Maintaining Thermal Equilibrium

Our perception that our surroundings are too cold or too hot is based on many factors beyond the temperature of the air. The season, the clothes we are wearing, the amount of humidity and air movement, and the presence of heat given off by objects in the space all influence our comfort. Contact with surfaces or moving air, or with heat radiating from an object, produces the sensation of heat or cold. There is a wide range of temperatures that will be perceived as comfortable for one individual over time and in varying situations. We can regulate the body's heat loss with three layers of protection: the skin, clothing, and buildings.

The human body operates as an engine that produces heat. The fuel is the food we eat, in the form of proteins, carbohydrates, and fats. The digestive process uses chem icals, bacteria, and enzymes to break down food. Useful substances are pumped into the bloodstream and carried throughout the body. Waste products are filtered out during digestion and stored for elimination.

The normal internal body temperature is around 37°C (98.6°F). The internal temperature of the human body can't vary by more than a few degrees without causing physical distress. Our bodies turn only about one-fifth of the food energy we consume into mechanical work. The other four-fifths of this energy is given off as heat or stored as fat. The body requires continuous cooling to give off all this excess heat.

An individual's metabolism sets the rate at which energy is used. This metabolic rate changes with body weight, activity level, body surface area, health, sex, and age. The amount of clothing a person is wearing and the surrounding thermal and atmospheric conditions also influence the metabolic rate. It increases when we have a fever, during continuous activity, and in cold conditions if we are not wearing warm clothes. Our metabolic rates are highest at age 10, and lowest in old age. The weight of heavy winter clothing may add 10 to 15 percent to the metabolic rate. Pregnancy and lactation increase the rate by about 10 percent.

The amount of heat our bodies produce depends on what we are doing. An average-sized person who

70 W

is resting gives off about the same amount of heat as a 70-watt (70-W) incandescent lightbulb (Fig. 4-1). When that person is sitting at a desk, the heat generated rises to about that of a 100-W lightbulb (Fig. 4-2). The same person walking down the street at two miles per hour generates around the amount of heat given off by a 200-W lightbulb (Fig. 4-3). During vigorous exercise, the amount rises to between 300 and 870 W (Fig. 4-4). This is why a room full of people doing aerobic exercise heats up fairly quickly.

The set of conditions that allows our bodies to stay at the normal body temperature with the minimal amount of bodily regulation is called thermal equilibrium. We feel uncomfortable when the body works too hard to maintain its thermal equilibrium. We experience thermal comfort when heat production equals heat loss. Our mind feels alert, our body operates at maximum efficiency, and we are at our most productive. As designers of interior spaces, our goal is to create environ-

100 W

200 W

300-870 W

Figure 4-4 Person engaged in vigorous exercise.

The Human Body and the Built Environment 23

ments where people are neither too hot nor too cold to function comfortably and efficiently.

Studies have shown that industrial accidents increase at higher and at lower than normal temperatures, when our bodies struggle to run properly. When we are cold, we lose too much heat too quickly, especially from the back of the neck, the head, the back, and the arms and legs. When the body loses too much heat, we become lethargic and mentally dull. The heart pumps an increased amount of the blood directly to the skin and back to the heart, bypassing the brain and other organs. This puts an increased strain on the heart. Because we transfer heat from one part of the body to another through the bloodstream, it is sometimes difficult to figure out where the heat loss is actually occurring. We may need to wear a hat to keep our feet warm!

Our skin surface provides a layer of insulation between the body's interior and the environment, about equal in effect to putting on a light sweater. When the body loses more heat to a cold environment than it produces, it attempts to decrease the heat loss by constricting the outer blood vessels, reducing the blood flow to the outer surface of the skin. Goose bumps result when our skin tries to fluff up our meager body hairs to provide more insulation. If there continues to be too much heat loss, involuntary muscle action causes us to shiver, which increases heat production. We fold our arms and close our legs to reduce exposed area. When the level of heat loss is too great, muscle tension makes us hunch up, a strained posture that produces physical exhaustion. Ultimately, when deep body temperatures fall, we experience hypothermia, which can result in a coma or death. The slide toward hypothermia can be reversed by exercise to raise heat production, or by hot food and drink and a hot bath or sauna.

When we get too hot, the blood flow to the skin's surface increases, sweat glands secrete salt and water, and we lose body heat through evaporation of water from our skin. Water constantly evaporates from our respiratory passages and lungs; the air we exhale is usually saturated with water. In high humidity, evaporation is slow and the rate of perspiration increases as the body tries to compensate. When the surrounding air approaches body temperature, only evaporation by dry, moving air will lower our body temperature.

Overheating, like being too cold, increases fatigue and decreases our resistance to disease. If the body is not cooled, deep-body temperature rises and impairs metabolic functions, which can result in heat stroke and death. We will be looking at strategies for designing spaces that allow occupants to keep warm or cool enough to function in comfort.

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