GENERAL CHARACTERISTICS OF THE THERMAL ENVIRONMENT AND MECHANISMS OF THERMAL REGULATION

Humans tend to control their internal environment at about 37 o C (98.6 o F) although temperatures as high as 42 o C (108 o F) and as low as 18 o C (64 o F)have been reported in extreme cases.

The hypothalamus, human thermostat, controls thermal regulation in humans by acting as a thermal sensor, an integrator of information from other locations in the body, and as a controller of various effector mechanisms which are ready to either increase or decrease the body's ability to conserve or dissipate heat.

Anterior hypothalamus is the heat dissipation (loss) center. Posterior area of the hypothalamus is the heat conservation (gain) center. The two controller areas are reciprocally innervated, stimulation of one results in inhibition of the other.

Set Point of Hypothalamic Thermostat Is Affected By A Variety Of Factors Such As: 1.Fever - a pyrogen (viral or bacterial) elevates the set point. 2.Antipyrogenic agents (e.g., aspirin) - lower set point. 3.Circadian (24 h) rhythms - low set point in the early morning and high late- afternoon set point which corresponds to the usual light-dark cycle and usual pattern of metabolic activity. 4. Gender - women have a higher set point during the second half of the monthly menstrual cycle, which may be due to the anabolic effect of progesterone.

Heat Balance Equation is derived from the First Law of Thermodynamics (energy is neither created or destroyed): S = M - (+ W k ) - E + R + C + K S = Heat Storage; S = 0 at thermal equilibrium.

S = M - (+ W k ) - E + R + C + K M = Metabolism or metabolic heat production; total energy released by both the aerobic and anaerobic processes (VO 2 X approximately 5 kcal/L of VO 2 ; a little higher if CHO rather than fat is the fuel source). Note: 1 kcal = amount of heat required to raise 1 kg of water 1 o C. 1 MET = 3.5 ml/kg/min W k = Work: where + is positive work representing energy leaving the system or work against internal forces and - is negative or eccentric work or work against external forces; at rest, W = 0.

S = M - (+ W k ) - E + R + C + K E = Evaporation: insensible exchange of heat via vaporizing moisture. R = Radiation: sensible exchange of heat via electromagnetic waves. C = Convection: sensible exchange of heat via a circulating medium. K = Conduction: sensible exchange of heat via a static medium.

S = M - (+ W k ) - E + R + C + K Thermal Equilibrium exists when S = 0. The ability of an individual to maintain thermal equilibrium with the environment is a net result of the interaction of physics (e.g., clothing insulation or absorptivity) and physiology (esp., hydration levels). + flow or S = hyperthermia as an individual can not transfer excess body heat to environment. - flow or S = hypothermia as a individual can not effectively retain body heat as excessive amounts are being transferred to the environment.

Sensible or Dry Heat Exchange - it is a function of the measurable difference in temperature between an organism and the environment; includes convection, conduction, and radiation. Insensible or Moist Heat Exchange - it is a result of evaporation of water (sweat or perspiration) from the surface of the body.

CONVECTION Heat is transported by a stream of molecules from a warm object toward a cooler objective. The most common exchange of body heat by convection begins with heat loss from a warm body to a surrounding fluid (air or water). The heated fluid expands, becomes less dense, and rises taking heat with it. The area immediately adjacent to the skin is then replaced by a cooler, dense fluid, and the process is repeated. Note that heat gain can also occur through the opposite or reverse process.

CONVECTION Also occurs within the body in which warmed blood is cooled by cooler tissue and cooled blood is warmed by warmer, more metabolically active tissue; this is known as countercurrent heat exchange. Convective heat loss is greater for water than for air because water is more dense than air.

TWO TYPES OF CONVECTION Free convection - function of fluid density (decrease temperature, increase density) and is important in static or very slow flow rate. The concepts of free convection are most closely associated with the medium of water. Forced convection - function of fluid velocity and becomes increasing important at higher fluid speeds (i.e., fast wind speeds). Forced convection results in greater heat loss per unit of time than free convection.

Forced Convection and Laminar vs Turbulent Flow Laminar flow results in faster velocity; creates layers of increasing velocity flow above the surface. Turbulent Flow, which may be caused by rough surfaces, disrupts layers of flow bringing more opposing/diverse fluids of differing temperature in contact with the surface; increases in turbulence increases the potential for heat exchange by convection.

