2. Thermophysiological comfort and thermoregulation

#### 2.1. Thermophysiological comfort

Comfort is a relative and subjective category, which depends on individual reactions and perceptions. Hatch [3] defines the comfort as a neutral state, where there is no pain or discomfort. The thermophysiological comfort is a part of the physiological comfort, related to the reactions of the thermoregulatory system [4]. It is based on the sensors for warmth and cold in the body, susceptible to thermal environment, air velocity, temperature asymmetry, etc. and the consecutive reactions of the thermoregulatory system, which increases either the heat, produced by the body, or the heat losses to the surrounding environment. When the heat production is equal to the heat losses, the body is considered to be in a state of a thermophysiological comfort [4].

#### 2.2. Basics of human thermoregulation in the cold

The main goal of the thermoregulatory system is to maintain the temperature of the core body (the brain and organs in the torso) around 37°C. The sensation of cold is initiated by the reaction of cold receptors: specialized nerve endings in the skin. They send signals to the central nervous system together with signals, coming from the brain (Figure 1). The two signals are processed in the hypothalamus: the gland in the brain that is responsible for the reactions of the thermoregulatory system. Actually, the spinal cord also controls the thermoregulation in case of cold exposure [5].

When the hypothalamus is activated, it sends electrical signals that trigger different thermoregulatory mechanisms, related to the decrement of the heat losses from the body (skin and lungs) to the surrounding air and increment of the heat production (in muscles and liver) [6–10]. Figure 2 summarizes the basic physical and physiological reactions of the human body to the cold.

One of the first reactions of the body in cold environment is to provoke vasoconstriction: a decrement of the cross section of the blood vessels in the surface zones of the skin and the extremities. The vasoconstriction appears due to signals from the hypothalamus to the smooth muscles in the arterioles. The aim of the vasoconstriction is to impede the transfer of hot blood out of the core body, thus preserving as long as possible the accurate functioning of the heart, brain, and lungs. This process can increase the body temperature of 1–2°C, due to the reduction of heat losses through the three mechanisms of heat transfer: conduction, convection, and radiation.

Figure 1. Signals sent to the hypothalamus.

environment, which can be natural (outdoor) or artificial (indoor) cold environment. It is the clothing only (in the past) and special protective clothing (at present) that is the only barrier between the human body and the cold environment. Clothing, activity, and proper management of the occupational activities help and maintain the thermophysiological comfort of

The purpose of this chapter is to present details about the human thermoregulatory system and the thermophysiological reactions of the body in a cold environment. Natural cold environment and artificial cold environment are compared to the light of their different effects on the occupational activities and management of the cold exposure. Cold-related injuries as part of the thermoregulatory reactions of the body and the risk from cold exposure are summarized. Clothing and hi-tech garments for protection from extreme temperatures are discussed. Practical advices and standards for the management of the occupational activities in the cold

Comfort is a relative and subjective category, which depends on individual reactions and perceptions. Hatch [3] defines the comfort as a neutral state, where there is no pain or discomfort. The thermophysiological comfort is a part of the physiological comfort, related to the reactions of the thermoregulatory system [4]. It is based on the sensors for warmth and cold in the body, susceptible to thermal environment, air velocity, temperature asymmetry, etc. and the consecutive reactions of the thermoregulatory system, which increases either the heat, produced by the body, or the heat losses to the surrounding environment. When the heat production is equal to the heat losses, the body is considered to be in a state of a thermophysiological comfort [4].

The main goal of the thermoregulatory system is to maintain the temperature of the core body (the brain and organs in the torso) around 37°C. The sensation of cold is initiated by the reaction of cold receptors: specialized nerve endings in the skin. They send signals to the central nervous system together with signals, coming from the brain (Figure 1). The two signals are processed in the hypothalamus: the gland in the brain that is responsible for the reactions of the thermoregulatory system. Actually, the spinal cord also controls the thermo-

When the hypothalamus is activated, it sends electrical signals that trigger different thermoregulatory mechanisms, related to the decrement of the heat losses from the body (skin and lungs) to the surrounding air and increment of the heat production (in muscles and liver) [6–10]. Figure 2 summarizes the basic physical and physiological reactions of the human body to the cold.

