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Human Need of Air

Fresh air is required for respiration and for transport of heat and vapor emitted from the human body.

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An adult at rest breathes 15 - 18 respiration per minute - exchanging 0.5 liter - approximately 0.5 m3/h. With harder work the rate is 3 to 6 times more (1.5 - 3 m3/h).

The average composition of exhaled air is

  • Oxygen 16.5%
  • Carbon dioxide 4.0%
  • Nitrogen and Argon 79.5%

The quantity of carbon dioxide exhaled in 24 hrs is about 1 kg.

Equilibrium of Heat

Heat is generated within the human body by the combustion of food. The heat is lost from the body by

  • Conduction and convection - about 25%
  • Radiation - about 43%
  • Evaporation and moisture - about 30%
  • Exhaled air - about 2%

Evaporation prevails at high ambient temperatures. Conduction and convection prevails at low ambient temperatures. Heat is liberated in a rate maintaining the internal body temperature at 37oC.

Heat loss from human body

The total heat loss from an adult at normal activity is approximately 118 W (2 Met) in at room with temperatures between 19oC and 34oC.

Dry bulb temperature
Sensible heat (W) 135 112 100 95 84 59 29
Latent heat (W) 22 22 22 24 34 59 90
Total heat (W) 157 134 122 118 118 118 119
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