There is a range of subjective, non-quantifiable factors which affect the thermal preference of the individual by either changing the metabolic rate or influencing the heat dissipation mechanisms. There almost are no indices to go by, but it is useful to keep such factors in mind in situations where the intended design is for people that could be identified with one or more of the factors below.
It happens when one moves from one national-geographic region to another that is different in its climatic conditions. The body has to come to terms with the change in the environment.
Different researches done across a time-period of decades seem to indicate that there is no significant difference in the thermally neutral conditions between temperate and tropical locations. Or, expressed rather more cautiously, if a difference does exist between the comfort conditions, then it is slight, and not of engineering significance. What this means it that, for example, although acclimatized persons living in the tropics can better endure hot environments and have become used to accepting the discomfort due to heat, this seems to have only a slight influence, if any, on the thermal environment which they will actually prefer if given a choice.
Research also indicates that comfort conditions are not subject to any seasonal variations.
Age and sex:
Although it is widely expected that elderly persons prefer warmer environments, there are no experimental results to support the hypothesis. The reason for the existance of the above hypothesis is that the basal metabolic rate decreases slightly with age. However, so does the insensible perspiration for the elderly. These offset each other in the heat balance, and hence offer a reasonable explanation as to why there is no significant difference in the desired thermal comfort between the young adults and the elderly.
As might be expected, comfort level comparisons have not been done comprehensively globally. Of the few national-biogeographic regions it has been carried out (the Netherlands, the USA), the following has been observed.
No significant difference in comfort conditions between males and females has been found, except in the United States of America where the females seem to prefer temperature, on average, up to 1 deg C higher than that preferred by males. No reasonable explanation has been suggested. The explanation for no statistical difference in preferred comfort temperatures between the sexes elsewhere is the same as in the case of the elderly; of correlation between metabolic rate and insensible perspiration.
There are conflicting dialogues about the effect of obesity on thermal comfort. It is argued time and again that since a tall, thin body has a greater surface-to-volume ratio than a rounded figure, and so loses
proportionately more heat, a thin person prefers a higher temperature. Also, the fat under the skin acts as an insulator, reducing the conduction of heat from deep-body tissues to the surface. But no supporting experimental evidence has been cited. On the other hand, some early experiments actually indicate no significant influence of body build on the comfort conditions of sedentary subjects.
It should be remembered, however, that the metabolic rate for a certain work involving body movements will tend to be higher for an obese person, and for this reason alone he/she may prefer a cooler environment during activity.
Conditions of health:
When a person is ill the metabolic heat production rate may change without proper functioning of the regulatory mechanisms. Consequently, external means may have to be substituted for the normal internal controls. The environmental temperature should be changed inversely to the metabolic rate.
Food and drink: The intake of foods causes an increase in internal heat production. After a meal of mixed diet (in terms of proteins, carbohydrates and fat), there is an increase of 10-15% in SDA (specific dynamic action) of food which can be higher for protein-rich foods, decreasing the optimal temperature by upto 1 degC.
The consumption of alcohol does not change the production of internal heat noticeably, though by skin vasodilation, it gives a sensation of warmth. At high doses of alcohol the resulting hypaesthesia will, of course, make the concept of thermal sensation and comfort meaningless.
Various other factors to keep in mind are the presence of asymmetric radiant fields like warm and cold panels in a wall; presence of draughts; cold and warm floors, all of which can alter the perceived thermal conditions.
This concludes the first part of tutorial. You may wish to print out the definitions from the next section (Units & Dimensions) before you proceed. We will now look at the psychrometric chart and try to understand the processes it can represent.