One of the first reactions of the body in cold environment is to provoke vasoconstriction: a decrement of the cross section of the blood vessels in the surface zones of the skin and the

2. Thermophysiological comfort and thermoregulation

people, working in the cold.

2.1. Thermophysiological comfort

2.2. Basics of human thermoregulation in the cold

regulation in case of cold exposure [5].

are presented.

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Figure 2. Thermoregulatory mechanisms in cold environment.

Figure 3 presents thermograms of a human hand, before and 3 min after the exposure to a cold environment of −10°C. The isolines clearly show the vasoconstriction process that has started. Vasoconstriction creates a feeling of cold, causing muscle tremors and increased heat production. Heat production from the liver also adds to the process. Cold hands and feet are the most frequent complaints of people in low ambient temperatures. Many of cold-related injuries are referred to fingers, nose, ears, and extremities, due to the decreased blood flow to them.

Figure 3. Thermograms of human hand: (a) before the cold exposure and (b) after a cold exposure to −10°C.

The behavioral reaction of the person to add an additional layer(s) of clothing (or bedding) also contributes to the augment of the core body temperature and the reduction of heat losses from the skin. Thus, when exposed to cold environment, the body maintains its internal temperature via vasoconstriction, increased heat production, and behavioral changes.

Signals to the erector pili muscles in the skin provoke pricking of the skin. Though small as an effect, the reaction is related to the detention of motionless air as close to the skin surface as possible. The air near the skin can be an additional insulation layer for the clothed body, as the thermal resistance of the air is almost twice higher than the thermal resistance of the natural fibers, for example, used in textile and clothing production.

The skeletal muscles are triggered to shivering: a spontaneous movement of the muscles, aiming to increase the heat production. This reaction to the cold is temporary and depends on the glycogen, the "fuel" of the muscles. The glycogen depletion stops the heat production by shivering.

The signals from the hypothalamus to the thyroid and adrenal glands have to increase the metabolism, which is another source of heat production for the body. The metabolic reactions are performed on a cell level.

#### 2.3. Body reactions to cold

Despite its complex and sensitive mechanism for thermoregulation, the human body is coping with the aggressive impact of the environment in a relatively narrow range. The decrement of the core body temperature has an adverse effect on the function of the body and can lead to severe disability and even death.

Out of these threats, however, even in cases, when the protective mechanisms of the body are sufficiently effective and thermoregulatory processes are able to maintain the core body temperature at about 37°C, the cold environment can cause a sequence of events, associated with the so-called thermal discomfort (Figure 4). The low environmental temperature causes delay in reaction time, fatigue, and increased sleepiness of the individual [11].

These effects are, to their nature, pure physiological processes, i.e., the accelerated heart rate causes fatigue, and the lack of irrigation in the brain tissue causes sleepiness. All these can result in a larger number of subjective errors, compared to the occupational activities in thermal comfort environment (20–22°C ).

The behavioral reaction of the person to add an additional layer(s) of clothing (or bedding) also contributes to the augment of the core body temperature and the reduction of heat losses from the skin. Thus, when exposed to cold environment, the body maintains its internal tempera-

Figure 3. Thermograms of human hand: (a) before the cold exposure and (b) after a cold exposure to −10°C.

Signals to the erector pili muscles in the skin provoke pricking of the skin. Though small as an effect, the reaction is related to the detention of motionless air as close to the skin surface as possible. The air near the skin can be an additional insulation layer for the clothed body, as the thermal resistance of the air is almost twice higher than the thermal resistance of the natural

The skeletal muscles are triggered to shivering: a spontaneous movement of the muscles, aiming to increase the heat production. This reaction to the cold is temporary and depends on the glycogen, the "fuel" of the muscles. The glycogen depletion stops the heat production by

The signals from the hypothalamus to the thyroid and adrenal glands have to increase the metabolism, which is another source of heat production for the body. The metabolic reactions

ture via vasoconstriction, increased heat production, and behavioral changes.

fibers, for example, used in textile and clothing production.

shivering.

226 Occupational Health

are performed on a cell level.
