks
Mental Problems
Increase of errors:
•Loss of efficiency in skilled tasks
•More incidents
Pyscho-physiological
problems
Loss of performance:
•Disturbed water and electrolyte
balance
•Heavy load on heart and
circulation
•Fatigue and threat of exhaustion
Physiological problems
35 - 40°C Limit of high temperature tolerance

25. How does the body respond to
heat stress?
1. Internally generated metabolic
heat is carried to the surface of the
body via the blood stream. If the
ambient temperature is below
body temperature, any excess
heat is dissipated from the skin to
the air through convection.

26. How does the body respond to heat
stress?
2. Blood brought to the skin cannot be
cooled through convection to the
surrounding air if the ambient
temperature is as warm as or warmer
than the skin.
In this case, blood continues to be
pumped to the skin and evaporation
of perspiration becomes the primary
means of maintaining the body core
temperature at an acceptable level.

27. How does the body respond to heat
stress?
3. This situation is complicated if the
humidity is high. Perspiration is only
effective when it can evaporate and
high humidity retards evaporation.
When the environmental heat load is
high, the body’s cooling mechanism
can fail.

28. Heat acclimatization
The process of adjusting to a hot
environment takes about 10 days.
On the first day of working in the
hot environment, the body
temperature, pulse rate and
general discomfort are very
noticeable.
With each succeeding daily exposure, the symptoms will
gradually decrease. The worker should then be able to perform
the work required with minimal strain.
In general, a person who has become acclimatized to heat shows
significant variation in the amount of work that can be performed
in a hot or temperate environment.

29. CONTROL

30. • Increase aerobic capacity
– increased number of mitochondria /cell
– increased muscle glycogen stores
– training at intensity above 50% VO2max
provides for 1/2 of acclimatization needs
• Increased sweating capacity
– lower temperature threshold for vasodilatation
– increased volume
• Increased aldosterone production
– 10 - 25% increase in plasma volume
– lower sweat sodium conc. (65 - 5 mEq/L)
– increase potassium losses
Acclimatization

31. Source of heat
Location Source
Foundries, steel mills, bakeries,
smelters, glass factories, and
furnaces
hot or molten material
Outdoor occupations, such as
construction, road repair, open-pit
mining and agriculture
summer sunshine
Laundries, restaurant kitchens,
and canneries
high humidity

32. SAFETY PROBLEMS IN HOT ENV
 Increase of accidents due to slipperiness of sweaty palms
 Fogging of safety glasses
 Possibility of burn from hot surface, molten metal, etc.
 Decrease in mental alertness
 Decrease in physical performance
 Diversion of attention from hazardous tasks.

33. Heat Illness
1. Elevated skin temp: pain and tissue damage
2. Elevated body temp: heat stress syndromes

34. • Heat cramps
• Heat edema
• Heat syncope
Spectrum of Heat Illness
• Heat exhaustion
• Heat stroke

35. HEAT CRAMP
Heat cramps are muscle spasms which usually affect the arms,
legs, or stomach. Frequently they don't occur until sometime later
after work, at night, or when relaxing.
HEAT ILLNESS
HEAT CRAMP CAUSED BY
• Heavy sweating
• Especially when water is replaced by drinking, but not salt or potassium.
• Heat cramps can be quite painful,
• They usually don't result in permanent damage.
HEAT CRAMP PREVENTION
• Drink electrolyte solutions during the day
• Try eating more fruits like bananas.

36. Cause: temporary ineffective circulating volume
due to:
– dependent blood pooling
– peripheral vasodilatation
Treatment: resolves quickly with assumption of
horizontal position; hydration
Prevention: avoidance of protracted standing;
use of frequent muscle flexion and postural
changes; acclimatization
Heat Syncope / Fainting

37. HEAT EXHAUSTION
• Loss of large amounts of fluid by sweating.
• Excessive loss of salt.
• Extreme weakness or fatigue, nausea or headache .
• In more serious cases vomiting or loss of consciousness.
• Skin becomes clammy and moist, complexion is pale.
• Body temp. is normal or slightly high.
HEAT ILLNESS

38. • Symptoms: fatigue, vertigo, impaired judgment, thirst,
nausea, vomiting
• Core temperature < 40oC (104oF)
• Tachycardia, orthostatic hypotension
• Differentiation from heat stroke:
– modest temperature elevation (< 40oC)
– no coma
– normal liver function
Heat Exhaustion – Exercise Associated
Collapse
-- Diagnosis --

39. • Rest, move to cool environment
• Assess temperature
– Temp > 39oC, water sprinkling, ice packs
• Assess hydration
– BP, pulse, electrolytes
• Hydration
– Initial (oral): 1 - 2 L over 2 - 4 hrs.
– IV therapy - Saline
• Many cases are managed with oral hydration without
Emergency department visit
Heat Exhaustion – Exercise Associated Collapse
-- Treatment --

40. Predisposing conditions, Differential diagnosis,
Diagnostic Criteria, Treatment
Heat Stroke

41. HEAT STROKE
• Is a life threatening illness with a high death rate. It occurs when the body
has depleted its supply of water and salt, and the victim's body temperature
rises to deadly levels.
• Heat stroke is sometimes mistaken for heart attack. It is therefore very
important to be able to recognize the signs and symptoms of heat stroke -
and to check for them anytime an employee collapses while working in a
hot environment.
HEAT ILLNESS
HEAT STROKE
Cause: Partial or complete failure of sweating mechanism.
The body cannot get rid of excess heat, so the body core
temperature is rising.

