During PFTs (Pulmonary Function Testing), we take FEV1 (Forced Expiratory Volume in 1 second) and the divide it by FVC (Forced Vital Capacity).

What is so special about FEV in one second, as opposed to a different quantity of time? Why don’t we measure FEV0.5 or FEV1.5?

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    Welcome tapasXplore! I think this question would be greatly improved if you could spell out the acronyms and show us what your research has led you to so far. Even if you haven’t found anything, it’s helpful to state where you‘ve been looking. You can always edit your question. Also, feel free to take the tour.
    – Narusan
    Commented Oct 31, 2018 at 21:08
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    I think my edits clarify it enough to not require additional background info in this case. It's an interesting question, so I answered what is answerable.
    – DoctorWhom
    Commented Nov 1, 2018 at 6:13

1 Answer 1


In the case of spirometry, after taking the deepest possible breath, there are two values used most often:

  1. FVC (forced vital capacity) is the total volume of air that the person can exhale. There will still be some air in the lungs even after exhaling to the max possible.
  2. FEV1 (Forced Expiratory Volume in 1 second) is how much air a person can exhale in 1 second when trying to exhale as fast as they can. As you can see below, since the normal ratio of FEV1/FVC is 0.8, the majority of air can be expelled within one second normally.

Obstructive lung pathology, for example what happens in asthma and COPD, reduces the ability to rapidly expel air due to "air trapping" and therefore the FEV1 drops.

Restrictive patterns like fibrosis do not have that kind of air trapping, so the FEV1/FVC ratio is normal or even higher than normal - but FEV1 is lower because the overall lung volume is lower due to the restrictive pathology.

(Of course this is a simplification; there are mixed patterns and fine details that are out of scope of this question.)

Your actual question about why exactly 1 second as opposed to 0.5 or 1.5 is most likely not answerable beyond the above. Often during research on topics, values are chosen because they are simple to use, and then stick. There are a number of odd values in physiology that were chosen by someone who researched it and now we all memorize it.

Spirometry patterns

  • Can you share the link of orginal paper which says that FEV1/FVC should be less than 0.7
    – TapasX
    Commented Nov 15, 2018 at 10:29
  • I never said it should be less than 0.7. That would be an obstructive pattern, not normal... Plus my link has multiple different text resources.
    – DoctorWhom
    Commented Nov 16, 2018 at 21:40

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