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Heat Stress

What is heat stress? Heat stress is the combined effect of a work environment with high temperatures and the demands of the task being performed. It simply occurs when heat is absorbed from the environment faster than the body can get rid of it. The resulting total heat load overrides the body’s temperature regulating mechanism and heat disorders may occur. Any process or job that raises the body’s core temperature increases the risk of heat stress.

Heat disorders associated with heat stress.

Disorder Definition Symptoms
Heat stroke It occurs when the body’s temperature regulation fails and body temperature   rises to critical levels. It is a medical emergency that can lead to death. High body temperature; Hot, dry skin or profuse sweating ; Hallucinations; Chills; headaches; dizziness; slurred speech

Heat exhaustion

It’s   a result of the combination of excessive heat and dehydration. Untreated,   heat exhaustion can lead to heat stroke Heavy sweating; Extreme weakness or fatigue; Dizziness,   confusion; Nausea; Clammy, moist skin; Pale complexion; Muscle cramps;   elevated body temperature; Fast and shallow breathing.
Heat syncope A fainting (syncope) episode or dizziness that usually   occurs with prolonged standing or sudden rising from a sitting or lying   position caused by dehydration and lack of acclimatization. Light-headedness; Dizziness; Fainting.
Heat cramps are   usually the result of hard physical labour in a hot environment, often   resulting from an imbalance of electrolytes in the body Muscle pain or spasms usually in the abdomen, arms, or   legs.
Heat rash Is a common problem resulting from persistent wetting of clothing by   un-evaporated sweat. Red cluster of pimples or small blisters


Environmental and job related factors that contribute to health related problems. The most common environmental factors include:

  • High air temperatures: this reflects the amount heat in the air and affects the body’s ability to loose heat to the environment.
  • Higher relative humidity levels: the moisture content of the air, it influences the evaporation rate of sweat. The warmer the air, the more moisture it can hold. High humidity makes the body feel hotter because sweat does not evaporate off the skin.
  • Low air movement: the speed or velocity of air over the body affects the heat loss by convection and evaporation, this cooling provides relief in a hot environment as long as the moving air is cooler than the person
  • Radiant heat: sources such as direct sunlight, machinery that generates heat, hot water, heaters and open flames may radiate heat which can be absorbed by persons.

Job related factors which may increase the likelihood of developing heat stress include the lack of rest periods and inadequate cooling off especially in hard labor activity. Insufficient water consumption and inappropriate clothing also increases the risk of developing heat stress. Other individual factors which may influence heat loss include fitness, age, sex, alcohol consumption and medical conditions and medication that may affect body’s temperature regulation.

Typical industrial were heat stress is apparent include:

  • Outdoor operations in hot weather (mining, construction)
  • Farming operations
  • Iron, steel and nonferrous foundries
  • Brick-firing and ceramics operations
  • Glass products manufacturing plants
  • Rubber products manufacturing plants
  • Electrical utilities (particularly boiler rooms)
  • Bakeries
  • Confectioneries
  • Restaurant kitchens
  • Laundries
  •  Food canneries
  • Chemical manufacturing facilities
  • Mines
  • Smelters

Heat Stress Index: the most commonly used index is the wet bulb globe temperature index (WBGT index): it is a measurement of the four parameters that govern the way humans respond to the thermal environment (i.e. air temperature, air movement, humidity, and radiant heat).

The South African legislation (OHSAct 85 of 1993, Environmental Regulations For Work Places) prescribes that the time weighted average (TWA) WBGT index measured over a period hour should not exceed 30­ 0C.

(Outdoor environment = solar load).               WBGT= 0.7Tw + 0.2Tg + 0.1Ta

(Indoor environment = no solar load).             WBGT= 0.7Tw + 0.3Tg

Where: n= natural wet bulb temperature; g= globe temperature; a= air temperature

Tw = 24°C

Tg = 42°C

Ta = 40°C

Therefore the WBGT = (0.7 x 24) + (0.2 x 42) + (0.1 x 40) = 29.2 0C

TWA WBGT index =           WBGT1 x  t1 + WBGT 2 x t2 + ……..

t1   +   t2 +  …………Control of heat Stress

Individual workers need at least 4 to 7 working days to become fully acclimatized, but the process may take up to three weeks. A scheduled exposure is therefore recommended.

An employer is required to follow the hierarchy of hazard control when a hazard assessment suggests that measures should be taken to reduce the potential to harm for workers.

Engineering Controls Administrative Controls Personal Protective Equipment
Establish a cooling station where workers can rest in a ventilated and air-conditioned   spaceUse fans to   increase air movementinsulating hot   surfaces that generates the heat Reduce the physical effort needed for the taskAllow time for acclimatizationUse a work-rest schedule

Provide   appropriate training and education

Hot   jobs should be scheduled for the cooler part of the day


Specifically designed gel packs that fit in a hard hatUse reflective clothing that reflects radiant heatUse wetted clothing

Use commercially available ice vests