BASIC PRINCIPLES IN OCCUPATIONAL HYGIENE Day 4 15

BASIC PRINCIPLES IN OCCUPATIONAL HYGIENE Day 4

15 - THERMAL ENVIROMENT

Thermal Environment • In order to function effectively we need to maintain our bodies at a constant temperature within 36. 5 - 37. 5 o. C • 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

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

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

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

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

Psychological Responses to the Thermal Environment • People will often modify the way they work depending on 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

Heat Transfer from the Body S=M+C+R–E Where M = C = R = E = S = Rate of metabolic heat production Convective heat loss or gain Radiant heat loss or gain Evaporative heat loss 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

Factors Influencing Heat Balance 1 Person 2 Work rate (i. e. activity or metabolic rate) Clothing 3 Air temperature Environment 4 Radiant temperature 5 Air Velocity 6 Humidity (moisture) conditions

Evaluating the Thermal Environment

Metabolic Rate Activity Sleeping Metabolic Rate (W/m 2 body surface) 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

Personal Insulation Clothing Clo Value Naked 0 Shorts 0. 1 Light summer clothes 0. 5 Typical indoor clothes 1. 0 Heavy suit 1. 5 Polar clothing 3 -4 Practical maximum 5

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

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

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

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 – P 4 SR (Predicted Four Hour Sweat Rate): Calculated from charts and used to assess physiological limits

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.

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

Controlling the Thermal Environment

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

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

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

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