Spirometry measures how much air you can inhale and how much you can exhale; it is used to diagnose asthma, chronic obstructive pulmonary disease (COPD) and certain other conditions that affect breathing.
Two important measures are taken, forced vital capacity (FVC) which is the largest amount of air that you can forcefully exhale after breathing in as deeply as you can and Forced expiratory volume (FEV-1) which is how much you can force from your lungs in one second.
A fraction of your FEV-1 divided by your FVC to determine a percentage that's compared with the average results of people of your same height, age and sex to determine how well your lungs are functioning. A result of .8 or 80% falls in the category of normal lung function; however this does not guarantee you don't have asthma or another chronic lung condition.
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