A peak flow meter is a portable handheld device used to measure how well a person can blow air out of the lungs. To use it, a person blows into it and then checks the reading. Many people with asthma use it at home to determine how well their asthma is controlled. Asthma causes a person's airways to become narrow, making it hard for him or her to breathe. This device can tell if the airways are getting narrow even before a person has any symptoms. Peak flow measurements can help someone with asthma work together with a doctor to identify approaching asthma flare-ups, to develop a treatment plan, and to decide when to seek medical care.