Factors that Increase Convective Heat Loss Increase in the difference between T 1 and T 2 (i.e., air or water temperature is lower than skin temperature). Decrease in temperature of circulating medium. Increase in surface area, which is related to dimension and shape of body. Decrease in clothing covering the body increases the surface area exposed. Increase in thermal conductivity of the circulating medium. Increase in density of circulating medium.

Factors that Increase Convective Heat Loss Thermal conductivity is greater for water than air. Decrease in temperature of circulating medium increases the density of the circulating medium and its thermal conductivity. Increase in precipitation would increase free convective heat loss, but may increase or decrease forced convective heat loss depending on how air temperature is changed relative to skin temperature.

Factors that Increase Convective Heat Loss Decrease in the insulation of clothing when wet as the insulatory layer of air is replaced by the higher conductive medium of water. Decrease in altitude (i.e., convective heat loss is greater at lower elevations due to a higher air density; at altitude air density decreases and hence convective heat loss decreases). Increase in turbulent flow and/or a decrease in laminar flow of circulating medium.

Factors that Increase Convective Heat Loss Increase in velocity of circulating medium increases the heat loss per unit of time. Increase in air pollution due to an increase in the density of air. Increase in hyperbaria (i.e., underwater diving) as an increase in barometric pressure increases the density of water, water has a greater thermal conductivity that air, and water temperature is usually lower than air temperature.

Factors that Increase Convective Heat Loss Exercise and the associated increase in core temperature. In general, the opposite changes in the factors listed above would have just the opposite effect by decreasing convective loss or perhaps increasing convective heat gain.

Conduction (K) Conduction (K) of heat occurs whenever two surfaces with differing temperatures are in direct contact. Conductors are substances that conduct heat readily, such as metals, water, & muscle tissue; where as insulators are substances that do not conduct heat readily, such as still air, nonmetals, and fat tissue.

Conduction (K) Note: Trapped still air in clothing makes an excellent insulator due to low conductivity and the fact that it increases the thickness (distance) through which heat must be transferred in order to be lost.

Conduction (K) Generally, conductive heat loss represents only a minor percentage of total heat exchange between the body and environment as the skin surface area in direct contact with external objects is usually minimal and people usually avoid contact with highly conductive materials. However, body heat is conducted from the skin to clothing where it is dissipated from the outer surfaces of the clothing via evaporation, convection, or radiation depending on the vapor pressure (i.e. relative humidity), air movement, and the skin-clothing-ambient temperature gradients. Also, conductive heat transfer also occurs within the body from one area to another as well as from the core and muscle shell to the skin surface.

Factors that Increase Conductive Heat Exchange Increase in the difference between T 1 and T 2. Increase in surface area, which is related to dimension and shape of body. Decrease in clothing covering the body increases the surface area exposed. Increase in the thermal conductivity of tissue, clothing, or surfaces that contacts the body (e.g., metals, water, and muscle tissue have greater thermal conductivity than fat, air, and non-metals).

Factors that Increase Conductive Heat Exchange Decrease in the thickness or distance between two surfaces, areas, or static mediums. Exercise and the associated increase in core temperature. In general, the opposite changes in the factors listed above would have just the opposite effect by decreasing conductive heat exchange.

Radiation (R) Radiation (R) is the exchange of electromagnetic energy waves emitted from one object and absorbed by another; it is a complex term which represents the net effective radiation balance of an individual. The human body absorbs nearly all the radiation that falls upon it.

Understanding Radiation In understanding radiation, heat can be considered as photons or light particles emitted or absorbed by the body. An atom is like a miniature solar system. At the heart of the solar system is the nucleus of the atom with one or more electrons orbiting around the nucleus. The orbital path of the electrons can change as absorption of photons or light particles cause the electrons to move to an outer orbit and emitted photons cause the electrons to move to a closer, inner orbit around the nucleus.

