Heat stress is a condition where the body's ability to cool itself is exceeded by the amount of internal and external heat it experiences. It is measured using factors like air temperature, humidity, velocity, and radiant heat. The human heat balance equation accounts for radiant, conductive, evaporative, stored, and metabolic heat. Several heat stress indices exist, including effective temperature, corrected effective temperature, predicted 4-hour sweat rate, Belding-Hatch index, Oxford index, and wet bulb globe temperature, with each taking different climatic and individual factors into account to assess heat stress levels and safety limits.

1. EVALUATION OF HEAT STRESS

2. What is Heat Stress?
Heat stress is a condition in which net heat load on your
body from internal heat production and external sources
exceeds your body’s capacity to cool itself.
Temperature > 37.6 degree Celsius

3. Measurement of heat stress
Metabolic Factors
 Direct and Indirect Calorimetry
Environmental Factors
 Air temperature
 Air humidity
 Air velocity
 Radiant heat
Psychrometer
Katathermometer
Globe Thermometer

4. Human Heat Balance Equation
Where
R = Radiant heat
C = Conductive heat
E = Evaporative heat
S = Storage heat
M = Metabolic heat

5. Heat Stress Indices
1. Effective Temperature : Sensory scale of warmth, compiled from the readings of dry & wet
bulb temperature and air velocity from temperature and standard nomogram. Safe Limit is
30 degree Celsius
2. Corrected Effective Temperature: The effects of radiant heat can be included in an
assessment of effective temperature by using the globe thermometer temperature instead
of the dry-bulb temperature in those cases when the reading of the globe thermometer is
higher than the dry-bulb temperature. In such cases, the result is described as the
corrected effective temperature. Safe Limit is 29 degree Celsius.
3. Predicted 4-hour Sweat Rate: The amount of sweat that would be perspired by a
physically fit and acclimatized young man in the condition under review oral period of 4
hours. It takes into account the metabolic level and type of clothing in addition to the
climatic factors unlike other indices mentioned earlier. Normal Range 2.5 – 3 Liters per 4
hours

6. Continuation……
4. Belding – Hatch Index: Safe limit is 24 units, if more than 24 units muscular activity should
be discontinued.
5. Oxford Index: It is used to assess the severity of hot humid conditions of the working place
where ventilation is poor.
WD = 0.15d + 0.85 w
Where, WD= Weighted value, d&w= dry & wet bulb temperature
5. Wet Bulb Globe Temperature (WBGT) Index: It embraces in a single value the effect of
‘radiation’, ‘ambient air temperature’ and ‘humidity’. Safe Limit 30 degree Celsius.
Exposure to Sunlight = WBGT = 0.7wb+0.2 gt+0.1db
Non Exposure to Sunlight = WBGT = 0.7wb+0.3gt
Where, db =dry bulb
Advantage: It excludes the use of anemometers and eliminates the problem of obtaining
average air velocity.