42. Symptoms
• Body temperature of over 108°F.
• A distinct absence of sweating
• Hot red or flushed dry skin
• Rapid pulse
• Difficulty in breathing
• Constricted pupils
• High blood pressure
• convulsions
• Collapse
• Loss of consciousness
HEAT ILLNESS

43. • Signs
– Temperature > 41oC
– Profound CNS signs (coma, delirium - acute and
relatively sudden decline in attention-focus, perception, and
cognition)
– Hot, dry skin (sweating persists in 50%)
• Must assume Heat Stroke with CNS dysfunction
during periods of high environmental temperatures
Heat Stroke
-- Diagnostic Criteria --

44. Heat stroke
Treatment: Medical emergency! Call paramedics.
Start cooling victim immediately. Remove victim
to a cool area. Soak clothing and skin with cool
water; use a fan to create air movement. Shock
may occur. Medical treatment is imperative.
Prevention: Acclimatization, close monitoring of workers,
medical screening, drinking plenty of water.

45. MEASUREMENT OF HEAT STRESS
ENVIRONMENTAL FACTORS
• Air Temperature
• Air Humidity
• Air movement
• Radiant heat

46. MEASUREMENT OF HEAT STRESS
EQUIPMENT REQUIRED
• Sling Psychrometer
• Black metallic Globe
• Globe Thermometer
• Kata Thermometer

47. HEAT STRESS INDICES
• Effective Temperature
• Corrected Effective Temperature
• Oxford Index
• Predicted Four Hour Sweating Rate (P4SR)
• Wet Bulb Globe Temperature (WBGT) Index

48. HEAT STRESS INDICES
Effective Temperature (ET)
ET is a sensory scale of warmth that corresponds to
the temp of still air saturated with water vapour
which is supposed to give same sensation of
discomfort as that experienced in the environment in
question.
Corrected Effective Temperature (CET)
It is a situation where there is radiant heat source.

49. HEAT STRESS INDICES
Oxford Index
It is used to assess the severity of hot humid conditions of
the working places, particularly where ventilation is poor
WD = 0.15 d + 0.85 w
WD = Weighted value
d = dry bulb temp
w = wet bulb temp

50. HEAT STRESS INDICES
Predicted Four Hour Sweating Rate (P4SR)
The amount of sweat that would be perspired
by a physically fit and acclimatized young man
over a period of four hours.
Factors:
Climatic condition
Metabolic level
Type of clothing

51. HEAT STRESS INDICES
Wet Bulb Globe Temperature (WBGT) Index
• It represents the effects of ‘radiation’, ‘ambient
air temp’ and ‘humidity’.
• It is calculated using temp measurements
alone, and eliminating the need to measure air
velocity.

52. HEAT STRESS INDICES
Wet Bulb Globe Temperature (WBGT) Index
• Outdoor exposure with solar load
WBGT = 0.7 tnwb + 0.2 tg + 0.1 ta
Tnwb = Natural wet bulb temp
Tg= Black Globe temp
Ta = dry bulb temp

55. CONTROL OF HEAT STRESS
ENGINEERING CONTROLS
• Ensure all inside area have adequate
ventilation.
• Provide exhaust fans
• Cross ventilation by providing parallel louvers
• False ceiling / double roofing to reduce
environmental heat load.

56. ENGINEERING CONTROLS
• Provide cool air duct.
• Provide shade for outside work.
• Providing thermal barrier between heat source
and worker (Aluminum sheet metal coated
with aluminum paint, Calcium Silicate
insulation block ).
CONTROL OF HEAT STRESS

57. ADMINISTRATIVE CONTROLS
• Provide training / educative program to the
workers.
• Provide cold water for drinking and washing
facilities.
• Provide cool rest room.
• Rotation of workers during high heat
operations.
CONTROL OF HEAT STRESS

58. ADMINISTRATIVE CONTROLS
• Protective equipment.
• Provide PPE when appropriate
• Dietary supplement.
• Selection of workers.
• Periodical check up.
CONTROL OF HEAT STRESS

59. THANK YOU
Dr. Sanjib Kumar Das,
MPT(Musculoskeletal), P.G.D Hospital Management,
Fellow Doctoral, NITIE-Ergonomics and Human Factors,
P.P. Savani University, Surat, India
Mail: sanjib_bpt@yahoo.co.in
Contact No. :+91 8879485847