Airman Education Programs
Topics of Interest
Got Nothing But Blue Skies
By J.R. Brown
It is September 19, 1783. The place, Lyons, France. Preparations are being made for a journey. A journey that will eventually take man from his secure environment of terra firma, and place him in a hostile environment called the atmosphere.
The vehicle to be used is a hot air balloon. The brainchild behind this trek is a wealthy paper maker named Joseph Montgolfier. There has been much speculation over just how Montgolfier made the discovery of the hot air balloon. The most commonly-believed story is that his wife was standing too close to a fire and that the smoke caused her skirt to be inflated and lifted above her knees. This caused Montgolfier to wonder-if this smoke, and its magical lifting powers, could be captured in a very large container, it might rise and lift a passenger along with it.
So, Montgolfier went about building the first hot air balloon. In 1783, not much was known about the atmosphere and its effects on human beings.
As science would have it, the first balloon flight passengers were a sheep, a chicken, and a duck. The craft was launched and attained an altitude of about 7,000 feet and fell to the ground a mile and half away.
Upon examination of the occupants for any ill effects caused by this lofty height, it was discovered that the duck had a broken wing. Could this have been an effect of exposure to altitude? Actually, several observers noted that as the balloon left the ground, the sheep had an anxiety attack and kicked the duck.
Montgolfier reasoned that it would be safe for humans to ascend to altitude. So on November 21, 1783, Jean Francois Pilatre de Rozier (a surgeon) became the first aeronaut and flight surgeon.
Over 200 years have passed since that first flight. Technology has allowed us to ascend through the atmosphere and into space, but the hazards of high altitude flight (hypoxia, altitude-induced decompression sickness, and trapped gases) will always be present. That is because humans are best suited to live in what is known as the "physiological efficient zone". This zone extends from sea level to 12,000 feet. When humans are exposed to altitudes above this zone, they are subjected to physiological hazards beyond their natural ability to adapt.
One thing to keep in mind is that everything that occupies space and exerts weight is considered to be matter. All matter is made up of atoms and molecules in varying densities. These particles within the matter are kinetic and in constant motion. The slower the motion of the particles, the more dense the matter becomes. Also, as the particles are pushed closer together, the matter also becomes more dense. The best way to slow down kinetic molecules is to cool the matter. The best way to get them to move closer together is to add pressure to the matter. Inversely, when you remove the pressure or heat any material, the molecules within the material moves faster and further apart, thus making the material less dense.
The least dense form of matter is, of course, gas. If a gas is cooled and compressed, at some point it will become a liquid. If that liquid is then cooled further, then at some point it will become a solid. Also, when you take the pressure off any gas or liquid, that material will grow less dense and expand. This is essentially what happens to the gaseous molecules of our atmosphere.
Our atmosphere contains approximately 79% nitrogen and 21% oxygen, a constant ratio until you reach an altitude of about 270,000 feet. So the question that always comes up is; "If I have 21% oxygen at sea level and 21% at 40,000 feet, why do I succumb to the effects of hypoxia within 20 seconds at that altitude?"
The answer is, atmospheric pressure
. If you could picture all the gaseous nitrogen and oxygen molecules in the atmosphere, they would stack up from the surface of the earth to the fringe of space. All these molecules stacking on top each other create a great deal of weight, or pressure. At sea level, one square-inch of any surface has about 15 pounds of air sitting on top of it. At 18,000 feet, that same square inch has only 7.5 pounds per square-inch (psi) exerted on it. What has caused this atmospheric pressure drop? The answer is simple: There is more air stacked up at sea level than above 18,000 feet, and therefore, more weight.
As you recall, when molecules are subjected to this pressure, they are going to move closer together. This will make the air more dense with oxygen and nitrogen molecules. For example, if at sea level you take in a breath of air that has an atmospheric pressure of 15 psi, then that air may contain 500 billion molecules of oxygen (this a fictitious number to be used only as an example); if you go to 18,000 feet and take the same breath where atmospheric pressure is 7.5 psi, then you will pull in only 250 billion molecules of oxygen. But, you require 500 billion per breath to function normally, and you're getting only half of what you need. That's hypoxia
Not only do gaseous molecules in the atmosphere expand with reduced total pressure, gases in the human body are also subject to the same expansion. There are several areas in the body- ears, sinuses, lungs, gastro-intestinal tract, and teeth - where these gases can expand and cause a variety of problems. As long as the gas can expand and escape, there will be no problem. But if the gas becomes trapped, then pain will be the usual result.
As we have discussed earlier, the air we breathe contains about 79% nitrogen. Nitrogen is inhaled into the lungs and distributed and stored throughout the body. According to gas laws, gases of higher pressure always exert force towards areas of low pressure. When you inhale nitrogen, it will be stored at a pressure of about 12 psi (79% nitrogen) of 15 psi (total atmospheric pressure), equal to about 12 psi).
When you ascend to altitude and the pressure around your body begins to drop, this creates a pressure gradient (higher nitrogen in the body than outside the body) and the nitrogen will try to equalize and escape outside the body. Sometimes this nitrogen can leave so quickly and in such quantify that it may form a bubble. If this bubble forms at a body joint, the pain it causes is know as "the bends."
These are just a few of the problems that can occur when the human body is exposed to high altitude conditions. These problems will always be there for aviation. But through education and knowledge of the mechanisms that cause these problems, we can take steps toward protection and prevention so that your blue skies won't give you a case of the blues.
Mr. Brown is an Aviation Physiology instructor at CAMI's Aerospace Medical Education Division in Oklahoma City, OK.