This experiment was the first comprehensive assessment of human lung (pulmonary) function during space flight. It flew on Spacelab Life Sciences 1 (SLS-1) in June of 1991 and Spacelab Life Sciences 2 (SLS-2) in October of 1993.

On Earth, gravity affects the way the lungs operate and may even exaggerate some lung disorders such as emphysema and tuberculosis. For example, gravity causes the distribution of ventilation (the air supply to the lungs) to be greatest near the bottom of each lung and become progressively smaller toward the top. In 1-g, the weight of both the rib cage and the lungs distorts the anatomy of the lungs. In space, changes in lung anatomy may cause changes in lung performance.

In a series of tests using the Astronaut Lung Function Experiment (ALFE) hardware, the crew members measured blood flow through the lungs, the ability of the lung to take up oxygen, lung volumes, and other functions. As the subject inhaled and exhaled, a gas analyzer system identified and determined the changes in quantities of oxygen, carbon monoxide, carbon dioxide, nitrogen, argon, helium, sulfur hexafluoride, and nitrous oxide in the bag. By comparing these test results with results from tests on Earth, the investigators could determine the effects of microgravity on lung function. Other experiments that monitored cardiac responses augmented this study, such as experiment 066 which also used a re-breathing technique to study lung and heart function during periods of rest and exercise.