A branch of preventive medicine concerned with the physiological and psychological stresses on the human body in flight. The study of effects within the earth's atmosphere is called aviation medicine, and the study of effects beyond the atmosphere is called space medicine.

An astronaut undergoes self-conducted medical testing onboard the shuttle Challenger. Single electrodes monitor eye movement, while nonrelated narrow strip tape electrodes encircle the forehead, neck, and chest to monitor cardiac activity during movement.

NASA/Phototake NYC

Specialists in aviation medicine study the reactions of human beings to the stresses of air travel; they are concerned with the screening of candidates for flight training and with development of safe aircraft. Their research involves the effects on the human body of acceleration and deceleration, atmospheric pressure, and decompression. High acceleration or deceleration forces, such as those experienced when pulling out of a dive, can cause visual impairment or total blackout. Also, if the head is not properly supported during extreme deceleration, swelling of the sinuses and severe headaches can occur.

A critical consideration in air travel is the body's requirement for oxygen. One condition that results from acute oxygen deficiency is altitude sickness, known medically as hypoxidosis. Symptoms include mild intoxication and stimulation of the nervous system, followed by loss of judgment and unconsciousness. Prolonged lack of oxygen may damage the brain.

Another concern is that because of decreased air pressure at high altitudes, the body tissues can no longer retain atmospheric nitrogen. As a result, gas bubbles may enter the circulatory system and form obstructions in the blood vessels, causing a condition known medically as aeroembolism, commonly called the bends. Aeroembolism leads to confusion, paralysis, or neurocirculatory collapse. Another potential problem is rapid decompression, which results from a drop in cabin pressure at high altitudes and causes major damage to the heart and other organs.

Airsickness is a condition produced by a disturbance of the inner ear, although factors such as apprehension can also play a part. Another condition resulting from long-distance air travel is disturbance in the biological circadian rhythm. This disturbance can produce disorientation and reduce concentration, a condition known as jet lag. While troublesome to passengers, the problem is more acute for pilots, who may have to fly another assignment in a short time.

Most of the dangers in space travel are similar to those encountered in atmospheric flight, but two additional problems must be considered: increased radiation outside the atmosphere and weightlessness. Short orbital flights produce exposures to radiation about equal to one medical X ray, although space flights are planned to avoid periods when solar flares are expected to occur, as these can emit dangerous levels of gamma radiation.

Tests conducted using animals in the 1940s and 1950s suggested that few biological dangers existed in space flight. This understanding was confirmed when human space flight began in 1961 and few serious biological effects were noted. In the 1980s, however, when cosmonauts from the Union of Soviet Socialist Republics (USSR) began setting records for time spent in the gravity-free environment, the effects of weightlessness emerged as a serious problem. Long-term weightlessness resulted in conditions such as a loss of bone matter and muscle strength. Atrophy of muscles in the heart was especially dangerous. The blood was also found to be affected, showing a decrease in the number of oxygen-carrying cells.