Lung Function:
• Lung function normally peaks in the late teens and early twenties.
• After the early twenties, lung function declines about 1 percent a year over the rest of a person's lifetime.
• Lung function decreases about 2 percent a year for people who smoke.

Pulmonary Function Testing:
• Forced vital capacity (FVC). This measures the amount of air you can exhale with force after you inhale as deeply as possible.
• Forced expiratory volume (FEV). This measures the amount of air you can exhale with force in one breath. The amount of air you exhale may be measured at 1 second (FEV1), 2 seconds (FEV2), or 3 seconds (FEV3). FEV1 divided by FVC can also be determined.
• Forced expiratory flow 25% to 75% (FEF). This measures the air flow halfway through an exhale (FVC).
• Peak expiratory flow (PEF). This measures how quickly you can exhale. It is usually measured at the same time as your forced vital capacity (FVC).

Lung Disease and Dysfunction:
• The two main types of lung disease that can be found with lung function tests are obstructive and restrictive.

Obstructive Lung Disease:
• In obstructive lung conditions, the airways are narrowed, usually causing an increase in the time it takes to empty the lungs. Obstructive lung disease can be caused by conditions such as emphysema, bronchitis, infection (which produces inflammation), and asthma.

Restrictive Lung Disease:
In restrictive lung conditions, there is a loss of lung tissue, a decrease in the lungs' ability to expand, or a decrease in the lungs' ability to transfer oxygen to the blood (or carbon dioxide out of the blood). Restrictive lung disease can be caused by conditions such as pneumonia, lung cancer, scleroderma, pulmonary fibrosis, sarcoidosis, or multiple sclerosis. Other restrictive conditions include some chest injuries, being very overweight (obesity), pregnancy, and loss of lung tissue due to surgery.