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Safety at Workplace



Measuring heat stress


The degree of heat stress can be measured either by measuring the state of the hot zone or by measuring the body-state of someone in that zone.

The following are the four main variables which can influence the degree of heat stress :

  • Air temperature.
  • Relative humidity.
  • Globe thermometer temperature (radiant temperature).
  • Wind speed.

  • The following instruments are used to measure these variables :

    Thermometer : This is also called dry-bulb thermometer, is used to measure air temperature in the shade. Air temperature can vary from below zero to up to about 50°C. The bulb is protected from direct radiation by a polished cylinder which is open at both ends.

    Wet-bulb thermometer : This is used to measure the humidity with a dry-bulb thermometer. The lowest part, or bulb of the wet-bulb thermometer is kept wet by cotton gauze placed over the whole bulb, with the end of the gauze (but not the bulb) in a tiny can of water. When the air is dry, the water in the gauze will evaporate quickly and the temperature reading will fall. If the humidity is high, the water will evaporate slowly and the two temperature readings differ only a little. By taking the difference between the two readings and finding its value in the table you can find out the percentage relative humidity. The relative humidity can be between zero and 100 per cent.

    Globe thermometer : This is used to measure radiant temperature, which can be quite different from air temperature. The thermometer’s bulb is placed in the middle of a 15cm diameter hollow black copper sphere. The black globe absorbs radiation and warms the air inside.

    Anemometer : This is a wind speed gauge. An alternative method is to use a Kata thermometer which has only two marks on it. It is put in warm water until the mercury exceeds the top mark. It is then dried and exposed to the air. The time required for the mercury to fall from the top mark to the bottom mark is measured by a stopwatch. Using an attached table, you look up the wind speed from the time taken for the thermometer to cool.

    These four measures may be combined into a unified single indicator of heat stress in some cases. It is important to see whether the air temperature does not considerably deviate from the comfort zone, and whether the radiant temperature is minimised at the place where the work is conducted, especially when the relative humidity is high.

    Measuring the person

    The body temperature will raise when a worker is under heat stress. The body reacts by using its various mechanisms to keep the body temperature as constant as possible. The skin temperature will rise, the heart beat will increase, breathing will normally be deeper and the worker will perspire. However the body temperature should not change by more than about 1°C.

    The rate of perspiration indicates the strain produced on the body. The amount of perspiration in a working period can be measured by calculating the loss of body weight discounting the weight taken in by eating and drinking, weight lost when going to the lavatory, and any changes in weight of clothing. Perspiration contains salt. If there is a lot of perspiration, the body loses a lot of salt. In extreme cases, cramp occurs in muscles due to a shortage of salt in the body.

    The water and the salt lost by perspiration must be replaced. A fit young person in a very hot zone can lose more than one litre of perspiration in an hour. But normally he can tolerate losing up to about 4 litres in an 8-hour shift provided he replaces the water and the salt. The replacement should be by drinking either pure water or by drinking fruit juices. Adding salt to food and eating foods that contain natural salt and other minerals will also be helpful. Anyone not taking in enough water to replace perspiration loss will not pass very much urine; that which does pass will be strong and darker in colour. This is unhealthy.

    The body will never perspire unnecessarily. It is dangerous to try to reduce perspiration by drinking less fluids. Perspiring is essential. The body will never perspire unnecessarily. As losing a lot of water and salt by sweating is not healthy, we should try to create working conditions where we need not perspire much. It is dangerous to try to reduce perspiration by drinking less.

    Two medical effects of heat are :

    Heat exhaustion : Heat exhaustion is when you feel dizzy and faint and is due to a shortage of blood to the brain. Blood pressure falls. Lie down in a cool place so that the blood can flow to the head.

    Heat stroke : Heat stroke is when the workers skin is very hot and dry and is due to the perspiration mechanism not working properly. There is a rapid rise of body temperature to over 40°c, frequently with cessation of sweating. This is a medical emergency and medical assistance must be summoned immediately. While waiting it is vital to cool the body by sponging.

    Acclimatisation occurs when a person lives and works in a hot climate. It means that the person has adjusted and adapted to the hot climate. For fully acclimatised people, body temperature and the heart rate react less to working in heat. They perspire easily and the perspiration is le salty. They can also have greater blood circulation near the skin.


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