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Archived by Raymond J. Noonan, Ph.D., Health and Physical Education Department, Fashion Institute of Technology of the State University of New York (FIT-SUNY), and SexQuest/The Sex Institute, NYC, for the benefit of students and other researchers interested in the human aspects of the space life sciences. Return to first page for background information on these pages.
Space Biomedical Research Institute

Spacecraft Air

In a spacecraft or spacesuit, humans still need to breathe. Since there is no air in space, mechanical and chemical systems must supply breathing gas to the astronauts.

Physiologic Properties of Air

Air fulfills several physiologic needs for the human body. It provides oxygen, and it allows removal of carbon dioxide. It conducts heat away from the body. Air allows sweat to evaporate from the body, providing another means of heat rejection. It allows sound conduction, which enables speech and hearing. Air is transparent in the visible spectrum, enabling sight, but the Earth's atmosphere blocks solar ultraviolet radiation, which can cause cell damage and skin cancer. Air transports particles and gases, enabling the sense of smell. Air provides pressure which maintains the boiling point of the fluids in the human body well above the body temperature. The inert gas component of air, Nitrogen, helps prevent fires from spreading.

Air is a mixture of gases that is necessary to support life. Each time you inhale, air is drawn into your lungs, and oxygen is absorbed into your bloodstream. Hemoglobin in your red blood cells carries oxygen to the rest of your cells. Meanwhile, carbon dioxide is being expelled from your lungs each time you exhale. This breathing cycle continues through the lifetime of all human beings and other animals. We depend on the correct mixture of gases in the atmosphere to sustain our lives.

Air Pressure

Human beings also depend on the pressure of the atmosphere for breathing. Atmospheric pressure is a expressed in millimeters of mercury (mm Hg), or (in the older units used by the Space Shuttle Program) in pounds per square inch (PSI). This represents the amount of force (pounds) acting over a given area (square inch). At sea level, the atmosphere presses in with a force of 14.7 pounds on every square inch of surface. This is enough force to raise a column of mercury up to a height of 760 mm. This atmospheric pressure helps us to inhale and exhale and also makes it possible to keep certain gases dissolved within the bloodstream. If air pressure becomes too low, some gases can come out of solution, and body fluids could even begin to boil at normal body temperatures. The correct mixture of gases in the atmosphere and the pressure of the atmosphere must both be maintained in order for us to breathe.

On Earth, air is a mixture of nitrogen (79%), oxygen (20%), carbon dioxide (about 0.04%), and trace amounts of other gases at a pressure of 760 mm Hg. This gives a partial pressure of oxygen of about 152 mm Hg (20% of 760) (3.06 PSI). On Mercury, Gemini and Apollo missions, the atmospheric pressure in U.S. space vehicles decreased from 14.7 PSI to 5 PSI during liftoff, then maintained 5 PSI of pure oxygen for the duration of the flight. Since the percentage of oxygen was 100%, the partial pressure of oxygen was 5 PSI -- more than what is needed for human survival. On the Space Shuttle, the crew compartment life support system normally maintains the atmosphere at 14.7 PSI, with 79% nitrogen and 21% oxygen. The life support system in the Space Shuttle spacesuits maintains 4.3 PSI of pure oxygen. If air pressure is reduced too rapidly, several physiologic problems can result, including "the bends."

Air Temperature

In the spacecraft cabin and in spacesuits, air temperature must also be controlled. Humans function on Earth with a body temperature of about 37 degrees C (98.6 degrees F) and a narrow temperature band of comfort. Thus, the internal environment of both spacecraft and space suits must have active temperature regulation, involving both heating and cooling so that humans can survive space travel.

In a spacecraft, air is circulated through a heat exchanger which removes excess heat or, if necessary, adds heat to the air before returning it to the cabin. Usually, the heat generated by people and electronics in the cabin and the heat absorbed from the sun is more than enough to keep the cabin warm enough for humans, so heat is removed by the heat exchanger and radiated to space. On the Space Shuttle and Space Station, large radiators are employed to reject excess heat from the vehicle.


Low humidity causes drying of the eyes, skin, and the mucous membranes of the nose and throat. Low humidity also causes chapped lips. High humidity can make breathing difficult and can encourage growth of fungus and microorganisms. On the Space Shuttle, humidity is contolled so that water vapor pressure is between 0.12 and 0.27 PSI (6 to 14 mm Hg).


  1. Name a few of the physiologic properties of air.
  2. What is the temperature outside your room today? Inside?
  3. What is the humidity outside your room today? Inside?
  4. What is the barometric pressure today?

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Last modified: Sep 15, 1994

Author: Ken Jenks


Contact Info:
Raymond J. Noonan, Ph.D.
Health and Physical Education Department
Fashion Institute of Technology of the
State University of New York (FIT-SUNY);
SexQuest/The Sex Institute, NYC
P.O. Box 20166, New York, NY 10014
(212) 217-7460

Author of:

R. J. Noonan. (1998). A Philosophical Inquiry into the Role of Sexology in
Space Life Sciences Research and Human Factors
Considerations for Extended Spaceflight
Dr. Ray Noonan’s Dissertation Information Pages:
[Abstract] [Table of Contents] [Preface] [AsMA 2000 Presentation Abstract]


First published on the Web on June 14, 1998
This page was last changed on March 25, 2002; Ver. 3a
Copyright © 1998-2002 Raymond J. Noonan, Ph.D.

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