Understanding Radiation Molecules absorb and emit radiation in different ways than atoms; they increase or decrease their vibration due to changes in the atom. Photons coming from the sun at 186,000 miles per second are absorbed in the skin thereby increasing molecular vibrations (as absorption of photons or light particles cause the electrons to move to an outer orbit in the atoms) and warming the body. Heat is lost from molecules when the amount of molecular vibrations decreases (emitted photons cause the electrons to move to a closer, inner orbit around the nucleus in the atoms).

Understanding Radiation The wavelength of radiation determines whether we can see it or feel it. Long wavelength radiation is invisible and can only be perceived as heat. For example, you can feel the radiation emitted from the body as heat or the infrared radiation from a fire that has stopped glowing. Shorter wavelengths can be seen. The color shifts through dull red through yellow to white as the wavelength becomes shorter.

Radiation (R) Six Factors Affect Radiation 3 solar (sun) factors: direct, diffuse, & reflected (ground). 2 thermal or heat factors (ground and sky). 1 radiation factor emitted from the body.

Factors that Increase Radiate Heat Gain Increase in the difference between T skin surface temp and T environmental radiant temp. Increase in the surface area exposed, which is related to dimension and shape of body. Decrease in clothing covering the body increases the surface area exposed. Increase in dark colors relative to light colors that are exposed.

Factors that Increase Radiate Heat Gain Increase in smooth textured surfaces relative to rough textured surfaces of skin and clothing. Increase in altitude (i.e., higher elevations) due to a decrease air mass that increases solar radiation and an increase in snow, ice, and rocks that increases reflected solar radiation Decrease in air pollution which decreases the density of air. In general, the opposite changes in the factors listed above would have just the opposite effect by decreasing radiant heat gain.

Insensible or Evaporative Heat Exchange Insensible or Evaporative Heat Exchange is the result of evaporation or condensation of water on the body surface as water is changed from a liquid to gas; this process requires heat which is extracted from the immediate surroundings (i.e., skin) which results in cooling; the amount or degree of evaporation is determined by the water concentration gradient between the body surface area and the environment.

Sources of Evaporative Heat Loss 1.Insensible perspiration (diffusion of water through the skin). 2.Thermal and nonthermal (nervous) sweating. 3.Water losses from the respiratory tract during respiration. Rest - 30 ml/hr of water loss. High Environmental Temperatures and/or Strenuous Exercise sweating rates may be as high as 1.5 to 2.0 L/hr Evaporative Heat Losses from the Respiratory Tract (E res ) are usually minor, but may become physiological significant at high altitude and/or extremely cold and dry air, particularly during exercising conditions.

Evaporative Heat Loss Note: (1) the cooling of air decreases the capacity of air for moisture and therefore the concentration gradient for evaporation; (2) however, when cold air comes in contact with the body it's temperature increases thereby increasing it's moisture capacity and hence, dehydration can occur even during cold temperatures; (3) also, if clothing is not properly ventilated so that moisture can not pass directly into the air from the skin for evaporation, the warm skin air will be cooled and moisture will condense in the clothing thereby decreasing the insulatory effects of the clothing which may result in combined dehydration and hypothermia.

Factors That Increase Evaporative Heat Loss Increase in the difference between the vapor pressure in the air and the vapor pressure at the skin. Decrease in relative humidity decreases the vapor pressure in the air thereby increasing the gradient for evaporation. Increase in the surface area exposed, which is related to dimension and shape of body. Decrease in clothing covering the body increases the surface area exposed.

Factors That Increase Evaporative Heat Loss Increase in sweat rate. Increase in thermal conductivity of sweat; decrease in osmolarity of sweat (i.e., more dilute sweat) increases thermal conductivity of sweat. Increase in the surface area that is wetted. Increase in altitude (i.e., higher elevations) due to an increase in the capacity of the air for moisture. Increase in air velocity (i.e., wind speed). Exercise. Increase in core temperature increase the latent heat available to vaporize sweat from a liquid into a gas.

Factors That Increase Evaporative Heat Loss Increase in ventilation rate which increases heat loss by respiration. No precipitation as precipitation decreases evaporation as the air becomes completely saturated with moisture. Increase in air temperature increases the capacity of air for moisture. Hyperbaria (i.e., underwater diving) completely eliminates evaporative heat loss. In general, the opposite changes in the factors listed above would have just the opposite effect by decreasing evaporative heat loss.

