Normal values vary based on a person's age, sex, and size. Peak flow measurements are most useful when a person compares the number on a given day to his or her "personal best."

A fall in peak flow can signal the onset of a lung disease flare, especially when it occurs with symptoms such as:

• Increased cough
• Shortness of breath
• Wheezing

Early treatment may be needed to prevent complications.

Peak air flow during exhalation decreases when the airways are blocked (obstructed).

Many patients can use peak expiratory flow monitoring to monitor their lung function at home. This can be very helpful, especially in asthma patients whose peak flow reading sometimes gets low before they develop breathing symptoms. If the fall in peak flow is treated early, symptoms can be avoided.

If you note that your peak flow is decreasing, tell your health care provider.