Heat stress indices:
In order to assess the total heat stress imposed on man by any working environment, an integrated approach evaluating both the climate and non climatic factors which are likely to influence the, eat-exchange between man and his environment.. In view of this many heat stress indices have been evolved from time to time. These are in the form of either designing instruments which ad as integrating mimics of human body or developing formulae or monograms to estimate the stress inflected by a ‘wide range or conditions of work and climate of quantity the physiological stain in response to these stresses. The common heat stress indices which are mostly used for the assessment; of the thermal environment are as follows.
- Effective Temperature and Corrected Effective Temperature (E.T& C.E.T)
- Wet bulb temperature (WBGT)
- Oxford index
- Predicated -4 hourly sweat rate (P4SR)
- Belding-Hatch Index(HIS)
Effective and Corrected effective Temperature (ET/ CET) Scale:
Effective temperature’ (ET) is a sensory scale of warmth, compiled from the readings of dry e wet bulb temperature and air velocity from temperature and air velocity from standard monogram. The temperature thus arrived at corresponds to temperature of still air saturated with water vapor which is supposed to give the same sensation of warmth as that experienced in the environment in question, meaning thereby that if an individual is hypothetically exposed to this temperature under 100% RH and same sensation of discomfort as he is experiencing’ under the environmental conditions actually still air movement, the exposed individual will have the recorded.
In situations where radiant heat source, the black globe temperature is considered instead of dry bulb temperature and the scale thus constructed Corrected Effective Temperature.’ (CET).
The ET/CET scales modified depending upon the amount of clothing worn by the individual such as ‘Normal Scale’ and Basic Scale’ which, can constructed. from two. Different monograms The basic scale ‘refers to men stripped to the waist and normal to men who are fully clad in indoor clothing.
For climate having relative-humidity of less than 40% the scales cannot sued Moreover, the scales exaggerate the effects or high dry-bulb temperatures in air movement of unto 3.5 m/ sec. and underestimate the deleterious impact of low air-movement in hot and humid environment. It well known that widely different climates having same ET/ CET values do not impose the same, physiological stain. Furthermore, the scales do not provide any allowance for different rates of energy expenditure. This index of heat stress has been devised to assess the severity hot humid conditions of the working place particularly where the ventilation is poor.
This has expressed by a simple weighting as follows:
WD = 0.5D +0.85 W,
Where WD = Weighted value, d & w are dry and wet bulb temperature respectively.
Heat stress indices
Predicted Four Hourly Sweat Rate (PSR):
This index based on the assumption of the amount of sweat that would perspired by a physically fit and acclimatized young man in the condition under review cover a period of four hours. It takes into account the ‘metabolic level and type of clothing in addition to the climatic factors, unlike other indices mentioned earlier. But this as the drawback of the cumbersome monograms to referred to, -and thus lacks which is essential in a practical situation.
Since the physical activity level on the shop floor will remain almost constant; we may make use of the simple indices like CET /ET or WBGT in our control program.
It is thus evident that the heat stress indices like ET/CET, P4SR, and Oxford index, etc. even though regarded as the useful indices for the evaluation of stress have inherent shortcomings and limitations. As a matter of fact some of them require expensive equipment and/or are difficult to determine in industrial work. An index which has received much attention in recent years, and has been officially adopted by some countries, is the Wet Bulb-Globe temperature Index, It also has limitations, but has the definite advantages of being very easy to determine and of requiring simple and inexpensive equipment.
Heat stress indices
Wet Bulb Globe Temperature (WBG1) Index
It embraces in a single value the effect of radiation ambient air temperature and humidity. It is the weighted value of wet and dry bulb temperature and globe thermometer readings. Calculated using temperature measurements alone, thereby eliminating the need to measure air velocity.
The wet bulb-globe temperature index initially developed; to provide a simple method for the assessment of heat stress among the military personnel. The basis for this index is that the Wet Bulb-Globe Temperature; (representing the environmental heat load) combined with the work load; (representing the metabolic heat load) by plotting the values of both parameters; on a co-ordinate system and evaluating the resulting points in relation to curves; established according to the concept of prescribed zone, as described by Lind. For continuous exposure over 8 hours,
The limiting curve is the Upper Limit of the prescribed zone, as described by Lind. For continuous exposure over 8 hours, the limiting curve is the ‘Upper Limit of the Prescriptive Zone (ULPZ) Line’. This curve is such that it represents the upper limit for combinations of environmental conditions; and workloads that do not cause an increase in the core temperature to above 380c; in 95% of average acclimatized individuals. In the’ Prescriptive Zone’ the deep-body temperature determined; only by the workload (physical activity) and is practically independent from the government; while in the T Environmental Drive Zone’ the deep body temperature becomes sensitive to small changes in environment climatic conditions.
Equipment: The instruments required for the determination of the WBGT are
- Dry-Bulb Thermometer (for measurements outdoors I sun shine only) ta
- Globe Thermometer – ta
- Natural wet bulb thermometer -ta
One of the advantages is that it excludes the use of anemometers; and eliminates the problem of obtaining average air velocities (Which is not very practical in some situations).
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