Biometeorology of Humans in Desert Environments

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Biometeorology of Humans in Desert Environments

• Chapter 19

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The Heat Balance of the Human Body - Components

• Longwave radiation gain or loss
• Conductive and convective exchange with the
  atmosphere gain or loss
• Metabolic source - gain
• Direct, diffuse, and reflected solar radiation -
  gain
• Evaporation loss
• Conduction with the ground gain or loss

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The Energy Conservation Equation
QM QLR QSR QH QE QG ?QS
?QS is the net rate of heat gain or loss as a
result of an imbalance in the terms. ?QS not
equal to zero means that the body temperature
will increase or decrease. ?QS must remain small,
or be large for only a short time, in humans
because of the small range in tolerable body
temperatures.
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Long-wave (Infrared) Radiation

• Emitted by the skin surface
• - IR (lost) intensity sTskin4, with Tskin
• 92 F, 33.3 C
• - clothing absorbs some of the emitted IR and
  re- emits it back to the skin and to the
  environment.
• Absorbed by the skin surface
• - IR (gained) intensity sTenviron4, where
  Tenviron applies to the atmosphere, the ground,
  etc.
• - clothing absorbs some of the IR from the
  environment, and emits it to the skin and back
  to the environment
• Whether there is a net gain or loss by the skin
  depends on Tskin versus the average Tenviron

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Metabolic Source of Heat

• Body at rest generates about 80 Cal of heat per
  hour enough to heat 1 liter of ice water to
  boiling point.
• The greater the level of exercise, the more heat
  generated normal to fast walk generates 280
  Cal per hour.
• Over a typical work day, the metabolic energy
  generated in a day would be about 3 times that of
  a person at rest.

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The greater the activity level, and the greater
the body weight, the greater the perspiration
rate required to maintain a heat balance.
Surrogate for rate of heat gain
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Conductive and Convective Exchange of Heat
Between the Skin and the Atmosphere

• In a very shallow layer above the skin surface,
  conduction is responsible for the heat exchange
  the direction of heat transfer is from hot to
  cold.
• Outside this layer, convection transfers the
  heat.
• This is analogous to how heat is transferred
  between the ground surface and the atmosphere

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• Thus, heat is gained by the body if the air
  temperature is greater than the skin surface
  temperature.
• A shallow boundary layer forms over the skin
  surface, with this thin boundary-layer air having
  a temperature near the skin temperature reduces
  heat transfer.
• Wind strips the boundary layer away from the
  skin, increases the temperature contrast, and
  increases the heat transfer.
• Thus, heat is gained most rapidly on windy days
  with air T over 92 F, when no clothes protect
  the boundary layer from the wind.
• Note parallel with the concept of wind-chill
  factor.

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Rate of Heat Gain
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How might perspiration rate be estimated?
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Evaporation

• Takes place in the respiratory system and from
  the skin.
• Respiratory evaporative cooling is responsible
  for loss of only 25 of metabolic heat generated
  in resting person. In a cool environment, the
  rest of the heat is lost through conduction and
  radiation.
• In the hot desert where conduction/convection, IR
  radiation, and solar radiation all represent heat
  gains, or when metabolic heat generation is high,
  perspiration from the skin is the main/only way
  that the body loses heat.

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Rate of Evaporation From the Skin Depends On

• Humidity
• Wind speed
• Temperature

In the desert, these conditions are favorable
enough for evaporation that we often dont see
any moisture on the skin.
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Fluid balance
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Example of the Efficiency of Evaporation For Heat
Regulation - Sauna

• Dry air that is heated to 110 C (230 F)
• Pan of water taken into this environment will
  boil, and a steak will cook in the amount of time
  that a person is in the sauna.
• Why doesnt the body overheat? rapid
  evaporation into the hot, dry environment.
• Evidence of evaporative loss of heat?
• Blow on the skin to remove the boundary layer
  blister forms immediately
• Nostrils become raw
• Water thrown on hot rocks in sauna makes it feel
  hotter because it reduces evaporation rate.

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Solar Radiation Direct, Diffuse, Reflected

• Amount absorbed by the body depends
• Sun angle
• Cloud cover
• Dust content of atmosphere
• Albedo of surface, and slope
• Position of the body
• Clothing

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Maintaining the Heat Balance
QM QLR QSR QH QE QG ?QS
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What the Body Does To Maintain The Heat Balance
in Hot Environments

• Increase perspiration
• Dilation of arteries to increase capillary blood
  flow at the skin surface (if air T is less than
  blood T)
• But, even with unlimited water consumption,
  strenuous physical activity in the desert can
  cause dangerously high body temperatures
• heat is not transported fast enough from the
  interior of the body to the surface where the
  heat is lost
• not enough perspiration is generated to
  produce the required cooling

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High work rate
Low work rate
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Deaths per summer day in Shanghai
Threshold
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Acclimatization to Desert Heat- Worked 100 min
at 120 F -
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Physical Effects of Heat Stress and Dehydration
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Survival daily water requirement, July,
acclimatized person at rest
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Number of Days Without Water For Which It is
Possible to Walk At Night