human beings give off heat, around an average of 100 kcal (kilo-calories) or 400 BTU (british thermal unit) per hour per person, due to what is known as 'metabolism'. the temperature mechanism within the human body maintains a body temperature of around 36.9 degree C (celsius) or 98.4 degree F (fahrenheit). but the skin temperature varies according to the surrounding temperature and relative humidity. to dissipate the heat generated by metabolism in order to maintain the body temperature at the normal level, there must be a flow of heat from the skin to the surrounding air. if the surrounding temperature is slightly less than that of the body, there will be a steady flow of heat from the skin.
but if the surrounding temperature is very low, as on a cold winter day, the rate of heat flow from the body will be quite rapid, thus the person feels cold. that is why we wear woolen winter clothing, which impede the rapid flow of heat from the body. on the other hand on a hot summer day, the surrounding temperature is higher than that of the body, and so there cannot be flow of heat from the skin to the surroundings; thus the person feels hot. in such a situation water from the body evaporates at the skin surface dissipating the heat due to metabolism. this helps in maintaining normal body temperature. but if the surrounding air is not only hot but highly humid as well, very little evaporation of water can take place from the skin surface, and so the person feels hot and uncomfortable.
a movement of air (such as by a fan) over the body helps in slightly improving the rate of evaporation of water at the skin surface, thereby giving some relief. too high a level of relative humidity causes a damp unpleasant feeling because of the accumulation of moisture in clothing and also leads to emanation of body odours. while too low a level of relative humidity causes the skin, mouth and nose to become dry and parched. so to obtain comfort, the temperature, relative humidity and air movement within the room are maintained by air conditioning. this is done so that the heat dissipation from the body is steady to maintain a normal body temperature resulting in a sensation of comfort.
in addition to maintaining proper temperature and relative humidity, it is evident that there must also be certain amount of air movement in the room. here again, too much of air movement (or draft) can cause discomfort. so the movement of air within the room should be gentle and uniform. the body odours emanating from people can become unbearable and for this, sufficient ventilation has to be provided. this can be done by taking in a certain amount of fresh air for dilution of the body odours. removal of dirt/dust particles is important from the health point of view, and also to maintain a neat environment.
since the combination of temperature, relative humidity and air motion influence the rate of heat dissipation from the body, these three can be considered interrelated for creating the sensation of comfort. therefore different combinations of temperature, relative humidity and air motion will give the same sense of comfort. again the sense of comfort can vary from person to person and also depends on the nature of their activities. for example, persons involved in heavy manual labour in a factory may need a different temperature, humidity and air motion combination from those seated in an auditorium.
in general, the range of temperature and humidity conditions maintained in the air conditioned space during summer are 23.5 to 25.5 degree C (74 to 78 degree F) and 55 to 65 RH with an air movement of 4.5 to 12 m/min. (15 to 40 ft/min.).
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