Forced Expiratory Volume (FEV1) and Asthma

Forced expiratory volume in one second (FEV1) is the maximum amount of air you can forcefully exhale in one second. It is used to describe the degree of airway obstruction caused by asthma, as measured with pulmonary function testing, using spirometry.

FEV1 is calculated by converting the spirometer reading to a percentage of what would be predicted as normal based on standardization that's calculated for your height, age, gender, and race. For example, your FEV1 could be 80% of your predicted value.

FEV1 values define the degree of obstruction:

  • FEV1 greater than 80% of predicted = normal
  • FEV1 65% to 79% of predicted = mild obstruction
  • FEV1 50% to 64% of predicted = moderate obstruction
  • FEV1 less than 50% of predicted = severe obstruction
Man about to breath into a forced expiratory volume machine
Science Photo Library Collection / Brand X Pictures / Getty Images

FEV1 is one of the most common indices used to assess airway obstruction, which helps in defining the severity of asthma, as well as evaluating how well treatment is working.

FEV1 is not the same as peak flow, which is determined with a device called peak flow meter that measures the flow rate (speed) of the air blown out. When measured over time, changes in peak flow can be an indication of whether asthma is well controlled or not.

How FEV1 Is Used In Asthma Treatment

An FEV1 test is most likely to be ordered by a healthcare provider as part of complete pulmonary function testing. It may be done to assess symptoms before an asthma diagnosis has been made or to monitor asthma control.

Your healthcare provider may order pulmonary function testing if you have certain respiratory symptoms, such as:

Although originally the only way to measure forced exhalation volume was in a healthcare provider's office, it is now possible to do so using a home spirometer, allowing you and your healthcare provider to use FEV1 testing as part of your home monitoring for asthma.

Home Monitoring and Your Asthma Action Plan

Many asthma action plans use peak flows as one of the criteria for intervention on your part. You and your healthcare provider will determine what your personal best exhalation is and then base your action on a percentage of that number. So it is not really the actual number that is important in this case, but the relative changes that you see over time.

If you are going to use forced expiratory volume as part of your asthma action plan, you will need to talk with your healthcare provider about what sort of home spirometer might be best for you. There are a number of different models with different features and price points.

You will monitor and record your FEV1 over time, after which you and your healthcare provider will place specific FEV1 readings into your green, yellow, and red zones of the asthma action plan.

2 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Sharoon D, Edwards CW. Forced expiratory volume. In: StatPearls. Updated September 14, 2020.

  2. Globe G, Martin M, Schatz M, et al. Symptoms and markers of symptom severity in asthma--content validity of the asthma symptom diary. Health Qual Life Outcomes. 2015;13:21. doi:10.1186/s12955-015-0217-5

Additional Reading

By Pat Bass, MD
Dr. Bass is a board-certified internist, pediatrician, and a Fellow of the American Academy of Pediatrics and the American College of Physicians.