Heat stress is a common hazard in most of the industries. Heat stress produces heat strains which is directly proportional to the stress level. It is also well known that exposure to excessively high temperature may produce undesirable and debilitating effects on the individual. Problems of heat stress are very common in some industries such as Iron and Steel Mills, Glass and Ceramic Units, Forge shops, Foundries, Bricks and Tiles Factories, Thermometric plants, Cements, Coke ovens, Laundries, Mines and many others.

There are many work places in these units where artificial hot climates are deliberately created for the requirement of some processes. Whenever an individual is exposed to heat stress condition, there is a resulting strain due to considerable changes in many physiological reactions such as ‘sweat production’, ‘increased heart rate’ and ‘higher core temperature’ etc. The greater the stress level, the greater is the degree of strain experienced.

High heat stress disturbs the thermal equilibrium of the body, and consequently produces many adverse physiological reactions in man. The condition becomes very alarming when high degree of industrial heat combines with the metabolic heat arising out of heavy physical work performed by the workmen, particularly during the summer months. Work under such conditions not only produces undue strain and fatigue but also results in progressive decline in efficiency and productivity. It is, therefore, of prime importance to investigate this problem in order to ascertain and quantify various contributory factors, and mitigate them as far as possible, by suitable measures.

Heat Stress Exposure and Measurement

The environmental temperature depends on various factors air temperature, humidity, dew point, etc., Among these factors, human comfort depends on ambient temperature and relative humidity which ranges from 20°C to 27°C and 35–60%, respectively. Heat has been on record as a hazard to man since biblical times. However, it is only in the late few decades that national means have been developed for evaluating the stresses of hot environment identifying the contributing factors and predicting the resulting physiological strain.

Psychrometer

Instrument used for measuring air temperature and humidity. The psychrometer basically consists of two thermometers – a ‘dry bulb’ and a ‘wet bulb’–over which air passes at a certain speed. Dry bulb thermometer is just a liquid (most frequently used liquid is mercury) in glass thermometer. Wet bulb thermometer is a similar one, but having its bulb covered by a cotton wick. The wick which covers the bulb of one of the thermometers is fully wetted with distilled water.

The air movement which directly influences the evaporation of water from the wick will have a cooling effect, thus decreasing the temperature in the wet bulb, referred to as the ‘depression of the wet bulb’. The air velocity over the bulbs must be sufficient to ensure that equilibrium is reached rapidly. The temperature values in both thermometers are read and recorded as ‘dry bulb’ and ‘wet bulb’ temperature which are plotted on to a psychrometric chart, and the values for relative humidity, few point and absolute humidity can be obtained.

Sling Psychrometer (Whirling Hygrometer)

This is a very simple instrument in which the thermometers are mounted in a sling. The air movement over the bulbs is made by whirling the whole assembly. It is recommended to whirl at about 60 revolutions per minute. Usually one minute is enough to get the wet bulb thermometer to its lowest reading. After checking the reading, it is advisable to whirl a few more times, and check if the wet bulb temperature remains the same. If it continues to fall, the process should go on until the reading is stabilized.

Aspirating Psychrometer

This instrument is basically the same as the sling psychrometer. However, the air is circulated over the bulbs by means of an aspirator bulb to eliminate error from manual rotation. The same precautions as for the sling psychrometer should be applied. The air velocity is measured from the cooling power of the moving air. These are very useful for evaluation of heat stress because they also measure non-directional, turbulent air movement which is important for heat transfer.

Globe Thermometer (Vernon Globe)

This instrument consists of a hollow sphere made of metal ‘copper’ (Globe – with a diameter of 15 cm (or 6 inches) painted in a matte black to absorb as much as possible of the incident infra-red radiation. A thermometer is inserted in the globe so that the bulb (or sensing element is located at its centre). The globe is suspended, at the point of measurement, about 1.20 m (4 ft.) above the ground, with as little contact as possible with any other solids. Thus the globe gains heat by radiation and loses by convection (not conduction).