basic principles in occupational hygiene day 4. 15 - thermal enviroment

BASIC PRINCIPLES IN OCCUPATIONAL HYGIENE

Day 4

15 - THERMAL ENVIROMENT

THERMAL ENVIROMENT

• 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.

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.

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. – No physiological acclimatisation to 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 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.

Heat Transfer from the BodySix parameters that may be taken into account:

S = M ‑ W + C + R + K ‑ E

Where: S = Storage, which in the long term must equal 0M = MetabolismW = External work doneC = ConvectionR = RadiationK = ConductionE = Evaporation

Factors Influencing Heat Balance

1 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

Metabolic RateActivity 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

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 ENVIROMENT

• Dry Bulb Temperature

• Mean Radiant Temperature

• Air Velocity

• Personal monitoring.

Heat Stress IndicesVarious 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.

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

Specific Environmental Problems

High radiant components

Specific Environmental Problems

High humidity conditions

Specific Environmental ProblemsHot dry conditions