1. BASIC PRINCIPLES IN OCCUPATIONAL HYGIENE
Day 4

2. 15 - THERMAL ENVIROMENT

3. Thermal Environment
In order to function effectively we need to maintain our bodies at a constant temperature within oC
Temperature regulation centres in our brain are sensitive to small changes of blood temperature and also get feed back from sensory nerves at the skin
Our brains then use this information to adjust our bodies responses to heat

4. Heat Physiological responses to heat Blood vessels in skin expand
Pulse rate increases
Increases blood to the surface of the body
Sweating also increases heat loss due to latent heat of evaporation
In very hot conditions, sweating offers greatest potential for regulating body temperature

5. Heat Possible adverse effects of exposure to excessive heat include;
fatigue
behavioural modification
reduced concentration
heat cramps due to salt loss
fainting
heat exhaustion
heat stroke

6. Cold Physiological responses to cold Blood vessels in skin contract
Heat flow to the body surface is reduced
Heat production is increased by physical activity and shivering.
Evidence regarding physiological acclimatisation to cold is conflicting

7. Cold Possible adverse effects to excessive cold include;
lassitude/listlessness
chilblains
frost bite
hypothermia

8. Psychological Responses to the Thermal Environment
People will often modify the way they work depending on the thermal environment
Modify their local work environment
moving to a more comfortable area,
changing clothes,
increasing or decreasing ventilation
Performance and efficiency can also be affected by adverse thermal conditions

9. Heat Transfer from the Body
S = M + C + R – E
Where
M = Rate of metabolic heat production
C = Convective heat loss or gain
R = Radiant heat loss or gain
E = Evaporative heat loss
S = Heat gained or lost by the body
Two more parameters, W (external work done) and K (conduction) are usually small and not considered so the simplified form is often used

10. Factors Influencing Heat Balance1
Work rate (i.e. activity or metabolic rate)
Person 2
Clothing 3
Air temperature
Environment 4
Radiant temperature 5
Air Velocity 6
Humidity (moisture) conditions

11. Evaluating the Thermal Environment

12. Metabolic Rate Activity Metabolic Rate (W/m2 body surface) Sleeping 43
Resting 47
Sitting 60
Standing 70
Slow Walk (2.5 kph)
107
Walking (5 kph)
154
Running ( 16 kph)
600
Sprinting (25 kph)
2370

13. Personal Insulation Clothing Clo Value Naked Shorts 0.1
Shorts
0.1
Light summer clothes
0.5
Typical indoor clothes
1.0
Heavy suit
1.5
Polar clothing
3-4
Practical maximum 5

14. Thermal Environment
Duration of exposure – use work/rest tables to reduce risk of prolonged exposure to heat
Dry bulb temperature – dry sensor shielded from heat
Simple thermometer - inexpensive, fragile, slow to respond
Electrical thermometer – accurate, convenient
Globe temperature – black copper globe with a simple thermometer in centre

15. Thermal Environment
Air velocity – heat removed from the body by convection when air current is passed over unless the air temperature is higher than the temperature of the skin.
Also affects evaporation of moisture from skin
Vane anemometer - propeller type, directional, electrical or mechanical
Resistance anemometer
Kata thermometer
Tracer smoke – visualise air flow and measuring low air velocities

16. Thermal Environment Moisture Content
Convection and evaporation play a major role in dissipating body heat and thus the temperature and moisture content of the air are important parameters
Natural wet bulb – simple thermometer with bulb covered and dipped in distilled water
Personal monitoring – heart rate and core temperature. If less than 30 minutes of work allowed or high levels of impervious PPE – undertake personal monitoring

17. Heat Stress Indices
Various workers have devised indices to combine some of them into a single figure to which a standard could be applied. Some of these include:
Wet Bulb Globe Temperature: A simple index calculated after measuring the dry bulb, natural wet bulb and globe temperatures
HSI (Heat Stress Index): Calculated using a range of environmental measurements as well as work rate
P4SR (Predicted Four Hour Sweat Rate): Calculated from charts and used to assess physiological limits

18. Heat Stress Indices
Thermal Work Limit (TWL): uses five environmental parameters plus clothing factors to arrive at a prediction of a safe maximum continuously sustainable metabolic rate (Wm-2) for the conditions. Must be euhydrated and acclimatised
Predicted Heat Strain: adopted in ISO It describes a method for calculating the heat balance as well as the required sweat rate that the human body should produce to maintain this balnce in equilibrium.

19. Thermal Comfort
Very subjective and people will feel differently about what is the ‘ideal’ thermal environment
Much less extreme conditions than thermal stress
Indices have also been generated in an attempt to measure thermal comfort

20. Controlling the Thermal Environment

21. Controls Hot environments Cold Environment
Screen radiant heat
Increase air movement
Modify behavioural patterns
Work/rest regimes
Provide air-conditioned refuges
Increase distance from local 'hot spots'
Air cooling
Protective clothing
Provide readily accessible and palatable drinking water
Allow time for employees to acclimatise after time-off
Reduce exposure to wind chill – wind barrier or refuge
Covering metal handles
Local heating
Use mechanical aids so sweat less
Workplace designed for use with gloves
Workplace designed so no prolonged sitting or standing

22. High Radiant Components
Surface temperatures surrounding a worker are hotter than the ambient air
Boiler rooms, engine and compressor house, power generating stations, inside military vehicles (tanks and aircraft)
Increase air velocity
Air conditioning/chillers
Cooled clothing
Shielding in smelting, furnace, steel making ad foundry where surfaces are extremely high

23. High Humidity Conditions
Laundries, mine, textile and other manufacturing processes, wet bulb and dry bulb are both high (indicating high humidity)
Supply dehumidified air, projected into the area, increasing air velocity, improving comfort and reducing stress

24. Hot Dry Conditions
Deep dry mines, inside buildings (in tropics), and manufacturing where heat is emitted from plant
Increase air velocity
Cooled air