A peak flow meter is a hand-held device that measures your peak expiratory flow rate (PEFR), or how fast you can blow air out of your lungs. Measuring your peak flow regularly can help you tell whether your asthma is getting worse.

To use a peak flow meter, you will first need to find out your “personal best” peak flow. Take a deep breath and blow as hard as you can into the mouthpiece. Your personal best is the highest reading you get on the meter over a 2-week period when your asthma is under good control.