The rate of heat loss from the body may exceed the rate of metabolic heat generation in a cold environment. Because the average specific heat of the human body is 3.49 kJ/kg C, each 1-degree Celsius drop in body temperature corresponds to a deficit of 244 kJ in body heat content for a 70 kg man. A 0.5C drop in mean body temperature causes noticeable but manageable discomfort. A temperature drop of 2.6 degrees Celsius is extremely unpleasant. A sleeping person will awaken when his or her mean body temperature drops by 1.3 degrees Celsius (normally 0.5 degrees Celsius in the deep body and 3 degrees Celsius in the skin area). A drop in deep body temperature below 35 degrees Celsius may harm the body’s temperature regulation mechanism, while a drop below 28 degrees Celsius may be fatal. Sedentary people reported feeling comfortable at 33.3C, uncomfortably cold at 31C, shivering cold at 30C, and extremely cold at 29C. People who do heavy work reported feeling comfortable at much lower temperatures, demonstrating that activity level influences human performance and comfort. Cold weather has the greatest impact on the extremities of the body, such as the hands and feet, and their temperature is a better indicator of comfort and performance. A temperature of 20 degrees Celsius is considered uncomfortably cold, 15 degrees Celsius is considered extremely cold, and 5 degrees Celsius is considered painfully cold. Hands can perform useful work without difficulty as long as the skin temperature of the fingers remains above 16 degrees Celsius.
In a cold environment, the body’s first line of defense against excessive heat loss is to reduce skin temperature and thus the rate of heat loss from the skin by constricting veins and decreasing blood flow to the skin. This measure lowers the temperature of the tissues next to the skin while keeping the internal body temperature constant. The next preventive measure is to increase the rate of metabolic heat generation in the body by shivering unless the person does so voluntarily by increasing his or her level of activity or by wearing extra clothing. Shivering begins slowly in small muscle groups and can initially double the rate of metabolic heat production in the body. In the most severe cases of total body shivering, the rate of heat production can reach six times that of resting levels. If this measure is also insufficient, the deep body temperature begins to fall. Tissue damage is most likely in body parts farthest away from the core, such as the hands and feet.
This is actually part-3 of the HUMAN BODY AND THERMAL COMFORT series. part-2 is given in the link below.
Part-2: Thermal comfort in buildings.