To the human body, an aircraft represents an artificial environment. Our bodies undergo physiological reactions to extreme conditions like air pressure and elevation.
So what exactly does flying do to us?
The most hard-felt physiological effect of a flight, especially a long one, is dehydration. During a flight, relative humidity drops significantly. The human body is most comfortable with a 50 percent rate of humidity, and can even tolerate humidity as low as 25 percent. In-flight, humidity drops to 10 percent and below -- drier than a desert!
Your eyes and skin begin to feel dry, and the rest of the body reacts to the lack of moisture by compensating or adjusting its biochemical levels. Our bodies consist of over 50 percent water; when that water is depleted, every organ can be affected, and your entire system is thrown out of sync.
The low pressure inside the aircraft can also cause the nitrogen gas in your body to expand, and may affect the times and dosages of medications. Fingers, ankles and joints can also swell, and you may feel bloated.
The lower oxygen levels in the pressurized air can have the same effects as oxygen deprivation on the ground -- light-headedness, difficulty in concentrating, shallow breathing, aches in joints, impaired vision and, believe it or not, loss of mathematical skills.
Relieve the effects of low oxygen by asking the flight attendant for some oxygen for yourself, or requesting that the airflow in the cabin be increased. Airflow requires fuel usage, so many airlines intentionally limit it, especially on the ground.
The air you breathe in-flight is the air you see outside your window -- it is drawn in, compressed, and then pumped into the cabin. Since the air comes from outside the plane, whatever ozone levels and pollutants exist out there are pumped inside for all to inhale. Although you can't see them, your body will definitely feel their effects.
Lesser-felt influences on the body when traveling range from radiation of jet streams and cargo containers, to magnetic fields, to noise pollution.