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The Pressure's Off!
The view of Earth from outer space shows us a very thin layer of gases that surrounds our planet. Considering what this layer of air does for us, we surely should not take this air for granted! Standing on the ground, buffeted by wind and rain and snow, we think we’re looking up into a vast and endless mass of gas. In truth, our Earth’s air fills a very small container, a container defined not by plastic, glass, or rubber, but by Earth’s gravitational pull.

At only 365 miles straight up the Earth’s atmosphere stops! Above that, all the way to the moon and beyond, there are only occasional molecules and lonely specks of space debris. Until you reach our neighbors Venus or Mars, there is nothing—just an immense vacuum.

Our gas-surrounded Earth becomes even more interesting if we imagine what happens during a return trip from the moon. The first leg of the journey, some 238,500 miles, we encounter nothing! The 365 miles above Earth, we run into our first gas molecules. They don’t make much of an impression. At about 250 miles, where Space Station Alpha orbits, there are a few more gas molecules, but again we still can’t play outside without special suits and oxygen supplies.

As we approach Earth, the blanket of air above us grows denser, and the pressure of the gas molecules being pulled down toward earth grows greater. At 30 miles above Earth, 99.9 percent of all the gases around the earth are still beneath us. The mountain climbers we observe high on Mount Everest are wearing oxygen masks.

Below Everest’s peak, but still three, four, and even five miles up, where private airplanes fly, oxygen must be added to the plane’s cabin so that the passengers can breathe. At about one to two miles, or about 5,000-10,000 feet, the gas molecules in the air become dense enough to let us breathe comfortably. Finally, when we land, we realize that Earth is embraced by what is actually a very thin and, as it turns out, fragile blanket of gas.

Human Respiration
We inhale the gases of the air. Our body’s chemistry and physical makeup use these gases to keep us alive. This story begins when air is inhaled through the nose and mouth. The air moves down the throat, through the trachea, and into the bronchi. The two bronchial tubes enter the lungs where they branch into smaller tubes called bronchioles. The air follows this path and ends up in small air sacs found at the end of each bronchiole. The air sacs are called alveoli.

If there is enough air pressure, the oxygen will pass through the membranes of the alveoli into the tiny blood vessels, called capillaries. The oxygen attaches itself to the blood's hemoglobin and is carried throughout the body to the cells in our muscles, our organs, and our nervous system.

Smoking cigarettes doesn’t improve our physical condition. In fact, the tars contained in the smoke from cigarettes tend to coat, or clog, the lung’s alveoli. This affects the ability of oxygen in the air to pass through the membranes. Emphysema is one disease that results when alveoli are injured beyond repair.

Many human cells are tiny factories. They use oxygen to burn the carbohydrates, fats, and proteins from the food that was processed in our digestive systems. The cells produce the energy that keeps our bodies at an even, warm-blooded temperature and allows us to think clearly, study our homework, run, and chew and swallow more food.

Meanwhile, the burning of oxygen within the cells produces carbon dioxide. Carbon dioxide is a waste product from the process. The carbon dioxide enters the blood stream and moves back to the lungs. In the lungs it passes outward through the membranes. We get rid of this poisonous gas when we breathe out. The cell processes also produce water vapor. Along with the carbon dioxide, we breathe out small amounts of water vapor, which is why our breath will fog up a mirror.

The Space Station: A "Bubble" in Space
From what you have just read, do you understand why it is so important to pay close attention to the atmospheric conditions in the space station? The astronauts' minds and bodies continue to work and process oxygen and fuel and produce carbon dioxide and water vapor, just as they did on Earth. In the very, very thin atmosphere of space, the astronauts still require Earth-like conditions to keep their minds and bodies healthy. They need the same amount of oxygen to breathe that they had on Earth.

As they exhale, the astronauts pollute the space station’s atmosphere. Work and exercise turn them into factories without environmental control systems. Consider the gases they produce. Carbon dioxide, trace elements, and water vapor are either poisonous or can cause damage if too much condensation forms on the space station’s sensitive electrical equipment.

The astronauts also produce methane, ammonia, urea, and other poisons and noxious gases that can really make a mess of the space station’s air. If allowed to accumulate, these natural human byproducts may cause illness and equipment failure. The accumulation of carbon dioxide in the atmosphere can cause carbon dioxide poisoning (see table below).

The Effects of Carbon Dioxide as a Percentage of Total Air Pressure on Humans
At ppCO2 0.2508 mmHg / .03%	No side effects.
At ppCO2 0.456 mmHg / .055%	Air seems "stuffy."
At ppCO2 0.76 mmHg / .092%	(Critical Level) Some people may begin to experience shortness of breath, rapid pulse rate, headaches, hearing loss, hyperventilation, sweating, and fatigue. Astronauts are trained to recognize these symptoms.
At ppCO2 3.8 mmHg / .46%	Too much time spent in these conditions may be permanently dangerous to the astronauts' health, especially if the amount of oxygen decreases.
At ppCO2 11.4 mmHg / 1.38%	(Mission Danger Level) The astronauts suffer serious symptoms within an hour or two. These symptoms include nausea, dizziness, mental depression, physical convulsions, and problems seeing.
At ppCO2 22.8 mmHg / 2.75%	Immediate health symptoms. Loss of consciousness may occur. Increased concentration of gas may prove fatal.

Sustaining an Earth-like Environment
The space station’s scientists, engineers, and technicians designed the life support systems to create and maintain a clean, Earth-like environment within the space station. Computer sensors take constant readings and signal the equipment to adjust and maintain the correct gas composition of the air. Nitrogen and oxygen stored in tanks are added to the atmosphere as needed. Carbon dioxide scrubbers are turned on and off as needed. Dehumidifiers remove excess water vapor from the air.

Sensors are the eyes and ears of the space station's environmental controls and life support system. This system is a technology that supports the physical needs of the astronauts. Maintaining a healthy atmosphere in the space station is a constant challenge. In case of technical problems, there are multiple backup and safety devices on board. The atmosphere must be monitored constantly, because a change in the mixture of gases, in the pressure of the different gases, or of the total air pressure on board the space station could endanger the astronauts without their even knowing it.