A peak flow meter is a handheld device that measures the volume of air that you can forcibly exhale from your airways. This volume is referred to as your peak expiratory flow rate or PEFR. PEFR is the measurement from the peak flow meter after you take the deepest breath possible and blow into the meter with a single, forceful blast.

PEFR can indicate one aspect of airway obstruction, and is an inexpensive way to monitor your lung function at home. Measuring your PEFR can be very helpful in detecting changes in airway function, which can signal a potential asthma attack. Your PEFR will improve as your symptoms are better controlled. Your target PEFR is the number you should be able to achieve when your asthma is under control.

Here are the basic steps for using a peak flow meter:

1. Read the instructions that come with your peak flow meter carefully. They will tell you how to operate and clean your peak flow meter, record your daily PEFR, and make sure your peak flow meter is working properly.
2. Set the sliding indicator on your peak flow meter to the base of the meter.
3. Stand or sit up straight and remove any gum or candy from your mouth.
4. Take a deep breath to fully inflate your lungs.
5. Put the mouthpiece of the peak flow meter into your mouth and close your lips tightly around it.
6. As forcefully as possible, blow out as quickly as you can. The force of your breath moves the indicator along the numbered shaft of the peak flow meter. The number opposite the indicator represents the PEFR.
7. Repeat these steps two more times and write down the highest number that you reach. Be sure to take the measurement three times whenever you use the peak flow meter.

In an effort to simplify the results of peak flow meter readings, the National Asthma Education and Prevention Program has developed the following system of "zones," based on the colors of a traffic light.

The GREEN Zone
80 to 100 percent of target PEFR
Readings that fall into this zone indicate that no asthma symptoms are present and that your routine treatment plan for maintaining control can be followed. If you take medications every day and have consistent readings in the green zone, you may want to ask your provider about reducing medications.

The YELLOW Zone
50 to 80 percent of target PEFR
Readings in this zone indicate that you should use caution. When in the yellow zone, you may experience more symptoms of coughing, wheezing, chest tightness, or shortness of breath. You may find that you are waking at night due to asthma and cannot perform all of your usual activities. These symptoms may require a temporary increase in your medication. If you are frequently in the yellow zone, your provider may recommend an adjustment to your daily medication regimen. If you are in the yellow zone and not improving, you should contact your provider to make an appointment for evaluation and further treatment.

The RED Zone
Below 50 percent of target PEFR
Readings in this zone signal a medical alert. If your peak flow meter readings are in the red zone and you are experiencing symptoms of significant shortness of breath or difficulty breathing, or if you can not perform your usual daily activities, you should use your fast-acting bronchodilator immediately and seek prompt medical evaluation and care.