Ergonomics 4 Schools

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Temperature

Thermometer  
What temperature is 'comfortable'?

A suitable physical climate is needed if you are to feel comfortable and to be efficient at work. The environment feels comfortable when you are barely aware of the climatic conditions. It is only when the temperature decreases or increases beyond your comfort limits that you become aware of discomfort.

For temperature, the comfort zone is about 20-22C for a clothed person in the winter, and 20-24C in the summer. An increase in temperature above the comfort level may make you tired and sleepy. A decrease in temperature may make you restless and less attentive. People vary in their feelings about what is a comfortable temperature, and this depends on what they are doing and what they are wearing. If you are running in an overcoat, you will feel hotter than if you are sitting still in your underwear, even if you are in the same room!

Age and gender can also make a difference. Old people, people with disabilities, babies and young children typically feel more comfortable at higher temperatures. Women notice that they are feeling cool quicker than men, which may be related to their different body size, but is also related to a difference in the how quickly women respond to changes in temperature. Women reduce the blood flow to their arms, hands and feet faster than men if they cool down, resulting in colder fingers and toes. This reduction in blood flow is a way of regulating body temperature.
 

 

Body temperature variation

Body temperature regulation

Regulation of your body temperature is called thermoregulation. In a neutral climate, at rest, your body regulates its temperature to around 37C. This is by no means an exactly fixed temperature for all humans. When measured in the morning after bed rest, the average temperature of a large group of people will be around 36.7C, but individual values of healthy people will range from 36 to 38C. During the day, your temperature will increase, typically by about 0.8C, peaking in the late evening. It will drop again until early morning due to the circadian rhythm (changes in bodily functions over the course of the day and night). Also, exercise will cause an increase in body temperature, with temperatures around 38C typical for moderate work and values up to 39C and occasionally above 40C for heavy exercise, for example, marathon running. Increases by exercise up to 39C are seldom a problem to the body and are considered as normal in thermoregulation.

Heat Balance

When your body is not stressed by the temperature around it, its own temperature is quite stable. This is achieved by balancing the amounts of heat produced in your body with the amounts lost, and ensuring that the amount of heat stored in your body is stable.

Heat is produced in your body by metabolic (chemical) processes, such as absorbing nutrients and performing muscular work. When at rest, your body needs all this heat to enable its basic functions to work properly. For example, to provide your body cells with oxygen and nutrients during breathing and digestion. Heat balance Your body has several ways of losing heat. A minor role is taken by conduction - heat loss through contact with fluids or solids. Conductivity only becomes a relevant factor for people working in water, in special gas mixtures (prolonged deep-sea dives), handling cold products or lying still (for example on an operating table).
When working however, the active muscles need more oxygen and nutrients, and your metabolic activity, and therefore heat production increases.

In the cold, automatic closing down (constriction) of the blood vessels is not enough to maintain your temperature. Shivering produces additional heat. Opposing muscle groups are made to work against one another. This increase in activity increases the metabolic heat to between 2 and 4 times what it is at rest. Shivering is not enough to heat you up - it can only stop you from cooling down. The better physical condition you are in, the better you at maintaining shivering without tiring, and therefore the better you would be at surviving cold stress.

Your body also needs to get rid of any excess heat it produces, otherwise it will warm up to lethal levels. For example, if no cooling were possible, and you worked at moderate levels (metabolic rate 450 watt; the equivalent heat production of six 75 watt light bulbs), your body temperature would increase by around 1C every 10 minutes.

More important for heat loss is convection. When air flows along the skin, it is usually cooler than the skin. Heat will therefore be transferred from the skin to the air around it.

Also heat transfer through electro-magnetic radiation can be substantial. When there is a difference between your body's surface temperature and the temperature of the surfaces in the environment, heat will be exchanged by radiation.

Finally, the body possesses another avenue for heat loss - by evaporation. Due to your body's ability to sweat, moisture appearing on your skin can evaporate and large amounts of heat can be dissipated from your body.

Apart from convective and evaporative heat loss from the skin, these types of heat loss also take place from the lungs by respiration, as inhaled air is usually cooler and dryer than your lung's internal surface. By warming and moisturising the inhaled air, the body loses an amount of heat as the air is exhaled, which can be up to 10% of the total heat production of the body.

 
 

Adjusting to different temperatures

When you change from one environment to another with a different temperature, your body makes certain adjustments to ensure that you maintain a constant body temperature.

Adjusting to different temperaturesFrom warm to cool

If the loss of heat from your body is greater than heat production, your body temperature will fall. In order to prevent this, the following changes occur:
  • Your blood is directed away from your skin, and more to the central part of your body to maintain the temperature of those parts,
  • Your skin becomes cool as a result of this,
  • You get goose flesh, which increases the insulation capacity of your skin as rough surfaces lose less heat than smooth ones,
  • You might shiver which activates the muscles in order to produce additional heat,
  • You might adjust your behaviour - you might move to a warmer space, put on clothing and change your posture to preserve heat.

Your body may eventually stabilise in a cold environment. However, in extreme cold, for example immersion in water after a ship wreck, your body will cool so much that it is unable to maintain its central temperature. This may fall below 35C, a state known as hypothermia, and if it falls below 33C, the risk of death starts to increase significantly. At this temperature and below, your heart can have problems with its rhythm, your mental performance decreases (often leading to wrong decisions) and your body control is reduced.