Partitioning of Actual Heat Loss to the Environment

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BIOPHYSICS OF HEAT TRANSFER AND CLOTHING CONSIDERATIONS

HEAT TRANSFER Heat transfer is the analysis of the rate of heat transfer, flow, or exchange in a system, which is governed by the laws of thermodynamics; the modes of heat transfer in a system are radiation, convection, conduction, and evaporation; the combined interaction of these mechanisms results in the overall heat transfer within a system and consequently, heat storage, heat loss, or thermal balance. Heat transfer is the analysis of the rate of heat transfer, flow, or exchange in a system, which is governed by the laws of thermodynamics; the modes of heat transfer in a system are radiation, convection, conduction, and evaporation; the combined interaction of these mechanisms results in the overall heat transfer within a system and consequently, heat storage, heat loss, or thermal balance.

HEAT FLOW AND FLUX Heat always flows from the region of high temperature to a region of low temperature. Heat always flows from the region of high temperature to a region of low temperature. Heat flux is a term used to summarize the amount of heat transferred per unit of time. Heat flux is a term used to summarize the amount of heat transferred per unit of time.

HEAT BALANCE EQUATION Remember: S = M - (+ W) + K + C + R - E; In this equation M is equal to metabolic heat production (resting metabolic rate = 3.5 ml/kg/min or 50 kcal/hr/m2; for every L of VO2, approximately 5.0 kcal are expended); W is equal to work, which is either positive work representing energy leaving the system or work against internal forces OR negative or eccentric work or work against external forces; K, C, R, & E represent the mechanisms of heat transfer. Remember: S = M - (+ W) + K + C + R - E; In this equation M is equal to metabolic heat production (resting metabolic rate = 3.5 ml/kg/min or 50 kcal/hr/m2; for every L of VO2, approximately 5.0 kcal are expended); W is equal to work, which is either positive work representing energy leaving the system or work against internal forces OR negative or eccentric work or work against external forces; K, C, R, & E represent the mechanisms of heat transfer.

HEAT TRANSFER In addition to previously discussed information, insulation from air and clothing are factors which need to be taken into consideration when understanding the total impact of heat transfer.In addition to previously discussed information, insulation from air and clothing are factors which need to be taken into consideration when understanding the total impact of heat transfer.

Total Insulation = I clothing + I ambient air Thermal insulation is the resistance offered to the flow of heat between two surfaces and is determined by: Thermal insulation is the resistance offered to the flow of heat between two surfaces and is determined by: (T 1 - T 2 )/Flow of heat per unit of surface area. (T 1 - T 2 )/Flow of heat per unit of surface area. Note: The slower (i.e., lower) the flow of heat per unit of surface area or the smaller the difference between the temperatures of two surfaces, the greater the thermal insulation.

Insulation of Clothing 1 CLO = unit of clothing thermal insulation; the clothing necessary to insult in comfort (thermoneutrality) a resting subject at 21 C o (70 o F), air movement of 10 cm/s or 20 fpm (normal ventilation rate of a room), and a relative humidity of less than 50%.

Factors Affecting the Insulative Value of Clothing Fabric's thermal conduction, which is a function of the thickness of the clothing and extend of trapped air layers; the greater the air trapped and/or the thicker the clothing, the greater the insulation. Fabric's thermal conduction, which is a function of the thickness of the clothing and extend of trapped air layers; the greater the air trapped and/or the thicker the clothing, the greater the insulation. Fabric's dispersion over the skin surface area, which extends the total potential surface area open to the environment; the greater the dispersion of clothing over the skin surface area, the greater the insulation. Fabric's dispersion over the skin surface area, which extends the total potential surface area open to the environment; the greater the dispersion of clothing over the skin surface area, the greater the insulation.

Factors Affecting the Insulative Value of Clothing Variations in skin temperature distribution and heat flow at various sites. Variations in skin temperature distribution and heat flow at various sites.

