GENERAL CHARACTERISTICS OF THE THERMAL ENVIRONMENT AND MECHANISMS OF THERMAL REGULATION

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GENERAL CHARACTERISTICS OF THE THERMAL
ENVIRONMENT AND MECHANISMS OF THERMAL REGULATION
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• Humans tend to control their internal environment
  at about 37o C (98.6o F) although temperatures as
  high as 42o C (108o F) and as low as 18o C (64o
  F)have been reported in extreme cases.

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• 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.

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• 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.

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• 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.

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• Heat Balance Equation is derived from the First
  Law of Thermodynamics (energy is neither created
  or destroyed)
• S M - ( Wk) - E R C K
• S Heat Storage S 0 at thermal equilibrium.

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S M - ( Wk) - E R C K

• M Metabolism or metabolic heat production
  total energy released by both the aerobic and
  anaerobic processes (VO2 X approximately 5
  kcal/L of VO2 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 1o C.
• 1 MET 3.5 ml/kg/min
• Wk 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.

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S M - ( Wk) - 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.

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S M - ( Wk) - 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.

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• 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.

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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.

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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.

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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.

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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.

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Factors that Increase Convective Heat Loss

• Increase in the difference between T1 and T2
  (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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Factors that Increase Conductive Heat Exchange

• Increase in the difference between T1 and T2.
• 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).

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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 (Eres) are usually minor, but may become
  physiological significant at high altitude and/or
  extremely cold and dry air, particularly during
  exercising conditions.

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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.

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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.

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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.

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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.

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Partitioning of Actual Heat Loss to the
Environment
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BIOPHYSICS OF HEAT TRANSFER AND CLOTHING
CONSIDERATIONS
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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.

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HEAT FLOW AND FLUX

• 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.

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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.

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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.

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Total Insulation Iclothing Iambient air

• Thermal insulation is the resistance offered to
  the flow of heat between two surfaces and is
  determined by
• (T1 - T2)/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.

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Insulation of Clothing

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

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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 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.

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Factors Affecting the Insulative Value of Clothing

• Variations in skin temperature distribution and
  heat flow at various sites.

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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.
• 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.

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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 1.5-2.0 L/hr during exercise) and rain or snow
  if the garment is not water repellent will
  decrease the insulation of clothing.

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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.

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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.

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General Clothing Recommendations

• Use multiple layers.
• Outer layer should be wind and water resistant.
• Middle layer should trap air.
• Goose down
• Wool
• Polyester
• Polyolefrin

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General Clothing Recommendations

• 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.

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Efficiency Factor of Clothing

• 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.

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Insulation of Ambient Air

• A function of temperature, air velocity,
  altitude, relative humidity, and precipitation.

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FACTORS THAT DECREASE THE INSULATORY VALUE OF AIR

• Decrease in air temperature below skin
  temperature (Ta- Tsk).
• Increase in air velocity (exercise will increase
  air velocity).
• Decrease in relative humidity and/or
  precipitation.
• Decrease in elevation (i.e., low elevation). Air
  at high altitude provides better insulation.

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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.

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QUESTIONS?

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