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Peak Expiratory Flow and Peak Flow Meter

Published on November 5, 2025 at 3:00 / Updated on November 19, 2025 at 8:00

Peak Expiratory Flow

Peak expiratory flow (PEF) is the maximum speed at which a person can forcibly exhale air from the lungs in a single breath. It helps determine just how open the airways are. Normal PEF values vary depending on a person's age, sex, and height.

In healthy people, PEF does not vary much from day to day. In asthmatics, however, it will decrease during an asthma attack and when medication is not having the desired effect. This measurement can be useful for:

  • Establishing or confirming a diagnosis of asthma
  • Assessing the effectiveness of treatment
  • Closely monitoring the progression of asthma, such as when your treatment changes or when asthma becomes less well controlled

Short-term PEF monitoring, over a period of a few weeks, may be recommended in these situations. Long-term PEF monitoring is often unnecessary, except in certain cases of severe asthma or for people who do not experience symptoms when their lung function worsens.

Peak Flow Meter

PEF is measured with an instrument known as a peak flow meter: a small device fitted with a mouthpiece and a pointer that slides along a graded scale when air is forced out of the lungs.

How to use a peak flow meter

It is recommended that you use your peak flow meter while standing.The device should be held horizontally. Any object that could interfere with the measurement (such as gum or food in the mouth) should be removed.

  • Make sure the sliding marker is at zero.
  • Take a deep breath.
  • Close lips tightly around the mouthpiece. In one breath, blow as hard as you can.
  • Record the result, usually in liters per minute.
  • Repeat 2 times, making sure the sliding marker is at zero each time.
  • Keep the highest value of the three attempts and record it on your asthma diary.

Peak Flow Meter Maintenance

The peak flow meter should be cleaned at least once a week, or as recommended by the manufacturer of your device.

To clean the meter, soak it in warm soapy water for 5 to 10 minutes, then rinse and allow it to air dry.

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