Factors Affecting the Insulative Value of Clothing Variations in clothing surface covering the skin and skin blood flow: none of the hands and face, presence of arterial-venous anastomosis in the extremities, and vasodilatory activities in the face. Variations in clothing surface covering the skin and skin blood flow: none of the hands and face, presence of arterial-venous anastomosis in the extremities, and vasodilatory activities in the face. Air layer next to skin; an increase in movement will decrease the air layer and insulation around the skin. Air layer next to skin; an increase in movement will decrease the air layer and insulation around the skin. Exercise increases air movement, particularly if garment is not wind resistant. Exercise increases air movement, particularly if garment is not wind resistant.

Factors Affecting the Insulative Value of Clothing Wet clothing will decrease the insulation of clothing to 30%. Sweating (30 ml/hr at rest, up to L/hr during exercise) and rain or snow if the garment is not water repellent will decrease the insulation of clothing. Wet clothing will decrease the insulation of clothing to 30%. Sweating (30 ml/hr at rest, up to L/hr during exercise) and rain or snow if the garment is not water repellent will decrease the insulation of clothing.

Factors Affecting the Insulative Value of Clothing Compression of clothing material. Particularly true to the feet where compression of boots as a person stands on a stone floor has been reported to reduce insulation to that comparable to standing with naked feet. Also with the hands, the gripping of ski poles or bike handlebars will cause compression of the gloves and therefore, reduce the insulation of the gloves. Finally, water has been reported to decrease insulation of compressed clothes by up to 50%. Compression of clothing material. Particularly true to the feet where compression of boots as a person stands on a stone floor has been reported to reduce insulation to that comparable to standing with naked feet. Also with the hands, the gripping of ski poles or bike handlebars will cause compression of the gloves and therefore, reduce the insulation of the gloves. Finally, water has been reported to decrease insulation of compressed clothes by up to 50%.

Factors Affecting the Insulative Value of Clothing Air temperature. As air temperature increases above skin temperature, insulation increases which may lead to a hyperthermic (i.e., heat gain) response; as air temperature decreases below skin temperature, insulation decreases which may lead to a hypothermic (i.e., heat loss) response. Air temperature. As air temperature increases above skin temperature, insulation increases which may lead to a hyperthermic (i.e., heat gain) response; as air temperature decreases below skin temperature, insulation decreases which may lead to a hypothermic (i.e., heat loss) response.

General Clothing Recommendations Use multiple layers. Use multiple layers. Outer layer should be wind and water resistant. Outer layer should be wind and water resistant. Middle layer should trap air. Middle layer should trap air. Goose down WoolPolyesterPolyolefrin

General Clothing Recommendations Inner layer should also wick away moisture from the skin to prevent evaporative heat loss. Inner layer should also wick away moisture from the skin to prevent evaporative heat loss.Polypropylene Cotton Fishnet Most important to cover trunk and head during prolonged exposure to cold. Most important to cover trunk and head during prolonged exposure to cold.

Efficiency Factor of Clothing Ratio of: Ratio of: Thermal resistance between clothing surface and air Resistance between skin surface and air Higher the ratio the greater the efficiency factor of clothing or insulation and vice-versa.Higher the ratio the greater the efficiency factor of clothing or insulation and vice-versa.

Insulation of Ambient Air A function of temperature, air velocity, altitude, relative humidity, and precipitation. A function of temperature, air velocity, altitude, relative humidity, and precipitation.

FACTORS THAT DECREASE THE INSULATORY VALUE OF AIR Decrease in air temperature below skin temperature (T a- T sk ). Decrease in air temperature below skin temperature (T a- T sk ). Increase in air velocity (exercise will increase air velocity). Increase in air velocity (exercise will increase air velocity). Decrease in relative humidity and/or precipitation. Decrease in relative humidity and/or precipitation. Decrease in elevation (i.e., low elevation). Air at high altitude provides better insulation. Decrease in elevation (i.e., low elevation). Air at high altitude provides better insulation.

Altitude and Insulation of Ambient Air Since altitude decreases convective heat loss and increases radiant heat gain and evaporative heat loss, the increase in the insulatory value of air at high altitudes suggests that the decrease in convective heat loss and increase in radiant heat gain is greater than the increase in evaporative heat loss at high altitude. Since altitude decreases convective heat loss and increases radiant heat gain and evaporative heat loss, the increase in the insulatory value of air at high altitudes suggests that the decrease in convective heat loss and increase in radiant heat gain is greater than the increase in evaporative heat loss at high altitude.

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