With a cold body, there may be a chance of recovery however. Even if submersed in cold water without breathing for periods that would mean certain death on land due to lack of oxygen, people have made full recoveries. The reason for this is the lower oxygen usage of the brain when it is cooled. The lowest body core temperature from which full recovery has been reported is around 14C. Of course, chances of recovery are slim in such a case.

From cool to warm

When you move from a cool to a warmer environment, your body temperature will increase as:
  • More blood is directed to the surface of your body (skin),
  • Your skin surface is warmed as a result (and a result of radiant heat from the surrounding air,
  • Your heart rate and pulse rate increases,
  • You may start to sweat,
  • You may adjust your behaviour by moving to cooler areas, adjusting your clothing, reducing activity and changing posture.

Your body eventually stabilises in a warm environment. However, if the rate of heat gain continues to be greater than the loss, your body temperature will rise continuously, leading eventually to heat stress, and in the most extreme cases, to heat stroke. If your temperature rises to 42C, your brain and your nervous system will cease to function and death will follow.

Note: "Often people who suffer heat stroke are young and energetic and are under situations involving work-related, military or athletic pursuits. Frequently they have not felt a particular need to take care of themselves and have never considered the effects of strenuous activity undertaken in conditions of high temperature" (from Occupational Health and Safety July/August 1978).

Acclimatisation to heat and cold

Acclimatisation consists of a series of physiological adjustments that occur when you are habitually exposed to extreme thermal conditions, hot or cold. Acclimatisation refers to an ability to endure conditions with less feeling of discomfort, and an ability to perform work activities for a longer duration. Acclimatisation to heat involves a decrease in body core temperature, a decrease in pulse rate and an increase in sweating. Much of the acclimatisation to heat occurs within 4 to 7 days and it is usually complete in 12 to 14 successive days of heat exposure. Acclimatisation to cold is much slower and often less evident. It may take months, and it will not protect you from extreme cold.

 

 

Working in extreme temperatures

According to the Offices, Shops and Railway Premises Act 1963, the minimum temperature at which people should be expected to work is 16C. However, this is not a comfortable temperature for light or seated types of work.

The surface temperature of the walls, the windows, the ceiling and the floor should not differ from the room temperature by more than 2 or 3C. In some industrial production processes, workers may be exposed to either very hot (hot-dry or warm-moist) or very cold environments, for example, furnaces, laundry or textile dyeing, and the cold storage industry.

Hot conditions usually result in decreased efficiency and in physiological strain through an increase in pulse and heart rate, increase in skin temperature, thirst, weariness and faintness. In a hot-dry work environment, the heat often occurs in the form of radiation. Reducing this involves preventing the heat from falling on you, by wearing protective clothing or screening with aluminium foil. You should move out of the environment regularly to allow time for cooling off, for example, a 20 minute work period should be followed by a 10 minute cooling off period. In addition, small quantities of liquid should be drunk continuously to replace fluid lost by sweating.

In warm-moist environments work becomes difficult because of the high humidity, which reduces the evaporation of sweat. Ventilation and air conditioning improve the air quality.

In cold conditions, well-designed clothing insulates your body and slows down the release of metabolic heat. Your hands and feet are particularly susceptible to the cold, so they should be covered well so that manual dexterity and mobility are not impaired.

 

The temperature at home

According to some building design regulations, the temperature at home during the cold months of the year should not be less than the following:
 
Living Room 21C
Bedroom 18C
Kitchen 18C
Bathroom 20C
Toilet 16C
Corridor 18C
Entrance Hall 14C

This list is based on building design regulations, but in practice different values may be preferred, like a warmer bathroom or a cooler bedroom.

For the elderly, it is important to avoid large contrasts in temperature, for instance, when moving between one room and another, as this may affect their cardiovascular health.

 

 
Problem-solving
 

Problem Result Consequences Solution Optimum conditions
Air temperature - the surrounding (ambient) temperature of the place you are working
Too high The body can't dissipate its heat to the surrounding air. Increase in body temperature, increased heart action, reduced performance, more mistakes made, drowsiness. Increase room ventilation, induct cool air, exhaust hot air. Light office work
19-21C;
Light factory work
16-18C;
General factory work
14-16C;
Heavy factory work
12-14C.
Too low The body loses too much heat to the surrounding air. Shivering, stiffness of joints and muscles, reduced powers of concentration, complaints about draughts (even at low air velocities). Warm the air (heaters, radiators, etc.), supply radiated heat (ceiling or panel heating).
Radiation temperature - temperature of a source that is higher or lower than your body temperature, which causes you to feel hotter or colder
Too high The body absorbs too much heat from the source. As above Intercept radiation with sun blinds, double aluminium screens (or in extreme cases with special clothing), reduce the air temperature. In general, equal to or slightly above the air temperature.
Too low The body loses too much heat e.g. to cold walls or windows. As above Increase the air temperature.
Air velocity - air speed, for example, from draughty vents or windows; particularly felt around neck and ankles
Too high Local cooling of the skin. Muscular pains, complaints about draughts. Trace and eliminate cause. Office and assembly shops 0.15m/sec; Standing and walking work 0.15-0.5m/sec.
Too low Evaporation of sweat is reduced and therefore also dissipation of body heat. Clammy skin, complaints about stiffness. Install ventilators, induct air.
Humidity - amount of moisture in the air
Too high Evaporation of sweat impeded. Reduced resistance to high temperatures. Improve ventilation. 30-70% relative humidity.
Too low Excessive evaporation. Dryness of mucous membranes (of nose and mouth, for example). If necessary, artificially increase the humidity.
           
 

Acknowledgements
Content: George Havenith
Images: IMSI's MasterClips Collection