BASIC PRINCIPLES IN OCCUPATIONAL HYGIENE

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 36.5 - 37.5 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 Balance

11. Evaluating the Thermal Environment

12. Metabolic Rate

13. Personal Insulation

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 7933. 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 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 • 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

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