Sometimes your pattern of symptoms doesn't accurately tell you how severe your asthma is or when a flare-up is approaching. A good to measure where you stand on a day-to-day basis is to use a peak flow meter. This simple device measures the strength of your breath when you breathe out hard. As you blow quickly and deeply into the mouthpiece, you push a plastic piece along the measuring tube. The point at which the marker stops after your strongest breath is your peak flow.

Peak flow varies from person to person. In general, taller, heavier people have higher peak flows than do small, lighter people, and men usually have higher peak flows than do women. In general, good numbers for adult men are in the 550 to 650 range, whereas women tend to have numbers in the 400 to 500 range.

There are a few things you should know before you measure your peak flow, including:

Peak flow "zones." Peak flow measurements often can detect changes in your asthma even before you feel any changes in your breathing. The higher your peak flow is, the more open are your airways. The lower your peak flow is, the more narrowed your airways are, due to airway muscle spasm and inflammation.

Peak flow numbers can be difficult to remember. That's why many people use three colored zones instead. The colors carry the same messages that a traffic light does:

Your personal action plan should have clear instructions for what you need to do in each zone.

Defining the green zone is easy. On a day when your breathing is at its best, use the peak flow meter to see what your lungs are capable of doing. This number will be inside the green zone. Your health-care provider will help you define the other zones.

You can mark your zones directly on some peak flow meters. Other meters include a chart on which you can record these zones.

Peak flow readings are very helpful when you talk with a health care professional about your symptoms — the numbers allow him or her to quickly assess the severity of a flare-up.